Quantification of the pregnancy-specific beta1-glycoprotein (SP1) by immunoprecipitation techniques: the influence of a cross-reacting high molecular weight alpha2-protein.

Two serum proteins with SP1 determinants were separated by means of size chromatography and preparative zone electrophoresis. One of the proteins had beta1 electrophoretic mobility and an estimated molecular weight of 90,000, and the other had alpha2-mobility and a molecular weight around 400,000. When available antibody preparations to SP1 were used, the high molecular weight protein formed immunoprecipitates only when polyethyleneglycol (PEG) was added to the agarose. It is shown that the high molecular weight protein has a pronounced effect on quantification of SP1 by rocket immunoelectrophoresis, whereas the effect was less pronounced but still considerable when quantification was performed by radial immunodiffusion.