Role of phospholipase A2 enzymes in degradation of dipalmitoylphosphatidylcholine by granular pneumocytes.

The role of phospholipase A2 (PLA2) enzymes in the degradation of internalized dipalmitoylphospharidylcoline (DPPC) by rat granular pneumocytes was evaluated with cells in 24 h primary culture on microporous membranes. In cell sonicates and rat lung homogenates, the transition state analogue MJ33 inhibited acidic (pH 4), Ca(2+)-independent PLA2 (aiPLA2) while p-bromophenacylbromide (pBPB) inhibited alkaline (pH 8.5), Ca(2+)-dependent PLA2 and phospholipase C activities. With intact cells, degradation of [3H]methylcholine-labeled DPPC during 2 h incubation was inhibited 48% by MJ33, 20% by pBPB, and 69%by the combination. The inhibitors (20 microM pBPB, 3 mol% MJ33) had no effect on cellular dye exclusion, adherence to membranes, or uptake of DPPC. Arachidonyl trifuoromethylketone, a cytoplasmic PLA2 inhibitor, had no effect on cellular degradation of DPPC. Degradation was depressed approximately 20% by the addition of NH4Cl or methylamine to the medium, suggesting a role for an acidic intracellular compartment in DPPC metabolism. Subcellular fractions prepared by differential centrifugation of rat lung homogenates showed highest specific activity of aiPLA2 in the lamellar body and lysosomal fractions, lower activity in cytosol, and essentially no activity in mitochondria, microsomes, or plasma membranes. The results of this study indicate that aiPLA2 has the major role in the degradation of internalized DPPC by granular pneumocytes and they are compatible with participation of lysosomes/lamellar bodies in DPPC metabolism.