ENZYMES. 35 



selves undergo any permanent change, for even after they have 

 been acting for a long time, they can still go on doing their work 

 if fresh material be supplied upon which to act. These proper- 

 ties are explained by the fact that they act catalytically , just as 

 the oxides of nitrogen do in the manufacture of sulphuric acid. 

 That is to say, they do not really contribute anything to a chemi- 

 cal reaction, but merely serve as accelerators of reactions, which 

 however would occur, though very slowly, in their absence. Thus, 

 to take our example of starch again, if this were left for several 

 years in the presence of water, it would take up some of the water 

 and split into several molecules of sugar (p. 34). The enzyme 

 ptyalin in saliva merely acts by hurrying up or accelerating the 

 reaction so that it occurs in a few minutes. 



Enzymes differ from inorganic catalysers in the remarkable 

 specificity of their action, there being a special enzyme for prac- 

 tically every chemical change that occurs in the animal body. 

 Thus, if we act on any of the sugars called disaccharides (cane 

 sugar, lactose and maltose) with an inorganic catylytic agent, 

 such as hydrochloric acid, they will split up into their constitu- 

 ent monosaccharide molecules, whereas in the body, each disac- 

 charide requires a special or specific enzyme for itself. The en- 

 zyme acting on one of them, in other words, will be absolutely 

 inert towards the others. This specificity of action is explained 

 by supposing that each substance to be acted on (called the sub- 

 strat) is like a lock to open which the proper key (the enzyme) 

 must be fitted. 



Enzymes are peculiarly sensitive towards the chemical condi- 

 tion of the fluid in which they are acting, more particularly its 

 reaction. Thus the enzyme of saliva acts best in neutral reaction, 

 whereas the enzyme of gastric juice acts only in the presence of 

 acid, and those of pancreatic juice in the presence of alkali. 

 Enzymes may unfold this action either inside or outside of the 

 cells which produce them. Thus, the enzymes produced in the 

 digestive tract act outside the gland cells, but the enzyme of the 

 yeast cell acts in the cell itself and is never secreted. The former 

 are called extracellular enzymes and the latter intracellular. The 

 activities of intracellular enzymes are much more liable to be 



