48 BACTERIOLOGICAL CHEMISTRY 



effect on most enzymes, their activity being merely 

 temporarily inhibited or greatly reduced at low tempera- 

 tures with recovery to the normal rate on warming up 

 again. 



{d) pH. — The acid or alkaline reaction of the medium 

 in which an enzyme operates has a profound effect on its 

 activity. There is an optimum pK value for each enzjrme, 

 and any considerable departure from that value results 

 in inactivation of the enzyme. Thus pepsin, the proteolytic 

 enzyme of the stomach, is only active in acid solution 

 with an optimum at ^H 1-4, whilst the pancreatic enzyme, 

 trypsin, will only hydrolyse proteins in alkaline solution 

 with an optimum between pK 8-2 and 8-7. The salivary 

 amylase hydrolyses starch in slightly acid conditions, its 

 optimum being at ^^H 6-7 ; it is completely inhibited by 

 the acid conditions of the stomach in which pepsin is 

 most active. The variation of the activity of proteolytic 

 enzymes with change in ^H value in many cases appears 

 to run parallel with the calculated dissociation curve of the 

 protein substrate if it is assumed, for instance, that pepsin 

 reacts with the acid cation, and that trypsin reacts with 

 the basic anion of the protein. Papain, with an optimum 

 at ^H 7-0, appears to attack the undissociated protein 

 molecule. It is for this reason that the pH for optimum 

 activity of an enzyme varies somewhat from substrate to 

 substrate, the optimum corresponding with the 2^11 

 required for maximum ionisation in the right direction. 

 In these cases the active part of the enzyme appears to 

 be the un-ionised part. 



(e) Activators. — Often an inorganic or organic activator 

 is necessary before an enzyme can bring about its normal 

 effect. Thus papain, a proteolytic enzyme found in 

 melon seeds, must be activated by hydrocyanic acid or 

 hydrogen sulphide before it will attack peptones, although 

 more complex proteins are hydrolysed by it in the absence 

 of the activator. The animal amylase, ptyalin of saliva, 

 will only hydrolyse starch when chlorine ions are present ; 



