ACTIVATION 121 



Tr\'psiii (CjV. Tribein — to rub — prepared by rubbing pancreas with 

 sand and glycerol) = alkaline or pancreatic proteinase. 



Some writers prefer to use names which point to the splitting 

 power of the enzymes, e.g. 



proteolytic or proteoclastic enzymes act on proteins 

 amylolytic or amyloclastic ,; ,, starches 



lipolytic or lipoclastic ,, ,, fats. 



On the other hand, hydrolytic enzymes produce their effect by 

 adding or subtracting water. 



Some enzymes act in the cells while others are secreted by the 

 cells and act on a substrate outside the cell. The former, endo- 

 enzymes, have been little studied. An active suspension of them 

 may be prepared by grinding up tissue with sand and extracting 

 with watery glycerol. It is probable that all muscle cells contain 

 enzymes which act on protein - disintegration products, either 

 rebuilding proteins from amino acids or breaking down these 

 amino acids. Similarly, the regeneration and the disintegration 

 of carbohydrates and fats have been attributed to endo-enzymes. 

 There are also special enzymes to carry out oxidations and reduc- 

 tions in the cell. The various stages in the production of uric 

 acid from nucleoprotein have been studied exhaustively, and 

 each stage has been shown to have its enzyme or series of enzymes. 



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. 



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^. The moment that the juice comes in contact 

 with the substrate, digestion begins. 



Zymogen Secretion. 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, pseudo-activation. 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„. This is not a true 

 activation. Acid does not so much activate pepsinogen as form 

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

 demonstrated by neutralisation of the acid, with consequent loss 

 of activity in the enzyme. On reacidifying, digestive activity 



