THE SYNTHESIS OF PROTEINS 21 



is autocatalytic and hence is caused by pepsin itself. About 15 per cent 

 of the nitrogen is spHt off during this reaction. So far as is known, 

 pepsin attacks only peptide linkages, so there is reason to believe that 

 the rupture of one or more peptide links in the precursor leads to the 

 formation of the active enzyme. If swine pepsinogen is activated by 

 chicken pepsin, swine pepsin is formed (Herriott, Bartz, and Northrop, 

 1938) . The structure responsible for the species specificity of the enzyme 

 is therefore present in the precursor. 



Relation of Protein to Precursor. The exact changes in chemical 

 structure which occur during these reactions is not yet known. It is 

 known, however, that the protein formed may be quite distinct in chemi- 

 cal, physical, and immunological properties from its precursor. 



It is probable that all amino acids present in the enzyme must also be 

 present in the precursor although, in view of the remarkable reactions 

 discovered by Schoenheimer, even this conclusion may be questioned. 



Immunological Relationship of Chymo-trypsin, Chymo-tryp- 

 siNOGEN, and Beef and Pig Trypsin (Ten Broeck, 1934). The tests 

 were carried out by the Dale technique for anaphylactic reactions. 

 Chymo-trypsinogen was found to be distinct from chymo-trypsin. 

 Guinea pigs sensitized with chymo-trypsinogen were negative in most 

 cases w^hen tested with chymo-trypsin, although four animals out of 

 twelve gave cross-reactions. No cross-reactions were observed when the 

 pigs were sensitized against chymo-trypsin and tested with chymo-tryp- 

 sinogen. There is a slow formation of chymo-trypsin from chymo- 

 trypsinogen even in the absence of trypsin (Kunitz and Northrop, 1935) 

 and it is quite possible that enough chymo-trypsin was formed in this 

 way during the experiment to sensitize some animals, and so produce 

 the cross-reaction. 



Trypsin from beef pancreas was distinct from trypsin from pig 

 pancreas and both were distinct from chymo-trypsin and chymo-tryp- 

 sinogen. 



Pepsinogen and Pepsin (Seastone and Herriott, 1937). These 

 tests were carried out by the precipitin reaction. They are complicated 

 by the fact that pepsin is inactivated rapidly at the pH of the blood so 

 that the antibody formed is presumably caused by the denatured rather 

 than the native protein. It is also possible that traces of pepsin are formed 

 from pepsinogen after injection into the blood stream. 



Pepsinogen antisera reacted with pepsinogen but did not react with 

 pepsin nor with serum protein from the homologous species. 



Antipepsin sera reacted with pepsin and also to a small extent with 



