22 JOHN H. NORTHROP 



pepsinogen. They do not react with serum proteins of the homologous 

 species. 



These resuhs show that there is at best only a faint cross-reaction 

 between the enzyme and its precursor, even when the experiments were 

 carried out with concentrated solutions of the two pure proteins. Had the 

 tests been carried out with crude tissue extracts of the precursors it is 

 probable that no cross-reaction between the enzyme and its precursor 

 would have been observed, owing to the small quantities present. 



Specificity of the Reactions. Each enzyme, as a rule, has its own 

 precursor, but Bodine (1945) has reported that different tyrosinases 

 may be formed from protyrosinase by different methods. The enzyme 

 precursors isolated so far, therefore, do not represent the hypothetical 

 proteinogen, but must be considered a separate step between this and 

 the active enzyme. The formation of the precursor of tyrosinase has 

 been studied by Allen, Ray, and Bodine (1938) and found to be an 

 autocatalytic reaction. The enzymes whose precursors have been isolated 

 are secretory enzymes and are required in high concentration at special 

 times. It is possible that relatively large quantities of the precursors of 

 these enzymes are to be found in the organs for this reason, since they 

 serve as a convenient source of supply from which the active enzymes 

 can be obtained rapidly at any time. The cellular enzymes and the normal 

 proteins, on the other hand, are not required suddenly and there is no 

 necessity for a rapidly available reserve. This difference in the require- 

 ments may account for the fact that the precursors for the secretory 

 enzymes only have been found in quantity. 



III. Formation of Viruses 



There is good reason to believe that some viruses, at any rate, are pro- 

 teins (for a review of this work see Stanley, 1940). 



No experimental evidence exists, so far as the writer is aware, to dis- 

 tinguish the formation of virus from the formation of normal proteins. 

 Both are found only in or on living cells and virus production is accom- 

 panied usually, if not always, by protein production. Experimentally the 

 only difference between the two processes is that "normal" proteins are 

 always produced by the cells but that the production of some virus pro- 

 tein may be started by introduction of the protein from outside the 

 organism (Northrop, 1937; Darlington, 1944). Phage produced by 

 lysogenic strains of bacteria and "indigenous virus" of plants and 

 animals are always produced just as are "normal" proteins. It can, of 



