QUANTIFICATION OF THE INTERACTION BETWEEN LYSOLECITHIN AND PHOSPHOLIPASE-A(2)

The rate of hydrolysis of phosphatidylcholine bilayers by phospholipas e A2 may be either enhanced or inhibited by the presence of lysolecith in depending on the experimental conditions examined. To further under stand the relationship of lysolecithin to phospholipase A2 activity, t he binding of lysolecithin to phospholipase A2 from the venom of Agkis trodon piscivorus piscivorus was examined by fluorescence spectroscopy . The tryptophan emission intensity of the enzyme was enhanced by 70% upon addition of lysolecithin. The binding isotherm for lysolecithin t o the phospholipase A2 estimated from the fluorescence change was biph asic, with a clear break in the curve occurring at the critical micell e concentration of the lysolecithin. Several observations suggested th at the phospholipase A2 was also capable of hydrolyzing the lysolecith in although at a rate far below that of phospholipid hydrolysis. These experiments were repeated using several other species of phospholipas e A2, and the results were found to be general among the enzymes excep t the lys-49 isozyme from A. p. piscivorus which displayed neither the dependence on the critical micelle concentration for binding nor the ability to hydrolyze lysolecithin. These results were used as the basi s for a quantitative analysis of enzyme fluorescence changes that occu r during the time course of phospholipid hydrolysis and of the mechani sm whereby lysolecithin inhibits the hydrolysis of phosphatidylcholine bilayers by phospholipase A2.