Soluble complexes of antigen and antibody fragments.

The insolubility of antigen-antibody aggregates formed at most proportions of reactants has prevented physicochemical study of these systems of interacting proteins under conditions in which they remain associated. Only when the antigen is present in great excess and soluble complexes are formed have the products been examined by ultracentrifugation (14)) moving boundary electrophoresis (l-3), or by optical rotation methods (5). Considerable amounts of free antigen were by necessity present in the mixed products used in most of these studies. The preparation of an artificial univalent protein antigen enabled recently a physicochemical study of soluble antigen-antibody complexes over a wide range of antigen to antibody ratios (6). Our finding that immune y-globulin could be converted by waterinsoluble papain into a form susceptible to fragmentation by reduction, but still retaining completely its ability to precipitate homologous antigen, offered the possibility of preparing another type of soluble antigen-antibody complex without resort to the use of excess quantity of antigen (7). Immunological precipitates, formed by antibody pretreated briefly with water-insoluble papain, could be rendered soluble merely by reduction with thioglycolate (7). In this paper, evidence is presented that the faster of the two major sedimenting boundaries observed upon ultracentrifugation of such dissolved precipitates represents an antigen-antibody fragment complex consisting of antigen and y-globulin fragments with antibody activity. Complexes containing several different protein antigens have been isolated, their molecular weights have been determined, and they have been used for various dissociation studies. Dissolved precipitates that contained antigen and antibody in molar ratios varying over a wide range, from the region of antibody excess up to equivalence, have been investigated by means of velocity sedimentation methods, The findings obtained have been used to elucidate further the nature of the original insoluble antigenantibody aggregates.