Isojima and co-workers have reported that crude hCG preparations, extracted from the urine of pregnant women and trophoblastic tumor patients, contained two immunoreactive components designated as Antigen 1 (Ag1) and Antigen 2 (Ag2). It was apparent that Ag1 was identical to highly purified hCG from immunological and biological studies. Ag2 possessed a portion of the antigenic determinants of Ag1 by the double diffusion test but no biological activity by the immature female rat ovarian augmentation method.
Recently Ag2 was found in commercial hCG preparations and separated by gel filtration on Sephadex G-100 in our laboratory. In this study, Ag2 has been characterized and compared to subunits of hCG by SDS disc gel electrophoresis and immunoelectrophoresis using antisera to hCG and hCGβ. Immunoelectrophoresis using absorbed anti-hCG antiserum resulted in the precipitin arcs of Ag2 and hCGα being displaced toward the cathode, but no precipitin arc of hCGβ was obtained. The precipitin lines with a single spur of Ag2 and hCGα showed that the antigenicity of Ag2 had a partial identity with that of hCGα. By immunoelectrophoresis using anti-hCGβ antiserum, the precipitin arc of hCGβ was migrated at the origin and also fused with that of Ag2 with a single spur. When subjected to SDS disc gel electrophoresis, Ag2 showed only a single migrating band and its molecular weight was estimated at approximately 30,000.
From these results, it has appeared that Ag2 was not similar to subunits of hCG, but possessed a portion of their antigenic components.