Relationships between bilayer structure and phospholipase A(2) activity: Interactions among temperature, diacylglycerol, lysolecithin, palmitic acid, and dipalmitoylphosphatidylcholine

Bilayers composed of phosphatidylcholine initially resist catalysis by phospholipase A(2). However, after a latency period, they become susceptible when sufficient reaction products (lysolecithin and fatty acid) accumulate in the membrane. Temperatures near the main bilayer phase transition and saturated long-chain diacylglycerol in the bilayer modulate the effectiveness of the reaction products. The purpose of this study was to identify possible mechanisms for these effects of temperature and diacylglycerol. Various fluorescent probes were used to assess changes in the ability of the reaction products to perturb the bilayer and promote enzyme binding to the membrane surface. Temperature appeared to cause three effects. First, the degree of binding of enzyme at the end of the latency period was greatest near the phase transition temperature where the latency period was shortest. Second, the bilayer was more sensitive to perturbation by reaction products near the transition. Third, the disturbance provoked by the products was confined to the membrane surface below the transition but affected deeper regions at higher temperature where the latency period was greater. The latter two effects of temperature required the presence of calcium. Diacylglycerol promoted lateral segregation of reaction products in the bilayer. This effect corresponded with the tendency of diacylglycerol to reduce the length of the latency period at temperatures below the phase transition. Therefore, it appeared that temperature affects the latency period by altering the binding of the enzyme and the depth and magnitude of the bilayer perturbation caused by reaction products. Alternatively, diacylglycerol may enhance the effectiveness of reaction products by inducing them to segregate in the bilayer and thus create local regions of increased impact on the bilayer surface.