96 IMMUNO-CATALYSIS 



modified proteinase requires that this be a fact demonstrable in all 

 cases. The proponents of the "adaptive enzyme" concept assume the 

 existence of a freenzyme (Monod, 1942, 1944, 1945, 1947; Lwoff, 

 1946) with which a certain substrate combines and liberates the 

 "adaptive enzyme" (discussed further below). On the basis of a com- 

 plete analogy between Burnet's theory and the above theory of adaptive 

 enzymes, it is to be noted that there does not seem to exist such a 

 preenzyme, to adapt itself to synthesize antibody in contact with newly 

 introduced antibody. The results of experiments have shown that the 

 reinjection of antibody into a previously immunized host from which 

 the antibody is derived, does not stimulate or initiate the production of 

 more of the same antibody. 



c. Adaptive Enzymes — A Consideration of Facts and Assump- 

 tions. In a previous communication (Sevag, 1946), enzymatic, chemi- 

 cal and theoretical reasons were given to reject the concept that the 

 "production of adaptive enzymes" involves the synthesis of a new 

 enzyme protein. Let us summarize the reasons for the position we 

 maintain. 



First: The pertinent data show that the so-called "adaptive enzymes" 

 were present as integral parts of the cells prior to coming into contact 

 with the substrates in question. In the presence of large amount of 

 substrates, the necessary factors seem to be optimal for the maintenance 

 or the demonstration of a measurable activity of the enzyme. The 

 substitution of other structurally related substrates temporarily reduces 

 the activity of the "adaptive enzyme" to a minimum, but does not 

 abolish it. The reversal of the reduced activity to optimal activity with 

 the return of the particular substrate to the metabolic environment has 

 been claimed to be the production of "adaptive enzymes." The restora- 

 tion of the activity to metabolize a substrate is also accomplished when 

 the cells are grown either in the absence of that particular substrate, or 

 also in the absence of a different substrate whose metabolism exercises 

 a deleterious effect on the particular enzyme (Sevag and Swart, 1947). 

 But under no conditions is the particular enzyme completely elimi- 

 nated. 



Second: "Adaptive enzymes" are formed only with those substrates 

 which are configurationally related to a substrate which is more 

 actively metabolized than others. This indicates a "master-key enzyme" 

 for a group and not a specific enzyme for each substrate. It also means 



