Characterization of pregnancy-associated plasma protein-A: comparison with alpha 2-macroglobulin

Pregnancy-associated plasma protein-A (PAPP-A), like alpha 2-macroglobulin (alpha 2M), is a large protein with homotetrameric molecular conformation. Each monomer has Mr 205 kDa. Total carbohydrate content of PAPP-A (19.4%) exceeded that of alpha 2M (8.6%). In addition to glucose (9.4%), fucose (3.1%), mannose (2.3%) and galactose (0.8%), PAPP-A contained glucuronic acid (3.8%). The amino acid composition of PAPP-A differed most significantly from alpha 2M, in the content of glutamate, glycine and lysine. Although the peptide core of both proteins were of similar size, the difference in size, of native molecules was due to the carbohydrate moiety. Whereas alpha 2M monomer was autolytically cleaved into two smaller non-identical subunits (Mr 128 and 65 kDa), no such breakdown products were observed with PAPP-A. Unlike alpha 2M, PAPP-A is not a broad spectrum protease inhibitor. Both proteins inhibited human granulocyte elastase (HGE) in a dose dependent relationship, with PAPP-A (Ki 0.2 x 10(-6) M) being a more potent inhibitor than alpha 2M (Ki 1.02 x 10(-6) M). Since PAPP-A lacked internal thiolester groups, the mechanism of HGE inhibition was unlikely to be entrapment, as defined for alpha 2M. Inhibition kinetics of HGE for PAPP-A (noncompetitive inhibitor) and alpha 2M (uncompetitive inhibitor) were distinct. Thus, these findings do not support the tenet of a common ancestral protein for PAPP-A and alpha 2M.