An L-amino acid oxidase (LAAO),NA-LAAO,was purified from the venom of Naja atra.Its N-terminal sequence shows great similarity with LAAOs from other snake venoms.NALAAO dose-dependently induced aggregation of washed human platelets.However,it had no activity on platelets in platelet-rich plasma.A low concentration of NA-LAAO greatly promoted the effect of hydrogen peroxide,whereas hydrogen peroxide itself had little activation effect on platelets.NA-LAAO induced tyrosine phosphorylation of a number of platelet proteins including Src kinase,spleen tyrosine kinase,and phospholipase C γ2.Unlike convulxin,Fc receptor γ chain and T lymphocyte adapter protein are not phosphorylated in NA-LAAO activated platelets,suggesting an activation mechanism different from the glycoprotein VI pathway.Catalase inhibited the platelet aggregation and platelet protein phosphorylation induced by NA-LAAO.NA-LAAO bound to fixed platelets as well as to platelet lysates of Weste blots.Furthermore,affinity chromatography of platelet proteins on an NA-LAAO Sepharose 4B column isolated a few platelet membrane proteins,suggesting that binding of NA-LAAO to the platelet membrane might play a role in its action on platelets.