unaltered itself, accelerates a chemical reaction, otherwise going on very slowly. To elucidate with an example: fat is a chemical union of two compounds, one of which is called a fatty acid, and the other an alcohol. Fat, in the absence of a fat splitting enzyme, yields very small quantities of these two substances in the course of a long time. But, in the presence of a proper enzyme the fat yields considerable quantities of fatty acid and alcohol in a comparatively short time. The rapidity of the splitting is directly proportional to the amount of the enzyme added. A small amount of the enzyme will decompose just as much of the fat as a large quantity will, but a longer period of time is required. A quantity of the enzyme may be used over and over again for splitting any amount of fat, unless it is destroyed by bacteria, heat, chemicals, or some other deleterious agent.
When fat and an enzyme are placed in a test tube together, not all the fat is changed into its component parts. The reaction proceeds until more than half the fat is decomposed. Then there is a reversal of the chemical reaction. Fat is reformed from the fatty acid and alcohol; the splitting process proceeds very slowly if at all. The fat formation goes on as the predominant process in the tube until an excess of fat is formed, when a reversal again occurs, and fat decomposition becomes the chief reaction in the test tube. The alternate breaking down and building up goes on indefinitely, like the swinging back and forth of a pendulum. The sweep of the pendulum when first started may be broad, but if allowed to swing uninterruptedly, there is a gradual diminution of the distance traversed until the pendulum eventually comes to a standstill. Thus it is with the chemical reaction. This power of an enzyme to carry a chemical reaction in either direction is spoken of as the reversibility of enzymes. This has not been demonstrated to be true of all these bodies, but the physiologist delights in the speculation that it is; and many are the problems planned to demonstrate this characteristic in this or that enzyme.
Enzymes have important functions to perform in both animal and plant economy. Practically all of the chemical reactions, normally occurring in life processes, are believed to be aided by ferments. The distribution of these bodies in an organism is general. In man they are found, not only in the alimentary canal, but in the blood and lymph and presumably every cell of the body. The ferments of the alimentary canal are there for the purpose of splitting the food stuffs into their components, which are more readily absorbable than the original materials. Those of the liquids and cells of the body reform and build up the food elements into the vital tissues or protoplasm of the organism.
Another process in which enzymes play an omnipotent part is that of respiration. The oxidation of the protoplasmic constituents, from