66 PHYSIOLOGICAL CHEMISTRY 



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). 



This topic already has been discussed under "autocatalysis" 

 at an earlier point in the consideration of enzymes. 



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 

 enzyme for completion, the reaction taking place in successive 

 stages, each one of which is brought about by a different enzyme. 

 Thus the breakdown of starch into glucose requires at least two 

 and possibly more enzymes. The first breaks down the starch 

 through various stages to maltose, the second breaks the maltose 

 into glucose. 



Antienzymes Defensive Enzymes 



If a rennin solution is injected into the blood of an animal, a 

 substance appears in the blood which is capable of destroying 

 the rennin. Such a substance is called an antienzyme or para- 

 lyzer. If various other substances, foreign to the body, are 

 introduced into the blood, enzymes are produced capable of 

 destroying them. Abderhalden has called these substances ' ' de- 

 fensive enzymes." This is referred to later in the discussion 

 of the Abderhalden reaction for pregnancy. It truly is remark- 

 able that the body tissues should be able to produce such sub- 

 stances for defense against compounds which they surely never 

 can have been called upon to meet, either in the development 

 of the individual or of the race. The secret to this problem 

 may hold the explanation of the mode of enzyme production 

 and enzyme action. 



Summary. In summary it may be said that enzymes are sub- 



