48 PHYSIOLOGICAL CHEMISTRY 



stroyed, and cooling the solution will not restore its activity. 

 On the other hand, enzymes in general can be exposed to tem- 

 peratures near the freezing point. Their activity is retarded but 

 will return if the solution is warmed. 



Effect of Chemical Reaction. Enzymes are also very sensi- 

 tive to chemical reaction. They are destroyed by strong acids 

 or alkalies. The most favorable reaction differs with different 

 enzymes, some acting best in weak acid, others in weak alkaline 

 solution. Certain of the. enzymes which act in weak acid solu- 

 tion are destroyed by making the solution even faintly alkaline, 

 and the converse case is also true. Some, on the other hand, 

 will stand considerable variation in this respect, acting in weak 

 acid, in neutral or in weak alkaline solution. 



Reversibility. Some of the enzymes have the power of 

 causing a chemical reaction to go either way, that is, of de- 

 composing a compound, or under proper conditions of building 

 up the same compound from its decomposition products. Cer- 

 tain of the lipases are notable examples. The property is spoken 

 of as reversibility, and these enzymes are said to be reversible in 

 their action. 



Active and Inactive Form. In the form in which they are 

 secreted by cells, some of the enzymes are inactive, and become 

 capable of exerting their customary activity only after they have 

 been acted on by some other substance. The enzymes are thus 

 said to exist in inactive and active forms. 



Action Retarded by Products. The activity of an enzyme is 

 often retarded by the products which it produces from the sub- 

 strate. In life (in vivo) these products usually are removed, as 

 in absorption of the digestion products from the intestine. In 

 a laboratory experiment in a test tube or beaker, this may influ- 

 ence the extent of digestion by an enzyme to a considerable ex- 

 tent. This fact may be summed up by saying that enzyme ac- 

 tion often is incomplete in vitro (in glass). 



Progressive Action. The enzymes are often said to be pro- 

 gressive in their action. This is only a special case of their 

 specific nature. Many reactions depend upon more than one 



