Pregnancy-associated plasma protein B (PAPP-B): isolation from human late pregnancy serum and biochemical characterization.

The pregnancy associated plasma protein B (PAPP-B) was first described by Lin et al. as a glycoprotein with a molecular weight of about 1,000 kD and a serum half life of less than 24 hours after delivery. We isolated PAPP-B from late pregnancy serum using liquid chromatography including HPLC. PAPP-B was found to be a glycoprotein with a molecular weight of 1,300 kD and to be an octadecamer of identical subunits (74 kD each). Cleavage of the carbohydrates with trifluoromethanesulfonic acid reduced the molecular mass to 65 kD (by SDS PAGE, 12.2%). Isoelectrofocusing and titration curves fixed the isoelectric point at 5.3. Spectroscopy measurements between 230 and 360 nm yielded a trp/tyr ratio of 1.1. Amino acid analysis showed that each subunit contained five methionines; this was confirmed by a CNBr cleavage experiment.