That the typical digestive enzymes are very pronounced catalysts may be judged from the relatively large amounts of material which they are capable of digesting under favorable conditions. Thus Hammarsten's rennin coagulated 400,000 to 800,000 times its weight of casein; Petit described a pepsin powder which dissolved 500,000 times its weight of fibrin forming 1000 times its weight of peptone; the pancreatic amylase preparation of Sherman and Schlesinger digested 2,000,000 times its weight of starch with the production of 1,200,000 times its weight of maltose.

A catalyzer is usually considered to alter the velocity of a reaction but not to initiate it. Thus hydrogen peroxide decomposes spontaneously into water and oxygen. In a pure aqueous solution this change goes on slowly, but it is very greatly accelerated by the presence of a minute amount of colloidal platinum. Blood and tissue extracts contain enzymes which accelerate the decomposition of hydrogen peroxide apparently in much the same way as does platinum, and the present tendency is to regard the enzymes generally as acting quite like the inorganic catalyzers in altering by their presence the velocity of certain reactions. Some of the best-known enzyme actions, however, fit into this view only theoretically; for if the enzyme be considered as simply accelerating a reaction already taking place, it must also be considered that in the absence of the enzyme the reaction is so slow that it cannot be demonstrated.

It may perhaps be asked why, if enzymes act by catalysis, there should be any limit to the amount of substrate which the enzyme can hydrolyze. One reason that enzymes cannot hy-drolyze infinite amounts of substrate is that they are themselves unstable organic substances which undergo decomposition when kept in solution. In most cases the purer the enzyme the more rapidly its solutions lose their activity. Another reason that an enzyme does not continue to hydrolyze substrate indefinitely is that the reaction is progressively retarded by the accumulation of the products formed.

The activity of an enzyme may be stopped, even when all other conditions are favorable, by the accumulation of the product of its action; and in certain circumstances the action of the enzyme may be reversed so as to accelerate a change in the opposite direction to that in which it ordinarily acts. Thus Croft Hill showed it to be possible to reverse the ordinary action of maltase so as to make it bring about a conversion of mono-into di-saccharide; Pottevin synthesized triolein by means of the pancreas ferment, and Taylor and others have demonstrated a partial reversion of the tryptic digestion of proteins. While the exact significance of these experiments upon the reversibility of the actions brought about by the digestive enzymes has been questioned, there seems to be no doubt that hydrolytic enzymes are widely distributed in active cells and that many of the transformations which take place in the course of the metabolism of the foodstuffs in the body are best explained on the ground of the reversibility of enzyme action. Consideration of the tissue enzymes will be left until the study of the fate of the foodstuffs in metabolism is taken up. At this point it may be convenient to summarize in tabular form the occurrence and action of the chief digestive enzymes.

Summary Of Chief Digestive Enzymes

With this brief sketch of the nature and action of the digestive enzymes, the adequate discussion of which would require a volume in itself, we may now pass to a review of the digestive process, following the course of the food through the human alimentary tract and noting briefly both the mechanical and chemical treatment to which it is subjected.