Biomembrane-modulated, lysosomal phospholipase A2 contamination of chromaffin granule ghosts

It was reported that subcellular fractionation of bovine adrenal medulla results in the separation of distinct, non-calcium-dependent phospholipases A2--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 after liposomal substrates and/or lysosomal phospholipase A2 such that the enzyme now hydrolyzes them (at low cation concentration) with an alkaline pH optimum. In a lysosomal membrane fraction, phospholipase A2 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 A2 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 A2 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 A2 to biomembranes broadens its potential physiologic pH range and may also play a role in the regulation of this potentially deleterious activity.