72 A TEXTBOOK OF PHYSIOLOGY 



De-aminizing. A group of enzymes which remove the NH 2 group- 

 from bodies, substituting therefor an OH group and forming ammonia 

 e.g. : 



(1) CH 3 CH.NH 2 .COOH + H20 = CH 3 CHOH.COOH + NH 3 



A Ian in Water Lactic acid Ammonia 



(2) C 5 H 3 N 4 .NH 2 + H 2 O = C 5 H 4 N 4 O + NH 3 



Adenin Hypoxanthin Ammonia 



Coagulative. In this group are included " rennin,'' which helps 

 to bring about the clotting of milk, and " thrombin," which is believed 

 to play a part in the coagulation of the blood. These actions are 

 by no means identical ; for whereas the product of the rennin action 

 is still soluble, that of the thrombin action is insoluble : 



Rennin Thrombin 



Caseinogen Fibrinogen 



Soluble casein Fibrin (insoluble) 



4. The action of enzymes is inhibited by the accumulation of 

 the products of activity. This is best seen in test-tube experi- 

 ments. In the body such products are constantly being removed 

 by absorption. 



5. They are reversible in action. This has been shown to be true 

 for a great number of enzymes, not yet for all. Reversible action was 

 first shown with the sugar-splitting enzyme maltase. This enzyme 

 usually splits maltose into two molecules of dextrose. In a test-tube 

 experiment only a certain, amount of maltose is converted into dex- 

 trose, the accumulating dextrose tending to stop the action. A point 

 of equilibrium is therefore reached when there is present as the result 

 of the enzymic action a certain amount of maltose and a certain 

 amount of dextrose. If more maltose be added, the action goes on 

 until the same proportion is reached. If dextrose be added, the enzyme 

 reconverts some of the dextrose to maltose until the same point of 

 equilibrium is again reached. 



Another example is the enzyme (lipase) which, according to the 

 conditions, breaks ethyl butyrate down into ethylalcohol and butyric 

 acid, or builds up ethyl butyrate from these bodies. 



C 3 H.COOC 2 H 5 = C 3 H 7 COOH +C 2 H 5 OH 



Ethyl butyrate Butyric acid Ethyl alcohol 



This reversible action is particularly important in the case of intra- 

 cellular enzymes ; with varying conditions of the blood and tissue 

 fluids the cells of the body may at one time act in a building-up 

 (anabolic), and at another time act in a breaking-down (katabolic), 

 direction. 



