Discriminatory Recognition of Membrane Phospholipids by Lysine-49-phospholipases A₂ from Trimeresurus flavoviridis Venom

Basic proteins I and II (BP-I and BP-II) isolated from Trimeresurus flavoviridis venom, which are classified into a group of lysine-49-phospholipases A₂ (Lys-49-PLA₂), exhibited only limited lipolytic activity for the mixed micelles of various phospholipids. Based on the finding that BP-II elicits a strong contraction of guinea pig ileum due to the release of arachidonic acid, BP-II together with BP-I has been tested for their interaction with artificial phospholipid bilayer membranes. The dye leakage experiments indicated that BP-II interacts strongly with liposomes of β-arachidonoyl-γ-stearoyl-L-α-phosphatidylcholine. The perturbation of liposomes was observed only in the Ca²⁺-containing buffer, and as demonstrated by HPLC analyses, accompanied by the release of arachidonic acid. The concentration of Ca²⁺ which gave a half maximal activity of BP-II was 3.0×10^-4M, suggesting that the affinity of BP-II for Ca²⁺ is more than 10 times stronger than that of BP-II without liposomes. These observations clearly show that Lys-49-PLA₂ of BP-II is the enzyme responsible for the hydrolysis of membrane phospholipids and that Ca²⁺ is essential for such enzymatic activity. The interaction of BP-I with liposomes was much weaker than BP-II. BP-I and BP-II share a common sequence except for 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 phospholipid membranes.