122 ENZYMES 



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 has been rendered active its activity cannot be withdrawn or 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 fimction 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 middleman. 



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 

 instance, contains protein which is not digested by gastric protease 

 as long as the blood supply is intact. Occlusion of the blood 

 supply to any part leads to the formation of a gastric ulcer. 

 Parasitic worms live in contact with enzymes that would cause 

 rapid digestion in the event of their death. Neither Cohnheim 

 nor Bayliss is inclined to accept the anti-enzyme idea as correct. 

 (1) The latter has shown that the phenomenon can be explained 

 without any such hypothesis — e.g. by the adsorption of the enzyme 

 by another colloid. Agitation of a suspension of trypsin with 

 charcoal results in a loss of digestive activity due to the adsorption 

 of the enzyme by the charcoal. The charcoal here acts as an 

 anti-enzyme. (2) Enzymes as colloids are sensitive to any altera- 

 tion in their environment. A slight alteration in salt content, 

 colloid or water concentration, or P^ leads to alteration in their 

 power of adsorbing or being adsorbed by their substrate. 



We are now in a position to consider if enzymes should be 

 admitted as catalysts. Do they have the six characteristics 

 detailed on p. 117 ? 



(a) Minute Quantities. They undoubtedly accelerate reactions 

 when present in amounts even more minute than an inorganic 

 catalyst, e.g. in the reaction 



Lactose + water = glucose + galactose 



one might employ as catalyst either an enzyme or an acid. The 

 enzyme lactase is about 5,000 times as effective as an equal weight 



