The environment of tryptophan in pig pancreatic phospholipase A2 bound to bilayers

Binding of pig pancreatic phospholipase A2 to ternary codispersions of diacylphosphatidylcholine/lysophosphatidylcholine/fatty acid (100:22:22, mole ratio) is monitored by the increase in intrinsic fluorescence intensity of the single tryptophan residue. The fluorescence is quenched by the brominated fatty acid components in the ternary codispersions. The quenching efficiency is in the order: 11,12-dibromo- greater than 9,10-dibromo- greater than 6,7-dibromo- greater than 2-bromo fatty acid. The quenching efficiency of the 9,10-brominated derivatives of the three components in the ternary codispersions is in the order diacylphosphatidylcholine greater than fatty acid greater than lysophosphatidylcholine. Two isomers of diacylphosphatidylcholine with 9,10-dibromo substituents on chain 1 or 2 are equally efficient quenchers. While succinimide also quenches the fluorescence of the free and the membrane bound enzyme, the tryptophan residue in both systems is not accessible to 1-methylnicotinamide. These results are rationalized by a hypothesis that the acyl chains of the substrate interacts with the tryptophan residue of pig pancreatic phospholipase A2, which is readily accessible to water soluble neutral quenchers both in the free and the bound state.