ANTIBODY AS A SPECIFIC ENZYME INHIBITOR 157 



The specific catalytic action of antigens on globulin factors or a cer- 

 tain group of amino acids, or polypeptides, etc., produces the antibody 

 globulin molecule. Antibody is therefore a final reaction product with 

 specific affinity for the antigen. Similarly the specific catalytic action 

 of an enzyme on its substrate produces final reaction products with spe- 

 cific inhibitory affinity for the enzyme. 



Antibodies being final reaction products of a chain of catalytic re- 

 actions—associated with the synthesis of globulin— should be looked 

 upon as specific inhibitors of antigens. As inhibitors of antigens, or as 

 inhibitors of the biological activity (toxicity, etc.) of antigens they are 

 comparable to the specific enzyme inhibitors which result from the 

 action of enzymes on substrates. This is particularly true when the 

 enzyme reactions take place in a complex protein environment as 

 shown by Northrop (1922) and others. 



The combination between an antigen and its homologous antibody 

 is a stable one and usually results, under optimal conditions, in a pre- 

 cipitation or agglutination reaction. This reaction is reversible under 

 certain not too mild conditions, such as heating at 50°-60°C., disso- 

 ciation by 10 per cent sodium chloride or the separation of the anti- 

 body from its combination with antigen by dilute acids. The action 

 of antibody on antigen does not produce a permanent chemical change 

 in the latter. The neutralization of the toxic properties of toxins and 

 venoms is not one of destruction but is a process of blocking of active 

 groups in the same way that an inhibitor blocks enzymically active 

 groups. Marrack's contention that the action of antigens on antibodies 

 is to produce some change in antibodies resulting in insolubility con- 

 cerns a secondary and physico-chemical property of the antigen- 

 antibody complex rather than a change produced in the antibody mole- 

 cule. For antibodies freed from their combination with antigens have 

 been shown to exercise a neutralizing property identical in every 

 respect with that exercised previous to their combination with antigen. 

 The stable union between an antibody and antigen is therefore a stoi- 

 chiometrical reaction representing a simple combination between 

 multivalent radicals, and it obeys the mass action law, as often empha- 

 sized by Heidelberger and Kendall (1935; Heidelberger, 1939; Ken- 

 dall, 1942). 



In contrast, an enzyme molecule does not form a stable combination 

 with its substrate. The life of the combination between an enzyme 



