THE SYNTHESIS OF PROTEINS 35 



ucts of the substrate. The production of yeast invertase is brought about 

 by glucose as well as by sucrose (Eulcr and Cramer, 1913). 



The production of the enzyme is not inherited but stops when the 

 substrate is removed. 



The potential ability to produce the enzyme, however, is inherited as 

 a mendelian character (Lindegren, Spiegelman, and Lindegren, 1944). 



Yudkin suggests that the new enzyme is always present in minute 

 amounts in all cells and is in equilibrium with its precursor. Addition 

 of the substrate or products of the reaction or other compounds which 

 react with the enzyme will decrease the enzyme concentration. This 

 disturbs the equilibrium and hence more enzyme will be formed. This 

 explanation is adequate and also accounts for the fact that substances 

 which combine with the enzyme, other than the substrate, may bring 

 about its production. 



It includes the assumption that minute amounts of all possible "adap- 

 tive" enzymes exist in normal cells and also that the equilibrium is such 

 that very little enzyme is present. It is further necessary to assume that 

 equilibrium is reached slowly, since the new enzyme does not appear at 

 once. The hypothesis predicts that the new enzyme should be obtainable 

 in zntro if the precursor were available. 



Actually there is no experimental evidence of an equilibrium between 

 an enzyme and its precursor, since the known reactions of this type 

 run to completion {in vitro) as far as can be determined. It is possible 

 to assume equilibrium, but it must be very far in the direction of the 

 active enzyme. 



If the reaction were autocatalytic the combination of the enzyme with 

 the products of reaction would decrease the rate of reaction and hence 

 delay the attainment of equilibrium conditions. The formation of trypsin 

 from trypsinogen, for instance, is stopped by the addition of trypsin 

 inhibitor (Kunitz and Northrop, 1936). 



The assumption that the substrate acts by changing the course of 

 formation of the normal (closely related) enzyme appears to be equally 

 probable. This assumption has the advantage that closely analogous cases 

 are known and also that it may be applied to the formation of anti- 

 bodies, a similar reaction. It has the disadvantage that it does not explain 

 the specificity of the new enzyme quite so definitely. 



According to this hypothesis the normal enzymes are continuously 

 being formed from their precursors in the cell by an autocatalytic reac- 

 tion. When a new substrate or other compound which combines with 

 the enzyme is added, it may act as a co-enzyme and change the course 



