Investigations into the molecular heterogeneity of pregnancy-specific beta 1-glycoprotein (SP1)

The molecular heterogeneity of pregnancy-specific beta 1-glycoprotein (SP1) was examined by analytical immunoelectrophoresis and a radioimmunostaining technique of immunoelectrophoretic plates. Analytical crossed immunoelectrophoretic analysis and radioimmunostaining of these plates demonstrated the presence of normal human serum components in the alpha-mobile precipitate previously considered to be exclusively the high molecular pregnancy-specific protein, SP1 alpha. This observation suggested that two molecular populations were contributing to the alpha-mobile precipitate. Following fractionation of late-pregnancy serum by size chromatography, the radioimmunostaining technique further demonstrated the presence of normal serum components in the intermediate fraction but not in authentic SP1 (i.e., SP1 beta) or the high molecular weight form (SP1 alpha). We suggest that SP1 antigenic determinants are distributed in three different fractions of pregnancy serum, one of which (intermediate fraction) is a complex of authentic SP1 (SP1 beta) and a normal serum protein, whereas non-pregnancy serum components were not demonstrable in the remaining two (i.e., SP1 beta or SP1 alpha).