Pregnancy-associated plasma protein A (PAPP-A) specifically inhibits the third component of human complement (C3).

Pregnancy-associated plasma protein A (PAPP-A), a macromolecular glycoprotein associated with pregnancy, was shown to inhibit complement-induced haemolysis and to bind heparin reversibly. Because of the inhibitory effects of heparin on the complement cascade it was not clear if the inhibition of complement activity observed with PAPP-A (isolated from heparin plasma) was attributable to the heparin moiety bound to PAPP-A. This work demonstrates that heparin exerts an inhibitory effect on complement activity but that heparin-free PAPP-A is also inhibitory. PAPP-A specifically inhibits C3 by binding to this complement subcomponent and not by inhibiting C3 convertase as demonstrated for C3 inactivator.