Summary1. Cobra venom phospholipase A2 from three different sources has been fractionated into different isoenzymes by DEAE ion-exchange chromatography. 2. Treatment of intact human erythrocytes with the various isoenzymes revealed significant differences in the degree of phosphatidylcholine hydrolysis in those cells. 3. It is argued that the plateaus observed in dose-response curves for such treatments may be caused by an increase in lateral surface pressure within the outer half of the membrane due to the production of free fatty acids and lyso-compounds.Phospholipases A2 (EC 3.1.1.4) purified from snake venoms (in particular from Naja naja venom) have been widely used as tools to study the disposition of phospholipids in biological membranes (for a recent review see Ref. 1). These enzymes have been particularly useful in elucidating the localization of phospholipids in the erythrocyte membrane [2], and the results thus obtained for the localization of phosphatidylcholine in that mem. brane could be quantitatively confirmed by studies using phosphatidylcholine exchange protein [3,4]. The reason why phospholipases could successfully be applied became apparent only recently, when analyses involving 3'P-NMR showed that even after extensive phospholipase treatments of the membrane, the residual phospholipids and products of hydrolysis remain in a bilayer configuration [5]. Indeed, fatty acids and lysophosphatidylcholine associate to form bilayers [6]. While the reliability of the results and the conclusions drawn from the above studies have been questioned by Martin et
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