Biomembrane-modulated, lysosomal phospholipase A 2 contamination of chromaffin granule ghosts

Abstract

It was reported that subcellular fractionation of bovine adrenal medulla results in the separation of distinct, non-calcium-dependent phospholipases A 2 — one associated with chromaffin granule ghosts, another with lysosomes. The basis of this distinction is pH optimum: in routine assays utilizing neat liposomal substrates, the chromaffin granule ghost-associated enzyme is alkaline-active whereas the lysosomal enzyme is acid-active (Husebye, E.S. and Flatmark, T. (1987) Biochim. Biophys. Acta 920, 120–130). We now report that biomembranes alter liposomal substrates and/or lysosomal phospholipase A 2 such that the enzyme now hydrolyzes them (at low cation concentration) with an alkaline pH optimum. In a lysosomal membrane fraction, phospholipase A 2 activity at pH 7.5 relative to activity at pH 5.0 increases as increasing amounts of lysosomal membranes are assayed. The pH optimum of chromaffin granule ghost-associated phospholipase A 2 toward liposomal substrates is likewise biomembrane-dependent and, when assayed carefully, is indistinguishable on the basis of optimal pH from the lysosomal enzyme. Although chromaffin granule ghost-associated phospholipase A 2 is most likely a lysosomal contaminant, its broad, biomembrane-modulated pH range may still allow it to participate in catecholamine secretion. More importantly, however, sensitivity of adrenal medullary lysosomal phospholipase A 2 to biomembranes broadens its potential physiologic pH range and may also play a role in the regulation of this potentially deleterious activity.