DIGESTION 315 



them. Some of them act best in an alkaline, some in an acid 

 medium. They all agree in having an ' optimum ' temperature, 

 which is more favourable to their action than any other ; a low 

 temperature suspends their activity, and boiling abolishes it for 

 ever. The optimum temperatures of the majority of enzymes 

 lie between 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 all of them is 

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

 elements of water by the substance acted upon. The accumula- 

 tion of the products of the action first checks and then arrests it. 

 It has been demonstrated in the case of some enzymes that 

 this is due to their action being reversible. For example, lipase 

 (p. 334) not only decomposes ethyl butyrate or glycerin butyrate, 

 but also builds them up again from the decomposition products 

 e.g., glycerin butyrate from glycerin and butyric acid (Hanriot, 

 Kastle and Loevenhart). 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 con- 

 denses dextrose to maltose (Armstrong). 



Many of the ordinary substances of the laboratory will accelerate 

 a reaction which goes on slowly in their absence. These are called 

 catalysers. Some writers also speak of catalysers which retard 

 a reaction progressing quickly in their absence. The process by 

 which the reaction is accelerated (or retarded) is termed catalysis. 

 A typical catalyser can exert its action when it is present in 

 exceedingly small amount in comparison with the substance 

 acted upon. However it may enter into the reaction, it does 

 not take part in the formation of the final products nor contribute 

 to the energy changes, and for this reason is often apparently un- 

 altered at the end of the process. A classical instance of catalysis 

 is the inversion of cane-sugar by weak acids, i.e., the change of 

 the cane-sugar into a mixture of equal quantities of dextrose and 

 levulose a reaction which may 'be represented by the equation : 



Cane-sugar. Water. Dextrose. Levulose. 



This is a reaction which occurs also when the sugar is simply 

 dissolved in water, but with extreme slowness at the ordinary 



