Pregnancy-associated plasma protein-A: an inhibitor of the complement system.

Pregnancy-associated plasma protein-A (PAPP-A) was purified to homogenicity from third-trimester pregnancy plasma using ion exchange chromatography and affinity chromatography on con A-Sepharose and anti-human serum-Sepharose. The elution of PAPP-A, alpha 2-macroglobulin, Cl-inhibitor and alpha 1-antitrypsin was followed by immuno-diffusion. the eluates were tested for stimulation or inhibition of complement-induced haemolysis. PAPP-A markedly inhibits the haemolytic activity of complement.