Mechanism of Phospholipase A₂ Action Toward Mixed Micelles of Detergent and Phospholipids

Abstract

Phospholipase A₂ from cobra venom (Naja naja naja) is a homogeneous, heat-stable enzyme that has a monomer molecular weight of only 11,000 and contains one histidine and one tryptophan residue. This enzyme acts optimally on phospholipids contained in mixed micelles with the nonionic detergent Triton X-100; the interactions of this detergent yith pholiholipid in the mixed micelles have been elucidated with ¹H- and ¹³C-NMR. In reacting, the enzyme first associates with the mixed micelle and then exhibits “surface dilution kinetics” in its reaction with substrate. Recent studies show that various reagents completely inactivate this phospholipase A₂ resulting in the modification of the histidine in only one-half of the enzyme molecules. These results suggest that the histidine residue, which is essential for activity, exhibits “half-site reactivity.” These and other experiments are interpreted in terms of a model that suggests that the monomeric enzyme forms an asymmetric dimer or higher order aggregate at the lipid-water interface. The studies which are described on the interaction of the phospholipase A₂ with mixed micelles serve as a general model system for understanding detergent effects on the assay of lipid enzymes.