Snake Venom Constituents that Affect Platelet Function.

Snake venoms contain complex mixtures of proteins with biological activities. These venom proteins affect blood coagulation and platelet function in various ways. Many inducers and inhibitors of platelet aggregation have been isolated from snake venoms, especially from the Crotalidae and Viperidae families. According to their biochemical properties and modes of action, they can be classified into ten groups: (1) thrombin-like enzymes which show higher activity towards platelets than towards fibrinogen; (2) procoagulant enzymes which generate thrombin and then activate platelets indirectly; (3) noncoagulant glycoproteins which activate platelets independently of ADP release or thromboxane formation; (4) lectin-like peptides which cause agglutination of platelets through binding to a sugar moiety; (5) coagglutinins whose activities are dependent on the presence of von Willebrand factor (vWF); (6) membrane-active polypeptides which potentiate the aggregating action of other inducers by activation of platelet endogenous phospholipase A(2); (7) phospholipase A(2) enzymes which show biphasic aggregating and inhibitory effects on platelets; (8) α-fibrinogenases which degrade the α (A) chain of fibrinogen; (9) 5'-nucleotidase or ADPase which act on the ADP released from platelet dense bodies; and (10) fibrinogen-receptor antagonists which interfere with the interaction of fibrinogen and glycoprotein IIb/IIIa on activated platelets. These venom proteins are unique research tools for study of the haemostatic process and some of them are potential antithrombotic agents.