114 IMMUNO-CATALYSIS 



Any body of experimental data offered for the formidation of a 

 concept dealing with the mechanism of the m-ultiplication of a viral 

 or bacterial parasite, and that of the synthesis of viral or bacterial pro- 

 teins, must take into consideration the following well-kno%vn facts: 

 C«) no living cell as yet has been found to synthesize a protein which is 

 not species specifically related to itself; and (I?) no living unit with 

 distinct genetic makeup has been demonstrated to exist which is 

 capable of directing, in accordance with the image of its own species 

 specific characteristics, the synthetic facilities of another living unit 

 belonging to an entirely different species. 



d. Structural Specificity of Polypeptides of Low Molecular 

 Weight. The above considerations suggest that structural specificity 

 is introduced into protein molecules during their synthesis from 

 smaller building blocks. What is more fundamental is the fact that 

 enzymes of the smallest unit of molecular weight (15,000), that is the 

 smallest unit which cannot reversibly be split, possess biological 

 specificities. Polypeptides with a molecular weight of 6,000, such as 

 trypsin and pepsin inhibitors, and d-glutamic acid poh^peptide of 

 B. anthracis possess specific serological (Ivanovics and Bruckner, 1937, 

 1940) and, other activities. Pepsin inhibitor, for example, specifically 

 combines with pepsin, but has no demonstrable effect on the activity of 

 crystalline trypsin and on the milk clotting activity of crystalline 

 chymotrypsin or commercial rennet. Trypsin inhibitor isolated from 

 trypsinogen crystals (inhibitor-free trypsinogen crystals have not been 

 obtained, Kunitz and Northrop, 1936) inactivates trypsin, activates 

 chymotrypsinogen to chymotrypsin, but has no effect on the milk 

 clotting action of pepsin. These findings show that polypeptide mole- 

 cules of 6,000 molecular weight show a high degree of structural 

 specificity. Irrespective of their origin, these characteristics are in the 

 polypeptide molecules, indicating that they are introduced there during 

 their synthesis as independent units, or as parts of certain larger protein 

 molecules, split therefrom, perhaps, by enzyme hydrolysis. 



In this connection, it is of interest to note that Kunitz (1945, 1946, 

 1947) isolated a trypsin inhibitor of globulin nature from soy bean 

 which formed a crystalline inactive complex with trypsin. Lineweaver 

 and Murray (1947) isolated from egg-white an ovomucoid (mol. wt., 

 29,000) which caused 50 per cent inhibition of trypsin at equimolar 

 concentrations. It would be of pertinent interest to learn if these latter 



