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.
- Quoted from Bunge: "Text-book of Physiological and Pathological Chemistry," 2d edition, 1902, p. 252.