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

The rate of hydrolysis of phosphatidylcholine bilayers by soluble phos pholipase A(2) (PLA(2)) is greatly enhanced by the presence in the bil ayer of a threshold mole fraction of the reaction products: fatty acid and lysophosphatidylcholine (lyse-PC). The threshold requirement of t hese products appears to vary as a function of vesicle and calcium con centration. To further identify the roles of myristic acid, lyse-PC, a nd calcium in promoting optimal PLA(2) activity, we have quantified th e various interactions among these components and dimyristoylphosphati dylcholine large unilamellar vesicles. The bilayer/water partition coe fficient for myristic acid was obtained by competition of vesicles for the binding of the fatty acid to an acrylodan conjugate of an intesti nal fatty acid binding protein as monitored by the acrylodan fluoresce nce emission spectrum. The partition coefficient for lyse-PC was obtai ned by a similar procedure using the tryptophan emission spectrum of b ovine serum albumin. The effect of calcium concentration on these inte ractions was also quantified. These results were incorporated into an empirical model to describe the threshold requirements for these produ cts in the bilayer. This information is vital for elucidating the mech anism of activation of PLA, by the hydrolysis products.