Lysine-49-Phospholipases A2 from Trimeresurus flavoviridis Venom Are Membrane-Acting Enzymes

Basic proteins I and II (BP-I and BP-II) isolated from the venom of Trimeresurus flavoviridis (Habu snake) are isozymes of highly active Asp-49-phospholipase A2 (Asp-49-PLA2) and classified into the group Lys-49-PLA2. BP-II was found to elicit a strong contraction of guinea pig ileum, and this activity was inhibited completely by 1 μM indomethacin, an inhibitor of the arachidonate cascade. BP-II was inactive in the Ca2+ -free medium, and p-bromophenacy lated His-48-BP-II was also inactive. BP-II exhibited no binding affinity for the cells expressinng PLA2 receptors. These results indicated that the contraction elicited by BP-II is due to the hydrolytic action of BP-II, liberating archidonic acid from the ileum phospholipid biomembranes. In spite of its limited lipolytic activities (av. 0.9% of Asp-49-PLA2) against monomers and micelles of synthetic phospholipids, BP-II hydrolyzed considerably strongly the phospholipids in the artificial bilayer vesicles. Arachidonic acid released from liposomes of β-arachidonoyl-γ-stearoyl-l-α-phosphatidylcholic was determined by HPLC, and the activity of BP-II was estimated to be about 75% as compared to Asp--49-PLA2. Liposomes encapsulating carboxyflurescein exhibited a strong dye-leakage induced by BP-II in a concentration-dependent manner, only in the Ca2+-containing buffer. The net result from all these observations was that BP-II, a Lys-49-PLA2, is an enzyme that hyrolyzes the membrane phospholipids. In contrast to BP-II, BP-I was found to be considerably weak in hydrolyzing membrane phospholipids, although its activities were distinct. BP-I and BP-Ii share a common sequence with the sole exception of Asp-67 (BP-I) and Asn-67 (BP-II) in the aligned sequences. This implies that the amino acid at position 67 of Lys-49-PLA2s is the residue required for discriminatory recognition of β-arachidonoyl-phospholipid membranes.