Popular Science Monthly/Volume 72/May 1908/Some New View Points in Nutrition
|SOME NEW VIEW POINTS IN NUTRITION|
SHEFFIELD SCIENTIFIC SCHOOL OF YALE UNIVERSITY
IN the latter part of the seventeenth century, long prior to the discovery of oxygen, the English chemist, John Mayow, had laid hold of the important principle that there is something in the air necessary for combustion; that this something is capable of exerting its influence whether it exists free in the air or is combined in the substance undergoing combustion. Further, he pointed out that in the processes of burning and breathing there is a certain definite relationship in that both consist in the consumption of the so-called igneo-aerial particles of the air. He made clear by experiment that the views then prevailing regarding respiration, in which it was held that breathing serves to cool the heat of the heart or to facilitate the passage of the blood from the right to the left side of the heart were quite erroneous. He maintained that in breathing, something belonging to the air, something essential for sustaining life passes from the air into the blood. To quote from his own statement: "On the one hand it clearly appears that animals exhaust the air of certain vital particles which are of an elastic nature. On the other hand there can not be the slightest doubt but that some constituent of the air absolutely necessary to life enters into the blood in the act of breathing."
We fully understand to-day that Mayow's igneo-aerial particles were what we call oxygen, and that in some mysterious fashion he had arrived at a fairly accurate conception of the important part played by these particles in the processes of life. He recognized that the substance embodied in these particles "formed only a part of the atmosphere, that it was essential for burning, that it was essential for all the chemical changes on which life depends, that it was absorbed into the blood from the lungs, carried by the blood to the tissues, and in the tissues was the pivot, the essential factor of the chemical changes by which the vital activities of this or that tissue are manifested. It was essential in muscle to the occurrence of muscular contractions, it was essential in the brain to the development of animal spirits. This great truth was reached at a time when the men of chemistry were struggling with the spiritualistic fermentations of van Helmont on the one hand, and with the material effervescences of Sylvius on the other. It was reached by a young man of twenty-five years, who died a few years afterwards."
For nearly a hundred years this fundamental idea so skillfully worked out lay practically dormant and no material progress was made until, in the latter part of the eighteenth century, Priestley prepared his dephlogisticated air and Lavoisier discovered oxygen. Then came essentially a revival of Mayow's views concerning respiration, only with a clearer understanding of the nature of the process. As stated by Lavoisier and Laplace, "respiration is a combustion, slow it is true, but otherwise perfectly similar to the combustion of charcoal. It takes place in the interior of the lung without giving rise to sensible light because the matter of the fire (the caloric) as soon as it is set free, is forthwith absorbed by the humidity of these organs. The heat developed by this combustion is communicated to the blood which is traversing the lungs and from the lungs is distributed over the whole animal system."
In this conception of so-called respiration the fundamental errors as viewed from the standpoint of to-day are first: the idea that the process deals solely with the combustion of carbon and secondly, that the process is limited to the lungs where a hydrocarbonous fluid was supposed to be secreted, i. e., from or through the tubes of the lungs. Later, Lavoisier himself recognized that in this process of combustion in the animal body hydrogen (discovered by Cavendish in 1781) was likewise involved, and that water as well as carbon dioxide was a normal product of the oxidation that is associated with respiration. Still later, the Italian investigator, Spallanzani, through his experiments on animals broadened the conceptions then prevailing by proving that the individual tissues of the body, like the organism as a whole, respire, i. e., that they consume oxygen and produce carbon dioxide. This view, however, was slow in gaining ground, and for years it was generally held that oxidation as it occurs in the animal body takes place mainly in the lungs. Not until 1837 when Magnus, through use of the air pump, showed conclusively that both arterial and venous blood contain the two gases oxygen and carbon dioxide, though in different proportions, did the theory of respiration and its connection with oxidation take on its present form. Then, gradually, physiologists began to perceive that pulmonary or external respiration had to do primarily with the exchange of gaseous materials and that oxidation did not occur in the lungs, neither to any degree in the circulating blood, but rather in the different tissues and organs of the body where activity of various sorts prevails.
The processes of life, the processes of nutrition, soon came to be looked upon as essentially processes of oxidation. The work of Lavoisier in 1780, indicating as it did that animal heat is the result of a process of slow combustion in which oxygen is used up and carbon dioxide produced, a process analogous to that of the burning candle, naturally emphasized the idea of oxidation. As to the nature of the substance or substances undergoing combustion in the animal body, knowledge at that time was somewhat vague and indefinite. Later, when Chevreul had made his classical studies of fats and Mulder had essayed a description of protein, Liebig came forward with his theories of nutrition, among which was the view so long upheld, that the fats and carbohydrates of the food are burned up directly by the inspired oxygen, while the protein is used to replace the protein of the tissues, the latter being oxidized to furnish energy for muscle work. Liebig conceived that oxygen was the cause of oxidation, a view shown to be incorrect by the well-established fact that animals produce no more carbon dioxide in an atmosphere rich in oxygen than under ordinary atmospheric conditions, i. e., unlike the processes of combustion outside the organism a forced draft is without effect on the rate of burning. Moreover, it was found that oxidation would take place in a tissue independent of an intake of free oxygen, viz., that a contracting muscle, for example, would give out carbon dioxide even when made to contract in a vacuum, thus implying a decomposition or disassociation in which combined oxygen must have been made use of. Further, it was evident that in the tissues and organs of the body, oxidation proceeded gradually through a series of successive steps; the large, complex molecules of the food and tissues being slowly transformed into simpler molecules with ultimate formation of carbon dioxide and water, plus some nitrogen-containing compounds of a relatively simple nature.
During all these years, since the time of Mayow and the later discovery of oxygen, oxidation has been the key-note to which all the varied changes characteristic of life have been adjusted. In ultimate analysis, the breaking down of tissue material in the complicated processes of catabolism has been ascribed to oxidation. While this explanation is in a measure true, the passing years have brought to light many additional data which tend to show that simple oxidation is quite inadequate to account for the variety of transformations that pertain to nutrition. Many other factors are involved that give to these processes a totally different and at the same time broader scope than was formerly thought of. The simple views of Lavoisier and the later theories of Liebig do not suffice; they fail to explain the numerous and complicated reactions occurring during life.
Something was lacking in our knowledge during these earlier years, and that was an understanding of the role of the cell in nutrition. It was difficult for the chemists of this period to let go of the tempting hypothesis that oxidation in the animal body was akin to that of ordinary combustion and their eyes were apparently closed to the many inconsistencies that such a theory imposed. When, however, Virchow developed his cellular hypothesis and it became clear that the living cell was the morphological unit of the body, then it gradually dawned on the physiological world that the cell was likewise the seat of the many chemical transformations associated with nutrition. Oxidation could not occur in the lungs, it did not take place in the blood, there was no one particular spot where the fires of the body were located. On the contrary, they occurred everywhere, in every living cell, and all kinds of combustible or oxidizable material were burned. This conception, in which the living cells might well be compared to miniature laboratories, is now thoroughly justified by the facts at our disposal. Still, it is not the cell as a physiological unit that is to be considered as the cause of the varied decompositions that occur in the body. Enzymes of various types appear in the foreground whenever we attempt to unravel the nature of the processes associated with nutrition; and this is equally true whether we are dealing with the changes incidental to digestion or with those more subtle ones associated with the processes of metabolism. In the living cells of the body there are many agencies at command, enzymes or ferments of divergent forms endowed with the power of inciting and carrying forward chemical changes of differing degrees of magnitude, by means of which complex organic matter is made to undergo alteration and decomposition.
Turn for a time to the changes which the protein or albuminous foods undergo in digestion. Here we have what was for a long period considered as a simple process of transformation or polymerization, brought about mainly through the agency of the two enzymes pepsin and trypsin, of the gastric and pancreatic juice, respectively. Physiologists for years believed they understood the purport of this process, which was merely to transform the protein foods into soluble and more or less diffusible modifications adapted for absorption into the circulating blood. The various forms of vegetable and animal proteins supposedly underwent transmutation without much chemical change, into closely allied substances which by simple osmosis or diffusion could enter the circulation and thus be distributed throughout the body. We see in these views a striking example of how preconceived ideas stand in the way of progress. Advance of knowledge is frequently held back by our proneness to interpret observations or data in harmony with our conception of what they should signify. The old-time physiologists knew full well that the protein of the blood and tissues was made good by the protein of the food, and digestion as they understood it was adequate for the purpose, viz., to transform the protein into a soluble and diffusible form. Anything beyond this was not only unnecessary, but uneconomical and wasteful. I well remember an experience of my own twenty-five years ago in Germany when I was at work on the so-called primary and secondary proteoses formed in gastric digestion, substances at that time just discovered as products of digestive action; how an eminent physiologist from Dorpat happened in the laboratory one day, and after looking at some of the products he turned to Kühne, who was standing near-by and remarked, "It is certainly interesting to see what changes pepsin is capable of producing, but of course they have little bearing on the processes that take place in the stomach and intestine, where naturally the sole aim is to fit protein for absorption."
Another eminent physiologist, whose name has long been known to every student of the science, remarked once in my hearing that the acid-albumin stage, the first product formed by pepsin-acid, was as far as gastric digestion need extend, since this substance was easily absorbed and it was a useless waste of energy for protein to undergo conversion into primary and secondary proteoses and peptone. I recall also what controversies arose when it was established that artificial pancreatic juice could break down protein into leucine and tyrosine; those two crystalline amino-acids now so well known as decomposition products of most proteins. When the fact was established beyond a shadow of doubt, physiologists were still disinclined to believe that any appreciable amount of these relatively simple substances could be formed in ordinary digestion, because such a view was so strongly opposed to the general purpose of protein digestion as then held. We were not inclined to follow the path which experiment was opening up simply because our eyes were blinded by preconceived ideas. I recall an early experiment made by Kühne and myself in the Heidelberg laboratory where a dog was fed a large amount of meat and then after a suitable time chloroformed, the small intestine ligatured and the contents analyzed. We found a gram or more of leucine and tyrosine, which we weighed and identified, thus proving to our satisfaction at least that these two amino-acids were formed in ordinary intestinal digestion. But such data twenty-five years ago, and indeed up to very recent times, failed to attract much attention or were misinterpreted. Physiologists hastened to formulate a theory which would harmonize with existing views, and so arose the theory of "luxus consumption," in which it was held that when an excess of protein food was taken, far larger than the demands of the body called for, the organism was able to protect itself by virtue of this power possessed by trypsin of breaking down protein matter into simple decomposition products easily got rid of with less strain upon liver, kidneys and other organs and tissues.
Many of you doubtless remember the experiments of Schmidt-Mülheim and of Fano, who attempted to determine the amounts of proteoses and peptones present in the blood of dogs after a hearty meal of protein food; and how the negative results they obtained led finally to experiments on the injection of these substances directly into the blood, in which it was found that marked physiological action followed. In other words, proteoses and peptones are not normal constituents of the blood, even though there be a large amount of them in the intestine. They are plainly not absorbed as such, and this fact led to the theory—apparently supported by experiment—that proteoses and peptones in the very act of absorption, in their passage through the epithelial cells of the intestinal wall, are transformed into the proteins of the blood. This made a convenient way of explaining the facts, and one could well imagine that the system took this method of reinforcing the proteins of blood, lymph and tissue. Data, however, have been slowly accumulating which do not admit of such easy interpretation. Physiological chemists interested in enzyme action and equally interested in the chemical constitution of protein matter have been gradually collecting evidence of much significance. A row of diamino-acids, arginine, lysine and histidine, together with alanine, proline, cystine, tryptophane, etc., have been discovered as hydrolytic decomposition products of proteins, both by the action of pancreatic juice and by boiling dilute acids, in addition to the earlier known leucine, tyrosine, glycocoll, aspartic and glutaminic acids, etc. Further, it has been shown that pancreatic juice in artificial digestion experiments, if sufficient time be allowed, is able to bring about a complete breaking down of the protein molecule into these relatively simple amino-acids, so that the biuret reaction, for example, entirely disappears. For a time, this extremely significant fact was not accredited with much importance physiologically; it was interesting; it testified to the general lability of the protein molecule and it threw light on the nature of the building stones which make up the protein complex. Then came, as many of you know, the comparatively recent discovery by Cohnheim of the enzyme erepsin in the duodenal mucous membrane; an enzyme which acts especially on secondary proteoses and peptone, breaking them down quickly into amino-acids. This enzyme is naturally present in the intestinal secretions, becomes mixed with pancreatic juice, and is able to reinforce the latter in the complete destruction of the food proteins in the intestine, the final products being simple amino-acids and their combinations known as polypeptides.
Now, we understand why proteoses and peptone are not normally present in the circulating blood, even of the portal vein. We see that it is no longer necessary to assume a construction of blood proteins from absorbed proteoses and peptone. The old dictum, so often quoted, that the proteins of our muscle tissue, for example, are simple transformation products of the various food proteins no longer satisfies us, since it is so out of harmony with observed facts. We see opening up before us a totally different conception of the process of digestion so far at least as it relates to protein food. It has been a long period of time since the discovery of the proteolytic enzyme of the gastric juice by Schwann, or the early work by Claude Bernard on pancreatic juice. Slowly, but surely, however, our knowledge has progressed, until to-day we are on the threshold of a new vista; new paths spread out before us filled with the light of truth and they bid us hasten to clear away the accumulated misconceptions of the preceding years.
Think for a moment what the new facts lead to in their bearing on nutrition! Recall how we have come to understand that the specific immunities and specific reactions of the blood of different species are properties which reside in the individual blood proteins, and that these peculiarities are associated with the chemical constitution of the proteins. Every physiologist knows how greatly blood from different species varies, and we may safely say that each species of animal probably possesses blood characterized by a personal coefficient in the constitution of its proteins, upon which rests in some measure at least its physiological individuality. In the light of such suggestions is it not difficult to conceive of the varying food proteins of our daily diet being merged into the specific proteins of blood and tissue through simple transformation into closely related proteoses and peptones? Can their individuality be so easily lost by such a superficial alteration? No, the facts at our disposal to-day clearly indicate that the proteins taken as food can not find a place in the economy of the animal body until (as aptly expressed by Leathes) they have been, as it were, melted down and recast. Stated in different language, it is apparently the purpose of digestion, through the enzymes, pepsin, trypsin and erepsin, to thoroughly dismember the protein molecule so that no vestige of its original structure remains; while out of the many fragments or chemical groups so split apart the body can reconstruct proteins adapted to its own particular needs. This means synthesis of a most marked kind, quite different in character from that implied by the hypothetical transformation of absorbed proteoses and peptone in the mucous membrane of the intestine. Further, there is suggested a far-reaching application of this synthetical power in the construction of tissue protein throughout the body.
Let us grant that in the intestinal walls or elsewhere the serum albumin and globulin of the blood are constructed de novo from the many simple fragments split off during the processes of digestion, what then is the origin of the many different forms of protein, nucleoprotein, etc., which characterize the various tissues and organs of the animal body? Do these result from simple transformation of the blood proteins which, as we are wont to say, nourish the active cells of the body and serve as pabulum for the hungry tissues?
In attempting to answer this question, I need only call to your attention the many data collected during the past few years concerning autolysis in general; in which it has been found that practically all the organs of the body are capable under suitable conditions of undergoing auto-digestion, with formation of essentially the same cleavage products as result from the breaking down of proteins in the gastro-intestinal tract. Further, the ferments or enzymes that are responsible for these autolytic transformations have been in some measure isolated and separated from each other. When these facts were first brought to light, it was assumed that the changes in question were mainly at least the result of post-mortem conditions, but there is no justification for such an assumption. Intracellular enzymes are a part of the natural equipment of living cells, and metabolic events, nutritional changes, such as characterize the life and activity of tissues and organs in general, are undoubtedly due to the power of these agents, normally controlled, however, by a variety of conditions that must tend to balance conflicting interests. We can well imagine that in the life and death of tissue cells autolytic decompositions are constantly taking place whereby cell protein is broken down into its component parts, while at the same time a synthesis of protein may be occurring from other amino-acids brought by blood or lymph with a possible utilization of some of the fragments liberated by the autolysis. In other words, our conception of nutritional changes to-day embodies the hypothesis of a synthesis of protein throughout the body; that it is a function of every living cell, "each one for itself, and that the material out of which all proteins in the body are made is not protein in any form, but the fragments derived from proteins by hydrolysis, probably the amino-acids, which in different combinations and different proportions are found in all proteins, and into which they are all resolved by the processes, autolytic or digestive, which can be carried out in every cell of the body" (Leathes).
Here, we have presented several points of view which are radically unlike the old-time traditions concerning nutrition. Objects and methods are both out of harmony with the long prevalent conception of the plan of the living organism in which oxidation was confessedly the ruling power. Profound and progressive hydrolytic cleavage is now seen to be the purpose of digestion, as well as of the autolytic processes which are associated with all the tissues and organs of the body; a cleavage which proceeds until the complex protein is split apart into its simplest chemical groups or components. In this process there is no sign of direct oxidation, but hydrolysis appears in the foreground, and by this means the rock protein is broken asunder into small fragments of definite shape, each of which can be used in the construction of fresh protein. Especially noteworthy is the harmony of action between the enzymes of the digestive tract and those of the living tissues of the body. Both have the same object in view; there may be differences in the rate of action, but in the end essentially the same simple fragments result. Again, what a suggestion of broad constructive power in the cells of the animal body; what a powerful synthetical process that by which the animal cell manufactures the most complex substance of its body protoplasm! For years the chlorophyll-containing cell of the plant world enjoyed the distinction of being the main laboratory in nature for the synthesis of organic compounds. The animal body could transform and modify, it could even accomplish a mild form of synthesis, such as a combination of two large molecules to form a still larger conjugate, but anything like a true synthesis, i. e., the formation of a well-defined complex, such as protein, out of simple amino-acids was far beyond our imagination, until now accumulated facts seem to open up a new point of view.
Are we really warranted in accepting this modern conception of protein synthesis in the animal body? Are the facts available sufficient to substantiate the claim advanced? Physiologists quickly recognized the necessity of confirmatory evidence on this important matter, for a hypothesis so far-reaching in its significance demands careful consideration before it can be given much credence. We understand full well that protein is an essential foodstuff, without which life can not be maintained. We have been accustomed to consider that no other form of nitrogen than protein-nitrogen can supply the physiological needs of the body. If, however, it is true that in normal digestion the protein molecule is completely broken down into relatively simple fragments, non-protein in nature, i. e., into amino-acids and polypeptides, and that from these fragments specific proteins are reconstructed, then it is plain that animals fed on a diet free from protein, but with a proper amount of these nitrogenous cleavage products, should live and thrive, assuming, of course, a sufficient addition of non-nitrogenous food. Experiments after this order have been tried by various investigators, with very interesting results. Dogs, for example, fed on a mixture of protein cleavage products with suitable addition of fat and carbohydrate, maintained nitrogen equilibrium and even stored up nitrogen, presumably in the form of protein, thus indicating that the animals were able to utilize these end products of protein decomposition in much the same way as protein food would be utilized. The only logical supposition is that the dogs were able to manufacture body protein out of this composite of protein fragments; i. e., a synthesis of protein took place; otherwise it would have been impossible to maintain a condition of nitrogenous equilibrium even for a day. Similarly, white rats gained in weight and stored up nitrogen on a diet in which the protein of their food was entirely replaced by the digestion products formed by trypsin and erepsin. Abderhalden and Rona, using a dog as subject and feeding the products formed by pancreatic digestion of casein, viz., the amino-acids and other biuret-free products, found it possible to prevent completely the loss of body protein, a further proof of the power of the animal organism to synthesize protein from its final cleavage products.
Experimental evidence of this character forces us, whatever our preconceived notions, to admit the power of the animal body to build up its needed protein out of the relatively simple decomposition products into which the various forms of food protein are broken down by the processes of digestion. Oxidation does not appear—on the surface, at least—but progressive hydrolytic cleavage is the key-note. The food protein, like a crystalline geode, is split apart into numerous crystalline fragments by action of the several digestive enzymes to which it is exposed in the gastro-intestinal tract, and from these fragments the body cells apparently select such as are required to construct the specific proteins needed for the replacement of those used up in the processes of life. The hydrolytic cleavage induced by these digestive enzymes is, however, in some measure peculiar, in that the fragments are not wholly akin to those formed by hydrolysis with acids. When a given protein is boiled with a dilute acid it is speedily broken down and many of the fragments are identical with those produced by the action of trypsin and erepsin. Thus tyrosine, leucine, arginine, lysine, histidine, etc., appear in both cases, but the enzymes presumably leave intact certain groups or combinations which acids break up or in some way modify. As a result, it is found that the cleavage products formed from casein, for example, by acids, will not take the place of casein, or the products formed by trypsin proteolysis, in meeting the needs of the body. On a diet of protein cleavage products formed in this manner the animal steadily loses nitrogen; it is impossible to maintain a condition of nitrogenous equilibrium, since for some reason the tissue cells can not synthesize protein from the mixture of fragments produced by acids. We may conjecture that while in acid hydrolysis the products are all simple amino-acids, in enzymolysis combinations of the amino-acids, i. e., polypeptides, remain intact. There is experimental evidence that such is actually the case, but equally good evidence seems to show that the presence of polypeptides is not essential for the synthesis of protein by the animal body. Thus, Henriques and Hansen found that by treating the mixed products of pancreatic digestion with phosphotungstic acid, which precipitates, so far as at present known, all the basic bodies including polypeptides, the mixture of monoamino-acids and possibly other nitrogenous substances contained in the filtrate is apparently able when fed to keep animals in a condition of nitrogen equilibrium. Further, the same investigators found, on treating the nitrogenous products (free from biuret reaction) of pancreatic digestion with strong alcohol, thereby separating the substances into two portions, the alcohol-soluble part was quite able to maintain animals in nitrogen equilibrium, i. e., it was equivalent in action to the original protein, while the portion insoluble in alcohol was wholly ineffective. It is thus apparent that all of the fragments resulting from proteolysis are not needed for the synthesis of protein; there are apparently certain products that are not essential or not immediately necessary. On the other hand, it is equally apparent that in the more profound breaking down of proteins by acids, something is done which constitutes a physiological obstacle to utilization of the products in the synthesis of protein by the animal body.
As stated by Leathes:
Whether this conception of the matter is wholly correct we can not say, but at all events it is a suggestion as plausible as any that can be offered at the present time.
Just here we may advantageously consider the nature and proportion of the chemical components present in the protein molecule so far as has been ascertained by hydrolysis with acids. Recently, much work has been done on this subject, especially by Dr. Thomas B. Osborne at New Haven. The accompanying table gives the results with eight typical proteins from the animal and vegetable kingdoms, which may be taken as representative of the present state of knowledge.
Perhaps the most impressive fact, certainly the one most quickly discernible, is that a large fraction of the protein molecule, 28-50 per cent., is of unknown composition. It is not unaltered protein, of that we can be sure, because all protein is destroyed in the hydrolysis. It is presumably composed of small fragments of some kind, not yet recognized by chemists. The next most noticeable feature is that no two of these proteins are alike in their chemical make-up. Proteins from the same grain are distinctly unlike; gliadin of wheat contains no lysine, while leucosin from the same kernel contains 2.75 per cent. of this basic substance; gliadin likewise contains 37 per cent. of glutaminic acid, while leucosin has less than 7 per cent.; gliadin shows 5.6 per cent. of leucine, and leucosin twice that amount; gliadin contains no glycocoll, while leucosin has nearly 1 per cent. of this amino-acid. Such marked differences in chemical composition speak plainly regarding the individuality of proteins, even of those which are associated in the same seed. Comparison of casein from cow's milk, as a typical animal product, with any of the vegetable proteins, brings to light equally strong points of difference, while in gelatin we see many of the familiar amino-acids reduced to a minimum or entirely lacking. While it is undoubtedly true that all proteins possess certain features in common, it is becoming strikingly manifest that they are more or less divergent in chemical constitution. It has been the custom of physiologists in the past to lay stress upon the general rule that proteins are substances capable of meeting the physiological necessities of the body and that their nitrogen exists in a form suited to the needs of the organism. We have been accustomed to point to gelatin as the one exception to the rule, and have classed it as a protein-like substance, with as much nitrogen or even more than most proteins, but not truly a protein, since it can not support life. I fancy, however, that many true proteins may prove, when taken alone, unable to support life. As a matter of fact, few isolated proteins have been tested in this respect. Most of our feeding experiments have been made with mixtures of proteins, and consequently a considerable variety of protein cleavage products have been available for nutritive purposes. Take, as an illustration, the zein of corn meal, which contains no tryptophane, glycocoll nor lysine whatever, and only 1.5 per cent. of arginine and histidine combined, but with 18.6 per cent. of leucine, to say nothing of other peculiarities of chemical structure. Is it not reasonable to suppose that such a protein, with so many of the ordinary chemical groups missing or in greatly diminished quantity, will prove inadequate to meet the demands of protein synthesis? Experiment with animals has, indeed, shown this to be the case.
Data along these lines are bound to bring us more definite information than we at present possess regarding the real merits of vegetarianism as contrasted with the use of animal foods. At present, so-called vegetarianism rests mainly upon sentiment, reinforced by the belief that the quantitative needs of the body for protein food are more satisfactorily met by a liberal addition of vegetable matter with its larger calorific or heat-producing power and smaller nitrogen content. In view, however, of what has been stated concerning the divergent chemical structure of individual proteins, it is obvious that a new standard of comparison is at hand, the suggestions it may offer to be tested by appropriate feeding experiments on man and animals. Truly, no chapter of nutrition is more deserving of careful consideration, both from a scientific standpoint and from its bearing on the welfare of the human race, than that which deals with the relative capabilities of the various proteins of animal and vegetable origin.
In all that has been said we see emphasized the ability of the living organism to break down its complex food material, as well as the corresponding material of its tissues and organs, into the simplest of chemical fragments, coupled with the capacity to construct equally complex tissue material out of the fragments so produced. Profound and progressive hydrolysis, rather than simple oxidation, is the method of decomposition, for many of the fragments at least are to be carefully conserved for future use. Oxygen, however, may play its part in connection with the smaller groups, though even here enzyme intermediation may still be found a ruling factor. Enzymes are to be detected on all sides, both inside and outside the cells of the individual tissues and organs, and it is through their agency that the varied processes of life are carried forward. The present realization of the profound part played by enzymes in the reactions of the animal body is completely transforming our views of life. The so-called vital activities of living tissue or its component cells are no longer shrouded in that mystery which defies explanation, but we see within our reach tangible means of unraveling the complexities of cellular activity. One by one, the old views of living matter and organic structure are giving place to truly scientific conceptions that admit of logical interpretation. It is not long since, when chemists and physiologists alike viewed with enthusiasm, akin to awe, the production of an organic compound by synthesis in the laboratory. I well remember meeting the renowned Wöhler, then an old man, in one of my early visits to Göttingen. Yet, Wöhler was the first to make an organic substance by synthesis. Up to his time, physiologists all believed that organic substances, whether simple or complex, could be formed only through the agency of a living organism. To-day, however, there is almost no limit to our power of producing organic substances by purely chemical synthesis. In the hands of the chemist, many of the reactions of living matter may be duplicated and we are led to see that the living organism makes use of processes which are merely a counterpart of those we have learned to control in the laboratory.
When Buchner a few years ago, by simple pressure, forced from the yeast cell a little limpid fluid and with this was able to induce the same chemical reactions that the living yeast plant produces when brought in contact with a sugar solution, it became clear that the typical formation of alcohol and carbon dioxide is not the result of the life of the yeast plant as formerly supposed, but is instead to be attributed to something—a chemical substance—easily separable from the yeast cell, and quite capable of causing the fermentation of sugar. This reaction, which had for so long been looked upon as a typical illustration of the power of life in inducing chemical change, is merely a simple process of enzymolysis. The yeast plant, it is true, produces the enzyme; but the isolated ferment, once formed, is just as capable of decomposing the sugar as the yeast plant itself. Indeed, the latter is able to accomplish this chemical reaction, solely because of the presence of the enzyme or ferment, now called zymase. In all forms of animal and vegetable tissue, intra- and extra-cellular enzymes abound; enzymes of varied nature, endowed with the power of inducing chemical reactions of diversified character. Those previously referred to in the breaking down of protein material, both in digestion and in autolysis, are typical of what may be found in many of the fluids and in most of the tissues of living organisms. Enzymes which induce hydrolytic cleavage are especially abundant; sugars, proteins and fats all falling as prey to their power of breaking down the respective molecules into smaller and simpler ones better fitted for distribution or utilization. Further, enzymes of the amidase type, which have the power of removing nitrogen from nitrogenous compounds, are equally conspicuous in many phases of intermediary metabolism, especially where changes of nuclein material are involved. In this reaction the elements of water are apparently alone involved, but in some mysterious fashion the enzyme causes a retention of oxygen while the hydrogen passes off with one atom of nitrogen in the form of ammonia, thus leading to the formation of a new substance with one more atom of oxygen than the body from which it was formed and with one less atom of nitrogen and of hydrogen. In this way, gradual oxidation results without free oxygen being involved, while at the same time the content of nitrogen is reduced. Again, there are enzymes separable from the tissues of the body which bring about the destruction of uric acid, not, however, by a process of annihilation, as might be implied by the above statement, but by a method of cleavage in which new bodies less complex are formed. Equally manifest is the action of enzymes which bring about glycolysis, i. e., the destruction of sugar as in the blood; while the separation of the amido group from amino-acids, the oxidation of aromatic aldehydes, the splitting apart of a substance like arginine into urea and ornithine, and a host of kindred reactions, all testify to the multitude of chemical changes that the enzymes of the animal body are capable of producing.
Turn for a moment to the oxidation of an amino-acid such as leucine, which is well known as a product of pancreatic digestion. As this process is carried out in the body under the influence of specific enzymes it is quite different from the ordinary conception of oxidation or combustion. Instead of a complete destruction of the molecule, there is first a removal of ammonia and of carbon dioxide, followed by the formation of an aldehyde and an acid free from nitrogen, together with acetone. The same kind of a reaction can be induced outside the body by some mild form of oxidation, as with hydrogen peroxide, as has recently been shown by Dakin. Here we have a series of reactions, in which an amino-acid by successive oxidation yields a row of non-nitrogenous substances such as are found in the intermediary metabolism of the body, i. e., an aldehyde, an acid, and finally acetone, nitrogen being removed from the molecule in the early stage of the process. Such facts as these throw light upon the methods of oxidation as they occur in the living organism, and they teach us to understand that animal oxidation is quite different in character from the old-time conception of the process. The amount or volume of oxygen has no influence on the character of the change produced, but the specific enzyme exercises a controlling power by means of which a progressive, gradual change is induced leading to the formation of a row of kindred substances of more or less physiological significance.
In other words, the oxygen so freely drawn into the lungs at every inspiration is not directly responsible for the oxidations that take place in the body. Animal oxidation is a roundabout process, in which food and tissue material are first through the agency of numerous enzymes subjected to a variety of changes whereby easily oxidizable decomposition products are formed, which may eventually succumb to the influence of oxygen; even here, however, enzymes of the oxidase type may prove to be the controlling factor in determining whether or not oxidation results.
In the animal body, however, there is no such accumulation of decomposition products as is implied here, but the principle involved may possibly admit of application.
It is generally understood that muscular energy comes primarily from the decomposition or oxidation of non-nitrogenous material, either of the food or of the tissues; and in man we are accustomed to measure the amount of muscular work performed by the amount of oxygen consumed and the amount of carbon dioxide thrown out. In other words, the potential energy of the foodstuffs is made available through oxidation. This, however, is not always the case. Thus, in Ascaris, a round worm inhabiting the intestine of some of the higher animals, we have an animal that can live and show extremely active movements for days at a time without any appreciable amount of oxygen. Carbon dioxide is given off abundantly, however, thereby implying a cleavage or process of disintegration in which energy is freely liberated for the necessities of the animal's machinery. It is quite apparent, however, that oxidation is not the source of muscular energy in these organisms. It may be claimed, and perhaps justly, that such an illustration as this can not be applied legitimately to animals higher in the scale of life, yet there is experimental evidence from various sources pointing in the same direction. Thus, Stoklasa has shown that organs from the higher animals, notably the lungs, liver, pancreas and muscle, yield on pressure fluids, from which by precipitation with alcohol and ether enzymes can be separated, having the power of producing in perfectly sterile solutions of sugar—and with exclusion of micro-organisms—alcoholic fermentation. The proportion of carbon dioxide and alcohol formed under these conditions is the same as produced by yeast. Remembering that in alcoholic fermentation sugar is simply split apart into alcohol and carbon dioxide, it is readily seen that the liberation of a certain amount of energy is possible by simple cleavage of the sugar molecule and without the intervention of oxygen. This then is a form of anaerobic metabolism or respiration, possibly analogous to that which occurs in Ascaris, where carbohydrates are broken down and energy set free for the needs of the organism. Again, Hermann years ago proved that a freshly excised muscle, from which all free oxygen had been separated by exposure to a vacuum, when placed in an oxygen-free medium could be made to work and give off carbon dioxide. Other data of a similar nature might be presented showing quite conclusively the power of animal tissues to carry on various decompositions of complex organic matter with an output of carbon dioxide and with consequent liberation of energy where free oxygen is entirely wanting. These are facts, however, well known to physiologists, but they serve to emphasize the validity of the present point of view, viz., that the processes of animal metabolism are peculiar and are by no means always concomitant with ordinary oxidation. Outside the animal body, the customary components of our daily food, the proteins, fats and carbohydrates, are not affected by oxygen even at the body temperature or by long exposure to the gas. Catalytic action is a necessary prelude to their oxidation, and it is the smaller molecules resulting from the action of enzymes working through catalysis that are mainly burned up or broken down with liberation of their contained energy.
The processes of nutrition are truly complicated, and we can readily conjecture that their harmonious working is dependent in large measure upon the integrity of many closely related operations. Enzymes must be elaborated in due proportions, both in digestive secretions and in tissue cells; proper conditions for enzymolysis must prevail at the places where the reactions take place, since enzymes are extremely sensitive to their environment and fail to work unless all the requirements are fully met; proper conditions of circulation of blood and lymph must be maintained, in order to supply fresh pabulum and to prevent undue accumulation of the products of enzymolysis. In short, there are a multitude of accessory reactions to be preserved in their proper sequence and normal rhythm if perversions of nutrition are to be avoided. Many a substance known to have a deleterious effect upon nutrition does so in virtue of its action upon some one or more enzymes with which it may be brought in contact in the body. Take, for example, the well-known influence of alcohol as a factor in the causation of gout. In this disease, there is an increased amount of uric acid in the system, due in part to an inhibition of its oxidation and consequent destruction. When alcoholic fluids are taken, together with an excess of meat or kindred animal foods, the kidneys at once excrete increased amounts of uric acid, in harmony with the increased content in the blood. It is a well-known fact that alcohol interferes with the oxidative processes in the liver. It is equally well known to-day that the liver and other organs contain an enzyme, or more specifically an oxidase, which has the power of oxidizing uric acid to urea and other products. After the ingestion of alcohol and animal foods rich in uric acid precursors, the notable increase of uric acid in the blood and urine is considered as due to the inhibitory action of alcohol on this oxidase, which under normal conditions causes more or less destruction of uric acid. The failure of the enzyme to accomplish its ordinary duty naturally results in an accumulation of uric acid in the system, although the kidneys plainly endeavor to meet the new conditions by increased elimination. Hence, we see that the predisposition to the development of gout caused by the ingestion of a high protein diet reinforced by alcohol is to be explained in part at least by the direct influence of alcohol on this oxidizing ferment which is normally charged with the destruction of any surplus of this deleterious substance. Here we have a definite and logical explanation of an abnormal condition where interference with the routine action of a tissue ferment or enzyme is one of the specific causes of the disturbance.
This is one of many illustrations that might be cited showing how alterations in the environment of the enzymes occurring in the body may modify the rate of action, either by stimulation or inhibition, and thereby pave the way for marked disturbances of nutrition. It is easy to see also how the many enzymes which rule the normal nutritional processes of the body may need control in order to prevent undue activity, or excessive enzymolysis, with consequent disturbance of the normal nutritional rhythm. Nature has apparently provided this protection by a row of anti-bodies widely distributed which serve as specific antiferments, and either prevent undue alteration or check entirely the action of a given enzyme in certain localities where its action would be detrimental. We find illustrations of such antiferments in the gastro-intestinal tract, by the presence of which the digestive enzymes are restrained from attacking the proteins of the tissue cells, composing the lining membranes of the intestine. Apparently, there is no reason why the enzymes pepsin, trypsin, etc., which digest so vigorously the various protein foodstuffs should not attack with equal avidity the related proteins present in the mucous membranes of the stomach and small intestine. This, however, does not occur during life, no matter how strong the digestive fluids that are secreted into the digestive tract, partly at least because of the inhibitory effect of the natural anti-bodies that are present in the membranes. Again, it is interesting to note that just as antitoxins are produced in the animal body by the injection of a proper amount of toxin into the system, so likewise antiferments can be formed by injection subcutaneously of specific enzymes. Thus, as Morgenroth found, if the enzyme rennin which coagulates milk be injected under the skin of an animal in small doses, after a time the blood serum of the animal so treated will contain something which hinders or prevents the coagulation of milk. In other words, an anti-rennin is formed, just as under similar conditions an antitoxin may be produced. We thus see a close similarity or analogy between the production of a specific immunity toward a given toxin and the formation of antiferments.
Finally, we may again emphasize the specific character of the many ferments that play such an important part in the nutritional processes of man and the higher animals. We readily understand that an enzyme capable of acting upon proteins is quite ineffective when brought in contact with a carbohydrate, or that an enzyme able to digest one form of sugar can not attack even a closely related sugar belonging to the same group. The specificity of enzymes, however, extends farther than this, being intimately connected with the chemical configuration of the molecule acted upon. As a rule, generally accepted to-day, it is understood that living organisms, both animal and vegetable, work mainly with optically active carbon compounds, i. e., compounds in which there is at least one asymmetrical carbon atom. As Kossel has expressed it, the asymmetry of the cell building stones begins the moment of the assimilation of carbon dioxide from the atmosphere by the chromophyl-containing plant cells, from whence it is carried directly to the herbivorous and indirectly to the carnivorous animals. In other words, enzymolysis as it occurs in the animal body is bound up with the chemical constitution and configuration of the substances undergoing change, so that only those substances can be transformed or decomposed that have a certain definite plan of structure. It is thus clear that the processes of nutrition are carefully ordered and clearly defined, while to follow their many paths and interpret aright the signs by the roadside requires accurate chemical and physiological knowledge.
From the early conceptions of nutrition as embodied in the work of Lavoisier and his immediate successors, we have traveled a long way. From vague generalizations based on erroneous views and faulty reasoning, we have passed to a period of scientific activity, where thoughtful observation and careful analysis have contributed to a broader and clearer understanding of the ways of nature. New points of view lie before us pregnant with meaning and full of suggestions for future work. Let us gather together all the facts available, search far and near for all the data that can be obtained bearing upon the question at issue, remembering that progress can come only from intensive and persistent investigation, and that conclusions bearing the imprint of truth must be based upon accurate knowledge. It is only when we lack knowledge that we are liable to be led astray by vain imaginings. How clearly this is illustrated by the experience of the renowned Harvey who when he was arriving at a true understanding of the circulation of the blood, by patient inquiry and still more patient dissecting, was constantly confronted by the crude and illogical views based entirely upon the speculation then prevalent! His many critics who lacked sufficient knowledge to be impressed by his careful demonstrations and who were moreover dominated by the prevalent belief in the spirits provoked from him this statement:
conflicting opinions, that it is not wonderful that the spirits, whose nature is thus left so wholly ambiguous, should serve as the common subterfuge of ignorance. Persons of limited information, when they are at a loss to assign a cause for anything, very commonly reply that it is done by the spirits; and so they bring the spirits into play upon all occasions; even as indifferent poets are always thrusting the gods upon the stage as a means of unraveling the plot, and bringing about the catastrophe.Fernelius, and many others, suppose that there are aerial spirits and invisible substances. Fernelius proves that there are animal spirits, by saying that the cells in the brain are apparently unoccupied, and as nature abhors a vacuum, he concludes that in the living body they are filled with spirits, just as Erasistratus had held that, because the arteries were empty of blood, therefore they must be filled with spirits. But medical schools admit three kinds of spirits: the natural spirits flowing through the veins, the vital spirits through the arteries, and the animal spirits through the nerves; whence physicians say, out of Galen, that sometimes the parts of the brain are oppressed by sympathy, because the faculty with the essence, i. e., the spirit, is overwhelmed; and sometimes this happens independently of the essence. Further, besides the three orders of influxive spirits adverted to a like number of implanted or stationary spirits seem to be acknowledged; but we have found none of all these spirits by dissection, neither in the veins, nerves, arteries, nor other parts of living animals.
Here we have the point of view of the true investigator, the true scientific spirit. Abide by the facts and base your reasoning upon careful observation. Although Harvey lived at a period when physiological knowledge, as we understand it to-day, was almost wholly unknown, and when the influence of the "spirits" dominated all thought, yet he applied rational methods of scientific study and drew logical conclusions from his observations, with the result that to him belongs the honor of discovering the motion of the heart and the circulation of the blood. What he could not see he had no faith in, and so the theories concerning the spirits of the body he laid aside as having no foundation in fact. Would that to all of us might be given that same true appreciation of the importance of scientific observation upon which depends the advance of exact knowledge.
- An address before the Sigma Xi societies of the universities of Missouri, Kansas, Nebraska, Iowa and Minnesota, February, 1908.
- Taken from Sir Michael Foster's "Lectures on the History of Physiology during the Sixteenth, Seventeenth and Eighteenth Centuries," Cambridge University Press, 1901, p. 194.
- Quoted from Sir Michael Foster, loc. cit., p. 198.
- Ibid., p. 249.
- Zeitschrift f. physiologischen Chemie, Band 44, p. 200.
- "Problems in Animal Metabolism," 1906, p. 132.
- Emil Fischer, P. A. Levene and R. H. Aders.
- Aberhalden. Cow's milk.
- Quoted from Bunge: "Text-book of Physiological and Pathological Chemistry," 2d edition, 1902, p. 252.
- Zentralblatt für Physiologie, Band 17, p. 465.
- Quoted from William Harvey: "An Anatomical Disquisition on the Motion of the Heart and Blood in Animals," translated from the Latin by Robert Willis. Everyman's Library, London and New York.