118 IMMUNO-CATALYSIS 



group are by themselves active; they are interdependent. In conjugated 

 antigens, vi^hile the determinant character of the hapten is dependent 

 on its combination wdth a protein, the antigenicity of the latter is 

 independent of the haptenic group. These basic differences in the 

 relationship of the coenzyme group to the specific protein in conjugated 

 enzymes on the one hand, and that of a determinant prosthetic group 

 to the protein component of a conjugated antigen on the other, 

 makes the assumption that antigens function as coenzymes en- 

 dowed with autocatalytic powers unwieldy. Furthermore, the results 

 of studies with bacteria and other cells fail to show that a coenzyme 

 group is capable, adaptively or otherwise, of initiating the synthesis of 

 a new protein in a cell specifically reactive with the particular 

 coenzyme where there is none to be found to start with. In contrast, an 

 antibody is produced in vivo only when there is an antigen present. 

 This contrast likewise shows the lack of an experimental basis for an 

 analogy between a coenzyme and an antigen. 



f. Specificity of Cleavage Products Derived from Antibody. How 

 far down in molecular size an antibody molecule can be brought 

 proteolytically, or otherwise, before it loses every vestige of reactivity 

 with homologous antigen is a pertinent fundamental question which 

 has not as yet been adaquately investigated. An antigenic protein 

 hydrolytically split into polypeptide units would lose the ability of 

 stimulating antibody formation, but would maintain the ability to 

 combine with antibody and thus inhibit its reaction with whole 

 antigen. The antibody combining abilities of smaller molecular entities 

 are therefore inherent in these as well as whole molecules from which 

 they are derived. Since these non-antigenic haptens are serologically 

 specific, they must differ in this respect from the corresponding entities 

 present in other proteins derived from the same species. It is reasonable 

 therefore, to assume that there must function enzyme reactions specific 

 for the synthesis of each protein and also for the synthesis of its 

 haptenic parts which are obtainable by the hydrolytic cleavage of the 

 whole molecule. 



In a consideration of the structural difference between y-globulin 

 and antibody globulin (page 139), it was pointed out that diphtheria 

 antitoxin (mol. wt., 184,000) on digestion with proteolytic enzyme 

 gave a crystalline product (mol. wt., 90,000) 90 per cent precipitable- 

 with diphtheria toxin (Northrop, 1942). Immunologically, this product. 



