64 PHYSIOLOGICAL CHEMISTRY 



Influence of Temperature. One of the characteristics of the 

 enzymes is their extreme sensitiveness to temperature. The 

 temperature of the solution in which an enzyme is acting will 

 be found to influence greatly the speed of the action. The tem- 

 perature at which different enzymes will act best is known as 

 their optimum temperature, and for the enzymes in the body 

 this is in the neighborhood of 37-40 C., in other words about 

 body temperature. The enzymes in some of the cold-blooded 

 animals, however, whose body temperature varies with that of 

 their surroundings, act well at temperatures much lower than 

 this, and it is obvious that enzymes in plant cells must work well 

 at temperatures much below that of the body. If a solution 

 containing an enzyme is heated to 60 -80 C., the enzyme is de- 

 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. 



In the existence of an optimum temperature for each enzyme, 

 these substances differ from inorganic catalysts, for which there 

 usually is no optimum temperature, and which usually are not 

 destroyed by heat, whereas all enzymes are destroyed in solu- 

 tion by heating to temperatures often much below 100 C. 



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. 



Tenth normal acid and alkali are the limits beyond which 

 enzyme action ceases. The optimum pH for an enzyme is much 

 influenced by a variety of conditions. For example, certain 

 proteolytic enzymes will digest one protein best at one pH, and 

 another protein at another pH. Also the presence of salts or 



