Serum amyloid P component: its role in platelet activation stimulated by sphingomyelinase D purified from the venom of the brown recluse spider (Loxosceles reclusa).

Serum amyloid P component or serum amyloid protein is a ubiquitous, highly conserved glycoprotein whose function is unknown. Although the related pentraxin, C-reactive protein, is an acute phase reactant in man, there is no direct evidence that human serum amyloid protein is involved in an inflammatory response. Here we show that serum amyloid protein is required by sphingomyelinase D, the principal necrotic agent of the venom of Loxosceles reclusa, for the in vitro-activation of human platelets. Furthermore, this platelet activation is dependent upon the presence of only serum amyloid protein; no other plasma components are necessary. Secretion of [3H]-serotonin and aggregation of platelets are nearly maximal following incubation of the platelets with purified sphingomyelinase D (0.3 micrograms/ml) and 5 micrograms/ml pure serum amyloid protein in the presence of calcium. Since the platelets are no longer activated when this 10% physiologic amount of serum amyloid protein is omitted, serum amyloid protein is likely to have a role in the necrosis caused by brown recluse spider venom.