Characterization of Phospholipase A Activity of β1-Bungarotoxin from Bungarus multicinctus Venom

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Abstract
β1-Bungarotoxin purified from the venom of Bungarus multicinctus has phospholipase A activity, which was assayed, by titrating fatty acids released from phosphatidyl choline with alkali (the pH-stat titration method) and by scanning carbonized spots of the fatty acids on TLC plates (the TLC analysis method). The toxin as the phospholipase was very stable in acidic media and retained full activity even after incubation at 37°C for 20 h, but the toxin was labile above pH 9. The loss of activity in alkaline media might be caused by precipitation near the isoelectric point (pI9.5) of the toxin followed by irreversible denaturation. The toxin as the enzyme was active over a broad pH range (pH 6–9), but the optimum pH for hydrolysis of dispersed phosphatidyl choline was 8.0 as determined by both methods, pH-stat titration and TLC analysis. The phospholipase activity of the toxin decreased on modification with p-bromophenacyl bromide (BPB). The inactivation followed pseudo-first-order kinetics. The complete loss of enzymatic activity of the toxin was accompanied by the loss of one histidine residue in the toxin. The pH dependence of the inactivation rate suggested that the modified residue has pKa 6.9. This pKa value also indicated that the modified residue might be a histidine participating in the phospholipase activity of the toxin.