332 DIGESTION 



37 and 53 C. ; the ' killing ' temperatures between 60 and 75 C. 

 when they are heated in solutions, but considerably higher when 

 they are heated dry. The action of the digestive enzymes is 

 hydrolytic i.e., it is accompanied with the taking up of the elements 

 of water by the substance acted upon. The accumulation of the 

 products of the action first checks and then arrests it. In many 

 cases this seems to be due to combination of the ferment with one 

 or other of the end products, and the consequent segregation of 

 the ferment from the reaction mixture. The enzyme is not affected 

 indiscriminately by any of the end products. On the contrary, 

 their action is curiously selective. Thus the hydrolysis of lactose 

 by lactase is retarded by galactose, but not by the other end 

 product dextrose. The hydrolysis of cane-sugar by invertase is 

 retarded by levulose, but not by dextrose. The splitting of the 

 dipeptid (p. 2) glycyl-/-tyrosin by a ferment in the expressed juice 

 of yeast-cells is greatly delayed by one of the products (/-tyrosin), 

 but not by the other (glycocoll) . Combination of the ferment with 

 an end product is not, however, the only way in which the reaction 

 may stop before the whole of the substrate, as the substance 

 acted on by the ferment is termed, has been changed. It has been 

 demonstrated in some cases that this is due to the action of the 

 enzymes being reversible. For example, lipase (p. 357) not only 

 decomposes the esters ethyl butyrate or glycerin butyrate, but 

 also builds them up again from the decomposition products ethyl 

 butyrate from ethyl alcohol and butyric acid, glycerin butyrate from 

 glycerin and butyric acid (Kastle and Loevenhart, Hanriot). Thus: 

 C 3 H 7 COOC 2 H 5 + H 2 0< >C 3 H 7 COOH + QI^OH. 



Ethyl butyrate. Water. Butyric acid. Ethyl alcohol. 



The action of the enzyme is merely to accelerate the establish- 

 ment of the proportions in which the four bodies entering into the 

 reaction are in equilibrium, and the point of equilibrium is the same 

 whether we start from one or the other side of the equation repre- 

 senting the reaction. Such reversible reactions in the presence of 

 enzymes seem to afford the key to the explanation of many of the 

 syntheses which are known to occur in the body. Sometimes the 

 action is not strictly reversible in the sense that precisely the original 

 material is reconstructed, but from the products of the hydrolysis 

 substances are synthesized or condensed, which are then incapable 

 of being split by the ferment. When a concentrated solution of 

 dextrose is acted on for a long time by yeast maltase, a ferment 

 obtained from yeast which changes maltose into dextrose, some of 

 the dextrose is reconverted into isomaltose and dextrin-like bodies. 

 Isomaltose is not again hydrolysed by maltase. The ferment 

 emulsin contained in almonds behaves in the converse way. It 

 hydrolyses isomaltose so as to form dextrose, and then condenses 

 dextrose to maltose (Armstrong). 



