PROTEIN-LIPID INTERACTIONS AND ENZYME REQUIREMENTS FOR LIGHT SUBTYPE GENERATION ON CYCLING RECONSTITUTED SURFACTANT

Surfactant convertase is required for conversion of heavy density (H) natural surfactant to light density (L) subtype during cycling in vitr o, a technique that reproduces surfactant metabolism. To study mechani sms of H to L conversion, we prepared liposomes of dipalmitoylphosphat idylcholine (DPPC) and phosphatidylglycerol (PG), or the phospholipids (PL) in combination with either surfactant protein A (SP-A), surfacta nt protein B (SP-B), or both SP-A and SP-B. Phospholipids alone showed time dependent conversion from heavy to light subtype on cycling in t he absence of convertase, which was decreased by adding SP-B, but not SP-A, to phospholipids (p<0.01 for PL+SP-B, or PL+SP-A+SP-B vs. FL, or PL+SP-A). The ultrastructure, surface activity, buoyant density, and L subtype generation on cycling PL+SP-A+SP-B with partially purified c onvertase or with phospholipase D were similar to those of natural TM. In conclusion, a reconstituted surfactant mimics the behavior of natu ral surfactant on cycling, and reveals that interaction of SP-B with p hospholipids decreases L subtype generation. In addition, esterase/pho spholipase D activity is required for conversion of heavy to light sub type on cycling. (C) 1998 Academic Press.