Inhibition of human platelet aggregation by L-amino acid oxidase purified from Naja naja kaouthia venom

L-Amino acid oxidase (LAO) widely exists in snake venoms. Purification of L AO from the Naja naja kaouthia (monocellate cobra) venom has been reported (Tan and Swaminathan, 1992), but its structural characterization and physio logical function remained to be determined. The function of snake venom LAO s in hemostasis, especially their effect on platelet aggregation, has been controversial. We determined the N-terminal amino acid sequence of the N. n . kaouthia LAO named K-LAO to be DDRRSPLEECFQQNDYEEFLEIAKNGLKKTxNPKHVXxV (3 8 residues). The protein data base search revealed that the enzyme had high similarities with other snake venom LAOs. Further, platelet aggregation st udies revealed that K-LAO functionally did not induce platelet aggregation in a platelet-rich plasma system, but that it inhibited platelet aggregatio n induced by agonists such as ADP, collagen and ristocetin in a dose-depend ent manner. K-LAO diminished platelet aggregation more intensely under low than high shear stress. This inhibitory activity of K-LAO on either ristoce tin-induced or shear-induced platelet aggregation was quenched by addition of catalase. These results indicate that K-LAO functions as an inhibitor to platelet aggregation through the formation of hydrogen peroxide. The enzym e may contribute to the development of a severe hematological disorder due to cobra envenomation. (C) 2001 Elsevier Science Ltd. All rights reserved.