302 FOOD AND DIGESTION 



tial factors in the secretions which produce the chemical changes in the 

 foods. Their predominant action is one of hydrolytic cleavage; that is, 

 the substance acted upon takes up water and then splits into two different 

 substances, usually of the same class. The chemical nature of the en- 

 zymes is as yet undetermined because of the difficulty of getting absolutely 

 pure specimens. Their mode of action is at present regarded in the nature 

 of catalysis. That is to say, the enzymes by their presence facilitate reactions 

 that would otherwise take place, but very slowly. Practically all are 

 secreted in the glands as zymogens, which bear the same relation to enzymes 

 as fibrinogen does to fibrin; they are transformed to enzymes by the proper 

 stimulus, but never exist as such in the glands. 



Each enzyme has a special point of temperature at which it acts best, and 

 any change in the temperature retards its action; the action is suspended at 

 a definite point of low temperature, but the enzyme is not destroyed by cold. 

 The action is suspended at a somewhat higher temperature, and at a still 

 higher point the enzyme is destroyed. Some enzymes act only in an alka- 

 line medium, being destroyed in an acid medium, and vice versa. Others 

 act in either alkaline, or neutral, or acid media. Enzymes are hindered in 

 their action by the accumulation of the products of their activity. Most of 

 them cease acting altogether when these products reach a certain concentra- 

 tion, but will begin acting again on the removal of these products or if the 

 mixture be simply diluted. 



The quantity of the enzyme determines the rapidity of the action, but not 

 the amount; a small quantity will digest as much as a large quantity, but will 

 take longer. The enzymes are not used up in the course of their activity, 

 as far as can be seen, and do not seem to undergo any change in their com- 

 position. 



Enzymes are more or less specific in their action. That is, each enzyme 

 is supposed to produce its change in only one particular substance, as in 

 starch, maltose, proteid, fat, etc. An enzyme that can cause cleavage of the 

 starch molecule will not act on fat or proteid or even on other members of 

 the starch group. This specific action is doubtless expressive of a definite 

 relation between the structure of the enzyme and the substance acted on. 



An interesting fact as to enzyme action is its reversibility a phenomenon 

 now well known and well established for carbohydrates and fats. Kastle and 

 Lowenhart have shown that lipase, which acts to split neutral fats into 

 fatty acid and glycerin, will also produce a synthesis, at least of butyric 

 acid and alcohol into ethylbutyrate. Taylor and Robertson in independ- 

 ent papers have recently made the far-reaching discovery that the proteid 

 molecule can be synthesized by the agency (apparent reversible action) of 

 enzymes. 



Enzymes are classified either according to the chemical nature of their 

 action or according to the class of substances on which they act; the former 



