NUTRITION 399 



cially on the effect of other substances which are present. It is not ahvays 

 evident just how a third body affects the rate at which one substance is 

 converted into another in a chemical reaction, and so doubtless many 

 effects of this sort pass unnoticed. But when the effect is pronounced, 

 the third body is spoken of as a catalyst, and the effect of the catalyst 

 on the reaction is known as catalysis. By such agents reactions,- so 

 slow as to be unnoticed, may be greatly accelerated and become evident; 

 and others, which might be very rapid, are retarded, even until they are 

 negligible. 



Enzymes. ■ — Among the catalytic agents (which are varied and not at 

 all confined to living beings) are certain substances produced by organ- 

 isms and called enzymes. These are widely different in their action, 

 though they all seem to be of protein nature, so far as their chemical char- 

 acter is made out. The great difficulty in doing this Hes in the impossi- 

 bility, up to date, of separating them from the other proteins of the cell 

 and obtaining them in any certain state of purity. In general they act 

 best within certain narrow limits of temperature, such as 30-45° C, 

 and most are totally destroyed at such temperatures as 60-75° C. Small 

 quantities of free acid or alkali may facilitate their action ; while certain 

 metallic ions, e.g. Hg, Cu, Ag, may retard or inhibit their ordinary 

 effect, just as they " poison " a live cell. 



There seems to be a great variety of enzymes, each producing an ap- 

 propriate effect upon certain foods; but others are known which have 

 to do with reactions quite apart from the digestive changes. The di- 

 gestive enzymes, then, are only part of a larger class of bodies, whose 

 number and variety are only imperfectly known. 



Reversible action. — The action of a number of enzymes is known to 

 be reversible; i.e. they not only, under certain conditions, hasten the 

 otherwise imperceptible decomposition of a particular substance into 

 two or more simpler compounds, but also, under other conditions, ac- 

 celerate the combination of the simpler substances into the more com- 

 plex one. Indeed, it seems likely that the constructive action of enzymes 

 may soon be shown to be as important as the destructive. This action 

 would be of the greatest importance in the making of complex foods from 

 simpler ones, such as the formation of starch from glucose, of cane sugar 

 from glucose and fructose, of proteins from amido-compounds, etc. 

 But the knowledge of this constructive action is yet very scanty. 



Carbohydrate enzymes. — Diastase is one of the most important and 

 widespread enzymes. It is found in practically all parts of plants, but 



