Phospholipase A 2 Action on Membrane Surfaces : Effect of Ionic Strength, Triton X-100, and Melittin

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enzyme are necessary (1). In previous studies using low concentrations of a highly purified homogeneous preparation of phospholipase A2 from cobra venom (Naja naja naja) and metabolically competent human erythrocytes, we found that under initial rate conditions the enzyme is barely active toward the outer surface of intact erythrocytes, but that the ghosting procedure results in an enormous activation of the enzyme toward phosphatidylethanolamine (PE) and phosphatidylcholine (PC) (1, 2). With phospholipid mixtures in Triton X-100 mixed micelles, we have found that phospholipase A2 activation toward PE is dependent on the direct binding of specific phosphorylcholine-containing activator lipids to the enzyme (3, 4). For a valid interpretation of the distribution of membrane phospholipids using phospholipase A2, keeping in mind possible phospholipid activation of the enzyme, we must assess the cause(s) of the low activity of phospholipase A2 toward intact erythrocytes, and whether activation toward the erythrocyte results from prehemolytic subtle alterations in the outer surface of the membrane or from posthemolytic exposure of the lipids at the inner membrane surface. We have now examined the action of cobra venom phospholipase A2 toward minimally perturbed human erythrocytes to see if hydrolysis can be activated in the absence of hemolysis.