Phospholipases A2 and C of human lung; subcellular distribution and substrate selectivity

The phospholipase activities of cell-free extracts of human lung were studied using sn-2-arachidonoyl phospholipids. Samples of human lung obtained during surgery were homogenized and separated by centrifugation into three fractions: P1, containing mitochondrial and lysosomal marker enzymes; P2, with microsomal enzymes; and S2, with cytosolic enzymes. The highest phospholipase activities were in the microsomal fraction, using any of the three substrates, [14C]arachidonoylphosphatidylcholine (PC), [14C]arachidonoylphosphatidylethanolamine (PE) and [14C]arachidonoylphosphatidylinositol (PI). From PC and PE, only free arachidonic acid was formed, suggesting the presence of a phospholipase A2 (PLA2)-like activity. From PI, two metabolites were produced, diacylglycerol and arachidonic acid, suggesting the presence of a PI-specific PLC activity. Rates of hydrolysis were highest for PI, followed by PE and then PC. Hydrolysis of [14C]arachidonoyl-PC was compared to that of [14C]oleoyl-PC and found to be similarly distributed and of comparable velocity. The distribution and relative activities of phospholipases in rat lung homogenates were very similar to those in human lung.