ACTIVATION 97 



The ecto-enzymes are secreted in the various digestive juices 

 and act on their substrates in some portion of the alimentary 

 canal. They really act outside the body and have one function 

 only to break down the food into a state in which it can pass 

 through the gut wall into the body. 



Zymase Secretion. Some of these enzymes seem to be secreted 

 ready for action. They themselves are in the active state, and 

 the juice of which they form a part contains the necessary salts 

 and has a suitable P H . The moment that the juice comes in 

 contact with the substrate, digestion begins. 



Zymogen Secretion. 



Pseudo- activation. Others, however, enter the alimentary canal 

 in an inactive state. Their inactivity is not due to the lack of 

 a suitable medium, but to the form in which the enzyme appears, 

 i.e. as a pro-enzyme or precursor of the enzyme. An activator 

 is required. For example, the active principle of gastric juice is 

 secreted as pepsinogen which becomes active pepsin on coming 

 into contact with a fluid of a certain P H . This is not a true 

 activation. Acid does not so much activate pepsinogen as form 

 a necessary concomitant for pepsin. That this is so may be 

 demonstrated by neutralisation of the acid, with consequent loss 

 of activity in the enzyme. On reacidifying digestive activity 

 restarts. Acid and pepsin have been termed co-enzymes a 

 misleading term. True activation is irreversible. Once an 

 enzyme has been rendered active its activity cannot be withdrawn 

 or restored at will. As an example of true activation, the pan- 

 creatic enzyme trypsin may be taken. Pancreatic juice drawn 

 from the duct contains trypsinogen. This precursor gives birth 

 to active trypsin on coming into contact with enterokinase of the 

 succus entericus. The mechanism of the change is unknown. 

 Enterokinase is an enzyme whose sole function is to act on the 

 zymogen form of trypsin. No other protease can be substituted. 

 The rate of activation is peculiar and suggests autocatalysis 

 i.e. it starts slowly at first and the rate rapidly increases with time. 

 Vernon suggests that a third enzyme, deuterase, acts as a middle- 

 man. 



A simpler explanation might be found in the adjustment of 

 equilibrium between two hydrophilic colloids with different 

 crystalloid contents. 



In order to explain the immunity from digestion of the living 

 cells, anti-enzymes have been postulated. The stomach wall, for 



