QUANTIFICATION OF THE INTERACTION OF LYSOLECITHIN WITH PHOSPHATIDYLCHOLINE VESICLES USING BOVINE SERUM-ALBUMIN - RELEVANCE TO THE ACTIVATION OF PHOSPHOLIPASE-A(2)

The activity of soluble phospholipase A2 to hydrolyze phosphatidylchol ine vesicles increases abruptly after a lag time of several minutes. T he onset of this apparent activation event probably results from the a ccumulation of a threshold mole fraction of the hydrolysis products (l ysolecithin and fatty acid) in the bilayer. One important observation relevant to the mechanism of this activation process is the biphasic d ependence of the lag time on vesicle concentration. To test whether th is dependence can be attributed entirely to the strength of partitioni ng of the lysolecithin into the phosphatidylcholine bilayer, we estima ted the apparent partition coefficient of lysophospholipid in the memb rane of phosphatidylcholine vesicles. Based on competition between bov ine serum albumin and the vesicles for the lysophospholipid, we estima ted the partition coefficient to be about 5 . 10(-7) or palmitoyl lipi ds at 39-degrees-C and about 9 . 10(-7) for myristoyl lipids at 22-deg rees-C. These values were able to rationalize the behavior of the lag time with dipalmitoylphosphatidylcholine vesicles, but they were unabl e to predict the behavior with dimyristoylphosphatidylcholine. Therefo re, it appears that the complete dependence of the lag phase on vesicl e concentration must be explained by additional means such as the poss ible contribution of nascent fatty acid or previously proposed kinetic activation mechanisms.