324 CELL HEREDITY 



TABLE 11.1 

 Inducers of /::i-Galactosidase 



(From Monod and C'ohn, 1952, Advances in EnzynwL, 13:102) 



1 2 3 



Column 1 

 Column 2 

 Column 3 



Activity induced by 10"^ M inducer. 

 Relative affinity for the purified enzyme. 

 Hydrolysis by the purified enzyme. 



enzyme adaptation involves the synthesis of new protein from constituent 

 amino acids. 



It must be noted, however, that the occurrence of enzyme synthesis 

 has been shown in only a few of the many systems involving enzyme 

 adaptation. The appearance of a new protein should be demonstrated 

 by chemical as well as enzymatic criteria. 



The nature of the inducer has been studied in a number of systems, 

 the data being most extensive for jS-galactosidase of E. coli. The 

 inducing ability of various compounds has been compared with their 

 capacity to serve as substrates of the enzyme, by measuring the relative 

 rates of induced enzyme formation in comparison with growth studies 

 employing the inducer as sole carbon source. Some results are shown in 

 Table 11.1. 



It can be seen that many kinds of galactosides can serve as inducers 

 and as substrates. However, not all substrates are inducers, and not all 

 inducers are substrates. This is a very important finding, for it indicates 

 that, although there is a close similarity, the active site of the enzyme 

 is not identical with the postulated site of action of the inducer. 



In consequence of these findings, the phenomenon of enzyme adapta- 

 tion has been redubbed more pointedly, "induced enzyme synthesis. ' 

 An example of the inducer-substrate relation is provided by comparing 

 two galactosides with their thiogalactoside analogues, as shown in Figure 

 11.9. Neither of the thiogalactosides can be hydrolyzed by the enzyme, 

 but the methyl derivative (TMG) is a potent inducer. The inducing 

 ability of TMG is shown dramatically by the growth of cells on a com- 



