Characterization of phospholipase A2 and acyltransferase activities in purified zymogen granule membranes

Phospholipase A2 and acyltransferase activities were identified in membranes associated with purified pancreatic zymogen granules. In homogenate and granule membranes, phospholipase activity was linearly related to protein concentration and was Ca2(+)-dependent with an alkaline pH optimum. The Ca2+ sensitivity was observed over the range of concentrations through which intracellular ionic Ca2+ is elevated by physiological stimuli in intact cells. Intact zymogen granules and granule membranes also demonstrated reacylating activity in the presence and absence of an exogenous acceptor. Reacylating activity was related to the concentration of lyosphospholipid added and was optimally activated at alkaline pH. A more rapid rate of reacylation was observed when [14C]arachidonoyl CoA was employed as the donor molecule rather than [3H]arachidonate (plus coenzyme A); this suggests the absence of acyl-CoA synthetase in the purified granule membranes. We conclude that granule membrane phospholipase A2 and acyltransferases may be involved in arachidonic acid turnover in exocrine pancreas and perhaps in membrane fusion events associated with exocytosis.