Asymmetric short-chain phosphatidylcholines: defining chain binding constraints in phospholipases

Several short-chain asymmetric lecithins with a total of 14 carbons in the acyl chains (ranging from 1-lauroyl-2-acetylphosphatidylcholine to 1-hexanoyl-2-octanoylphosphatidylcholine) have been synthesized and characterized. The specific activities of phospholipase A2 from cobra venom, phospholipase A2 from porcine pancreas, and phospholipase C from Bacillus cereus toward these lecithins as micelles have been determined. The results of these kinetic studies allow the definition of hydrophobic binding requirements in the active sites of these water-soluble phospholipases. For phospholipase C, with the exception of monomyristoylphosphatidylcholine, each of the asymmetric short-chain lecithins exhibits high activity, comparable to the 14-carbon symmetric short-chain species, diheptanoylphosphatidylcholine. Therefore, for phospholipase C, in addition to the acyl linkages, only a certain degree of hydrophobicity in the fatty acyl chains is requisite for substrate binding and appreciable hydrolysis; there is no chain specificity. The activity of phospholipase A2 from cobra venom toward the same asymmetric lecithins is quite different. As the sn-2 chain lengthens, activity is increased to a maximum for diheptanoyl-PC. Further increase in the number of carbons in the sn-2 chain has no effect on hydrolysis rates. For this enzyme, seven carbons in the sn-2 chain are necessary for optimal activity. In contrast, porcine pancreatic phospholipase A2 activity shows very little dependence on sn-2 chain length.