QUANTIFICATION OF THE INTERACTIONS AMONG FATTY-ACID, LYSOPHOSPHATIDYLCHOLINE, CALCIUM, DIMYRISTOYLPHOSPHATIDYLCHOLINE VESICLES, AND PHOSPHOLIPASE A(2)

The rate of hydrolysis of phosphatidylcholine bilayers by soluble phospholipase A(2) (PLA(2)) is greatly enhanced by the presence in the bilayer of a threshold mole fraction of the reaction products: fatty acid and lysophosphatidylcholine (lyse-PC). The threshold requirement of these products appears to vary as a function of vesicle and calcium concentration. To further identify the roles of myristic acid, lyse-PC, and calcium in promoting optimal PLA(2) activity, we have quantified the various interactions among these components and dimyristoylphosphatidylcholine large unilamellar vesicles. The bilayer/water partition coefficient for myristic acid was obtained by competition of vesicles for the binding of the fatty acid to an acrylodan conjugate of an intestinal fatty acid binding protein as monitored by the acrylodan fluorescence emission spectrum. The partition coefficient for lyse-PC was obtained by a similar procedure using the tryptophan emission spectrum of bovine serum albumin. The effect of calcium concentration on these interactions was also quantified. These results were incorporated into an empirical model to describe the threshold requirements for these products in the bilayer. This information is vital for elucidating the mechanism of activation of PLA, by the hydrolysis products.