Hydrolysis of endogenous phospholipids by rat brain microsomes

Phosphatidylcholine of rat brain microsomes was labeled in vivo by intracerebral injection of either [3H]oleic acid or [methyl-3H]choline chloride. These labeled microsomes served both as the enzyme source as well as a source of endogenously labeled substrate. Phospholipase D (PLD) activity was detected with these particles only in the presence of exogenous oleate, its activator. Ca2+ and the ionophore A 23187 inhibit PLD activity of oleate-labeled microsomes. In oleate-labeled particles, besides phosphatidic acid the product of PLD action radioactivity was also detected in diglyceride as a result of resident phosphatidate phosphohydrolase, which hydrolyzed the phosphatidic acid. The phosphatidate phosphohydrolase could not be completely inhibited by KF and propranolol. The release of endogenous fatty acids from labeled phospholipid by a mellitin-stimulated phospholipase A2 also present in these particulates produced minimal stimulation of endogenous PLD. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are hydrolyzed by 50% in the presence of mellitin and 90% of the radioactivity was found in the lyso-compounds. Mellitin and oleate together reduced the radioactivity found in lyso-PC and increased that in lyso-PE.