Regulation of Phosphatidylethanolamine Degradation by Enzyme(s) of Subcellular Fractions from Cerebral Cortex

Hydrolysis of 1-acyl-2-[14C]arachidonoyl-sn-glycero-3-phosphoethanolamine was studied in cerebral cortex homogenate and subcellular fractions. The enzyme(s) confined to the synaptic plasma membrane (SPM) hydrolyze(s) [14C-arachidonoyl]phosphatidylethanolamine (PE) in the presence of EGTA to [14C-arachidonoyl]diacylglycerol (DAG) and a small amount of [14C]arachidonic acid (AA). Degradation of PE is time-, protein- and substrate-dependent with a pH optimum of 7.8. The highest activity of PE degradation was observed in the presence of 10 mM EGTA. Under this condition GTPγS has no effect on PE hydrolysis. In the presence of Ca2+ ions degradation of PE was significantly lower as compared to the conditions with EGTA. However, the percentage distribution of free AA in the sum of both products of PE hydrolysis (AA + DAG) increases from 16 and 20% observed in the presence of EGTA 2 mM and 10 mM to 34% and 43% in the presence of 0.5 mM CaCl2 alone and together with GTPγS, respectively. Cytosolic enzymes also degrade PE in the presence of 2 mM EGTA with the formation of DAG and AA. Radioactivity in the AA represents about 80% of the total radioactivity of the products of PE degradation. The hydrolysis of PE by cytosolic enzymes is almost completely inhibited by neomycin but the hydrolysis by the SPM-bound enzyme(s) is inhibited only 70%. Other studies with quinacrine indicated that only a small pool of PE is degraded by SPM-bound Ca2+-independent phospholipase A2 (PLA2). All of these data suggest that PE in cerebral cortex is mainly degraded by cytosolic and SPM-bound Ca2+-independent phospholipase C. Further studies towards a better understanding of the mechanisms of cerebral degradation and the physiological significance of Ca2+-independent pathways of PE hydrolysis are necessary.