Action of phospholipases A 2 on phosphatidylcholine bilayers. Effects of the phase transition, bilayer curvature and structural defects

Abstract

We examined the action of porcine pancreatic and bee-venom phospholipase A 2 towards bilayers of phosphatidylcholine as a function of several physical characteristics of the lipid-water interface. 1. 1. Unsonicated liposomes of dimyristoyl phosphatidylcholine are degraded by both phospholipases in the temperature region of the phase transition only (cf. Op den Kamp et al. (1974) Biochim. Biophys. Acta 345, 253–256 and Op den Kamp et al. (1975) Biochim. Biophys. Acta 406, 169–177). With sonicates the temperature range in which hydrolysis occurs is much wider. This discrepancy between liposomes and sonicates cannot be ascribed entirely to differences in available substrate surface. 2. 2. Below the phase-transition temperature the phospholipases degrade dimyristoyl phosphatidylcholine single-bilayer vesicles with a strongly curved surface much more effectively than larger single-bilayer vesicles with a relatively low degree of curvature. 3. 3. Vesicles composed of egg phosphatidylcholine can be degraded by pancreatic phospholipase A 2 at 37°C, provided that the substrate bilayer is strongly curved. The bee-venom enzyme shows a similar, but less pronounced, preference for small substrate vesicles. 4. 4. In a limited temperature region just above the transition temperature of the substrate the action of both phospholipases initially proceeds with a gradually increasing velocity. This stimulation is presumably due to an increase of the transition temperature, effectuated by the products of the phospholipase action. 5. 5. Structural defects in the substrate bilayer, introduced by sonication below the phase-transition temperature (cf. Lawaczeck et al. (1976) Biochim. Biophys. Acta 443, 313–330) facilitate the action of both phospholipases.