Phospholipase D activity in subcellular membranes of rat ventricular myocardium

The phospholipase D (PL D), which catalyzes the formation of phosphatidic acid (PA), was studied in rat myocardium using 14C-labelled phosphatidylcholine (PC) as an exogenous substrate. Subcellular distribution experiments indicated the presence of PL D in particulate fractions only. Different procedures for the isolation of purified cardiac subcellular organelles showed the presence of PL D in sarcolemma (SL), sarcoplasmic reticulum (SR) and mitochondria with 14-, 11- and 5-fold enrichment when compared to the homogenate value, respectively. The activity of SL PL D was observed over a narrow acid pH range with an optimum at 6.5, and it showed a high specificity for PC while phosphatidylethanolamine and phosphatidylinositol showed a low rate of hydrolysis. Under optimal conditions, PA formation was linear for a 90-min period of incubation and the reaction rate was constant for 10 to 100 micrograms SL protein in the assay medium. The SR PL D displayed properties similar to those seen with the SL PL D. In membrane fractions PL D was also found to catalyze a transphosphatidylation reaction for the synthesis of phosphatidylglycerol. Assessment of the intramembranal levels of radioactive 1,2-diacylglycerol (DAG) in the absence or presence of KF suggested the presence of an active PA phosphohydrolase activity. This study indicates that a PC-specific PL D activity is localized in different membrane systems of the myocardium and may be associated with PA phosphohydrolase to act in a coordinated manner. The functional significance of PL D-dependent formation of PA in cardiac membranes is discussed.