468 ENZYMES 



its characteristic properties on standing for some time in the 

 cold. The preparation of the enzyme contained some iron 

 and also gave several reactions for aldehyde even after the 

 peroxidase had been temporarily inactivated by heat ; it was 

 found that a trace of iron contained in a solution of an aldehyde 

 added to a peroxidase increased its activity ; from this it was 

 concluded that the precursor of the peroxidase is an aldehyde 

 which, under the catalytic influence of iron, is converted into 

 the peroxidase. 



The above hypothesis coupled with the fact that Falk 

 reactivated lipase by means of manganese perhaps places the 

 significance of iron and manganese on a general basis ; their 

 influence would thus appear to be not so much upon the enzyme 

 itself as upon the production of the enzyme from its precursor 

 or zymogen. 



Below the temperature of inactivation, there is for enzymes 

 a temperature which is most favourable for their activity ; 

 this is termed the optimal temperature and varies within a 

 wide range for different enzymes ; for papain it is in the 

 neighbourhood of 60° C, whilst for maltase it is around 40'' C. 

 Constant values are hard to obtain, for the optimal temperature 

 hastens inactivation and, further, it is dependent on the 

 reaction of the medium, on the length of time of exposure 

 to the temperature, together with other factors such as 

 impurities. 



2. REACTION OF MEDIUM. 



The reaction of the medium has an important influence on 

 the activity of an enzyme. The optimum reaction is that 

 Pg value * at which the enzyme exhibits its greatest activity. 

 It varies with different enzymes and with their degree of purity. 

 This is illustrated by Willstatter,f who points out that the lipase 

 of the human stomach shows its optimum activity at Pjj 5-6 ; 

 but if purified by an adsorption process with kaolin, the opti- 

 mum activity is at P^ 8. 



Willstatter, however, points out that the optimum P^ is 

 not only dependent upon the enzyme but upon the substrate 



* See Appendix. 



t Willstatter : " J. Chem. Soc," 1927, 1359. 



