442 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



time is given, and provided also the products of action are removed. 

 The enzyme appears to take a share in intermediate reactions, and 

 there is some evidence that in certain stages it combines with the 

 substrate; but subsequently when the substrate breaks up into 

 simpler materials, the enzyme is liberated unchanged, and so ready 

 to similarly act on a fresh amount of substrate. 



Catalytic Action of Enzymes. The analogy of enzymic action is, 

 in fact, so close to that of inorganic catalysts, that the view at 

 present current regarding it is that the action is a catalytic one. 

 That is to say, the presence of the enzyme induces a chemical 

 reaction to occur rapidly, which in its absence also occurs, but so 

 slowly that any action at all is difficult to discover. To use the 

 technical phrase, its action is to increase the velocity of chemical 

 reactions. It is, for instance, quite conceivable that, if starch and 

 water were mixed together, the starch will in time take up the 

 water and split into its constituent molecules of sugar. But an 

 action of this kind would be so slow, occupying perchance many 

 years, that for practical purposes it does not take place at all. If 

 an inorganic catalyst is added, such as sulphuric acid, and the 

 temperature raised to boiling point, the action takes place in a few 

 minutes; if an organic catalyst, such as the enzyme ptyalin, is 

 added, the velocity of the change is even greater ; but what is of 

 more importance for the well-being of the animal, a moderate 

 temperature, namely that of the body, amply suffices. The organic 

 catalysts or enzymes are, however, colloidal in nature (possibly pro- 

 tein), and this explains their destructibility by high temperatures. 



Reversibility of Enzyme Action. On page 329 we have considered 

 the general laws of molecular reactions. The majority of enzymatic 

 reactions are unimolecular, or reactions of the first order ; that is to 

 say, one substance only, the substrate, undergoes transformation ; 

 the other substance, the enzyme, does not alter in concentration. 

 The law followed in such reactions is therefore the simple logarithmic 

 law. But in these reactions we meet with the peculiarity that 

 it is not quite completed when the reaction ceases. A certain 

 quantity of the substrate never disappears. Thus a small amount of 

 cane sugar remains unchanged whether the hydrolysis is brought 

 about by the action of an acid or of an enzyme. This phenomenon 

 is due to the fact that two reactions are always taking place in 

 opposite directions. Simultaneously with the splitting up the 

 synthetical reaction begins, and synthesis or building up increases in 

 proportion as the splitting of the compound advances. The velocity 

 of the splitting process decreases at the same rate as the velocity of 

 the synthetic process increases. At a certain point, both have the 

 same velocity, and therefore no further change occurs in the mixture 

 when this condition of equilibrium is reached. This rule is expressed 



