Phosphatidate phosphohydrolase activity in porcine pulmonary surfactant

The composition and enzymatic activity of porcine surfactant obtained by lung lavage was examined. The ratio of phospholipid to protein was found to be 13 mg per mg. Phosphatidate phosphohydrolase (PAPase) activity was present in lavage surfactant, lamellar bodies isolated from porcine lung tissue, and lamellar bodies isolated from human amniotic fluid. The optimal pH of PAPase in surfactant in lamellar bodies was 6.0. Cholinephosphotransferase activity was also present gradient centrifugation of amniotic fluid, the highest PAPase specific activity was found in the fraction that banded at the 0.65 M sucrose interface, similar to lamellar bodies obtained from porcine lung. From these results, we conclude that the enzyme PAPase remains closely associated with the surface active lipids during the process of storage and secretion of surfactant.