VELOCITY OF REACTION 9 



the active mass of the components of the system. If 

 water is present in abundance, the equiUbrium-point will 

 be almost at complete hydrolysis. But if ethyl acetate 

 be removed from the.system as soon as it is formed, complete 

 synthesis will take place. 



The question now arises whether enzymes behave hke 

 inorganic catalysts in this respect. Many reactions occur 

 reversibly in the body : the saponification and synthesis 

 of fats ; the intercon version of glycogen and glucose. Rever- 

 sibility of action has been proved for certain enzymes, 

 particularly for maltase and lipase. It is therefore probable 

 that in the body an enzyme accelerates a reversible reaction 

 in both directions, but that the actual change which takes 

 place depends upon the removal of certain products from 

 the sphere of action as soon as they are formed. When fat 

 is saponified in the intestine by the action of Upase the 

 process is complete, because the products of saponification 

 are rapidly absorbed. Within the intestinal epithehum 

 these accumulated products are resynthesised, probably 

 by the lipase which formed them. 



5. Velocity of Reaction. — When the amount of enzyme 

 is small compared with the amount of substrate, the rate 

 of reaction is, in the initial stages, directly proportional 

 to the amount of enzyme, and independent of the amount 

 of substrate. The enzyme, in other words, can only deal 

 with a certain amount of substrate at a time. But the 

 final result, given sufficient time, is the same whatever the 

 amount of enzyme ; that is to say, there is no quantitative 

 relation between the amount of enzyme and the amount 

 of substrate. This constitutes a useful criterion in deciding 

 whether a substance is a ferment or not. 



When the amount of enzyme is relatively large, the 

 velocity of the reaction undergoes a progressive diminution. 

 This is to be expected from the law of mass action, since 

 the concentration of the substrate is undergoing a constant 

 diminution. The falhng off, however, is usually more 

 rapid than would be expected from theoretical considera- 



