Heterogeneity of pregnancy-specific beta 1-glycoprotein following purification from maternal serum and partial characterization of a high-molecular-weight component.

Pregnancy-Specific beta 1-Glycoprotein (PSB1G) has been purified from maternal serum by physico-chemical and immunochemical methods. Marked instability of the protein was noted after partial purification which correlated with hydroxylapatite chromatography of PSB1G. Three molecular heterogeneous forms were demonstrated by gel filtration on Sephacryl S-200 which were designated as PSB1G-I, II and III. Each of the latter purified materials displayed altered immunoelectrophoretic mobility, i.e., altered from beta 1 to gamma mobility. In immunodiffusion experiments PSB1G-I reacted in complete immunological identity with 'native' serum PSB1G whereas PSB1G-II and III reacted in partial identity with each of the latter, suggesting that they are dissociation and/or degradation products of the parent PSB1G. The apparent molecular weights of PSB1G-I, II and III as determined by gel filtration were 280 000, 69 000 and 23 000, respectively, PSB1G-I was selected for structural analysis on the basis of its immunological identity with 'native' antigen. SDS-PAGE studies of PSB1G-I with and without reducing agent strongly suggest that PSB1G-I and 'native' serum antigen are composed of two dissimilar subunits of equal molecular size which are bonded together by non-covalent linkage. One of the subunits appears to represent a single polypeptide chain, whereas the other subunit appears to be composed of two polypeptide chains of unequal size that are linked together by covalent bonds. Investigations are continuing towards confirmation of the proposed structural model of PSB1G-I by an alternate methodology of protein structure analysis.