10 ENZYMES 



tions. Several factors contribute to this. There is the 

 gradual development of the reverse reaction. The enzyme 

 may be killed by the products of its own action. Again, 

 the products may cause a change in the reaction of the 

 medium which itself inhibits the action of the enzyme. 

 In the tryptic digestion of proteins, for instance, the amino- 

 acids formed, being many of them distinctly acidic, increase 

 the H-ion concentration, and thus tend to retard the action 

 of the enzyme. 



Is all metabolic activity due to the action of enzymes ? 

 At present this question cannot be answered decisively. 

 There are certain reactions which can be brought about 

 by hving cells, but not by enzymes. No enzymes, for 

 instance, have been discovered in the mammary gland 

 capable of forming the organic constituents of milk. Again, 

 antiseptics of a certain concentration are lethal to proto- 

 plasm but not to enzymes. It is possible that all stages 

 exist between simple enzyme action and protoplasmic 

 activity. 



