pH-dependent modulation of phospholipase A2 activity by alkaline cations and catecholamines in a granule-enriched fraction of adrenal medulla.

Phospholipase A activity was measured in the soluble fractions from bovine adrenal medullary granules and rat liver lysosomes. The adrenal medulla preparation, enriched 2.5-fold in chromaffin granules and lysosomes, hydrolyzes the phospholipids of [1-14C]oleate-labelled autoclaved Escherichia coli in the pH range 3.5-7.0 in an alkaline cation (Na+, K+, Ca2+, Mg2+)-dependent fashion. At low alkaline cation concentrations the apparent pH optimum is near 6.5 but decreases to about 4.5 with increasing cation concentrations. When measured at high alkaline cation concentrations phospholipase activity in the adrenal fraction has a pH profile and optimum similar to those of rat liver lysosomes. Amine-containing buffers, millimolar concentrations of catecholamines and micromolar concentrations of the amine-containing drug, trifluoperazine, modulate the adrenal medulla phospholipase activity in a pH- and alkaline cation-dependent manner. Studies with specifically labelled phosphatidylethanolamines confirm previous conclusions that activity at pH 6.4 is almost exclusively phospholipase A2; but in contrast to previous conclusions (Smith, A.D. and Winkler , H. (1968) Biochem. J. 108, 867-874) we find significant phospholipase A2 activity at pH 4.2.