An immunochemical study of proteins with SP1 determinants in native and acidified pregnancy serum.

The alpha and beta forms of the pregnancy-associated protein, SP1, have been studied in late pregnancy serum and in similar serum after acidification. In both sera only two forms of the protein, SP1 alpha and SP1 beta, could be found; both reacting with antisera against SP1. On gel chromatography these two forms could be separated, with the intermediate effluent containing a varying mixture of both proteins. Immunoelectrophoresis of the effluent fractions from gel chromatography showed rocket shaped immunoprecipitates whose morphology depended on the mixture of SP1 alpha and SP1 beta. Crossed immunoelectrophoresis confirmed that only two proteins could be defined with SP1 antisera; the front running protein having an alpha 2 electrophoretic mobility and a molecular weight of 430,000, while the slower moving component had the electrophoretic mobility of a beta 1 globulin and a molecular weight of 90,000.