Effect of phospholipids on fluorescence polarization and lifetimes of fluorescein-labeled phospholipase A2.

Phospholipids and divalent metal ions induce changes in the fluorescence polarization of fluorescein-labeled phospholipase A2 from the Indian cobra (Hazlett, T.L. and Dennis, E.A. (1985) Biochemistry 24, 6152-6158) which may be indicative of enzyme aggregation. Lifetimes and rotational correlation times of the conjugate in the presence of these and other ligands are now reported. The correlation times of monomeric phospholipase A2 were shorter than expected for a hydrated, 13,000 molecular weight protein, indicating a high degree of probe mobility. The increase in the enzyme correlation time upon phospholipid addition was found to be consistent with the formation of a phospholipase A2 dimer or higher-order aggregate. It was found that activator lipids, such as phosphatidylcholine, increase the correlation time and probably the size of the enzyme, while nonactivator lipids, such as phosphatidylethanolamine, did so to a lesser extent. This suggests a possible link between aggregation and phosphatidylcholine-induced activation of this enzyme.