138 On the Formation of Anthocyanin 



After a time a certain relative concentration of the four constituents 

 results and at this stage the velocities of the two reactions are equal 

 and equilibrium is established. 



If to a system of this kind, a catalyst, such as hydrochloric acid, is 

 added, the equilibrium position has been shown to remain unaltered. 

 From this it may be inferred that both the hydrolytic and synthetic 

 reactions are equally accelerated by the catalyst. 



In plants the greater number of reversible reactions are of a hydro- 

 lytic nature and are controlled by special catalysts, known as enzymes, 

 produced by the living organism. 



It is now known that a considerable number of these enzymes, as, 

 for example, invertase, maltase, lipase, diastase and emulsin, can be 

 extracted from the living tissues and their activities can be demon- 

 strated under artificial conditions outside the plant. It is then found 

 that in many cases the velocity of the hydrolytic reaction is so much 

 greater than that of the synthetic that the equilibrium position is very 

 near complete hydrolysis. When such is the case, we may infer that 

 there is some, though very little, reversibility of the reaction. Hence 

 if enzymes behave in the same way as inorganic catalysts, it should be 

 possible to show that they are able to again synthesise the products 

 they produce in hydrolysis if the right conditions can be found. 



Croft Hill was the first observer to give experimental proof that 

 enzymes accelerate synthetic processes, though in the special case 

 investigated by him the synthesised product was an isomeric form of the 

 compound hydrolysed. From a concentrated solution of glucose he 

 obtained, through the action of maltase, small quantities of isomaltose 

 which was again hydrolysed in dilute solution. 



Since then many other cases have been discovered, such as the 

 S3mthesis of the ester, ethyl butyrate, by lipase from a mixture of eth}^ 

 alcohol and butyric acid, of the glucoside, salicin, from saligenin and 

 glucose, and of cane-sugar by invertase from glucose and fructose. 



The value to the plant of even a slight reversible action has been 

 pointed out by Bayliss(lA), for if the synthesised product is removed 

 from the sphere of action as rapidly as it is formed, either owing to its 

 insolubility or by translocation, a considerable amount of synthesis may 

 eventually take place. 



There is also in many enzyme actions a special retarding influence 

 exerted by the respective products of action in addition to that due to 

 reversibility of the reaction. Usually the retarding effect exerted by 

 one of the products of action is greater than that exerted by the other ; 



