34 JOHN H, NORTHROP 



It is possible to assume, however, that the partially inactivated virus 

 reacts with the precursor and forms more "inactive virus," but this 

 virus is not liberated from the cell since no lysis occurs. From this point 

 of view the interference caused by the "inactivated" virus is similar to 

 the interference between two viruses and is due to the destruction of the 

 precursor. 



IV. Formation of Adaptive Enzymes 



The formation of normal proteins and enzymes and of viruses out- 

 lined in the preceding section is purely autocatalytic and results in the 

 formation of more of the same molecules. The formation of adaptive 

 enzymes or antibodies, however, is not purely autocatalytic, since the 

 molecules formed are different from those originally present. The hy- 

 pothesis, therefore, must be modified to account for the formation of 

 these compounds. 



Enzymes are sometimes formed as a specific response to the presence 

 of the corresponding substrate or its decomposition products. Enzymes 

 formed in this way are called "adaptive enzymes" (Karstrom, 1930). 



The production of galactase by yeast is the best known example but 

 many similar cases are known (cf. Yudkin, 1938; Knight, 1936; Dubos, 

 1939). "Normal" yeast cannot ferment galactose, but Dienert (1900) 

 found that yeast acquires this power if grown in the presence of galac- 

 tose. It was thought at first that this result was due to selection of yeast 

 cells during growth of the culture. Later work established the fact that 

 the enzyme can be formed without any increase in the number of cells. 

 If this observation is correct the fermentation cannot be due to selection 

 of yeast cells, but must be due to production® of a new enzyme." The 

 formation of the enzyme is always associated with the formation of 

 protoplasm, even though no cell division occurs (Dubos, 1939). 



In some cases, like the polysaccharide-splitting enzyme of Dubos 

 (1939), the enzyme displays extreme specificity, in fact the specificity 

 of this enzyme is more precise than the immunological tests. 



Formation of the enzymes may be caused by the decomposition prod- 



9 There does not appear to be any advantage in distinguishing between "production" 

 and "activation" of an enzyme. An inactive enzyme, strictly speaking, is not an enzyme 

 at all. 



1° Sevag (1946) considers that no new enzymes are actually formed but that the new 

 substrate increases the activity of the normal enzyme in some unexplained way. Ac- 

 cording to this point of view the phenomenon is simply a change in the specificity of the 

 reaction similar to that which occurs when various amino acids are added to peptidase 

 systems (cf. page 11). 



