Isolation and characterization of rabbit lung lamellar bodies

A method has been devised for the isolation of a highly purified preparation of lamellar bodies from rabbit lung. The purity of the preparation was confirmed by electron microscopy, marker enzymes, phospholipid composition, and isopycnic centrifugation on continuous density sucrose gradients. Contamination of the lamellar bodies by such subcellular components as mitochondria, nuclei, lysosomes and plasma membranes could be excluded; however, reduced nicotinamide adenine dinucleotide phosphate (NADPH) cytochrome c reductase, an enzyme specific for the endoplasmic reticulum components was a persistent contaminant in the preparation of the isolated lamellar bodies. When the lamellar bodies were subject to isopycnic centrifugation, all of the NADPH cytochrome c reductase activity was associated with the lamellar bodies in the low density peak; no reductase activity could be detected in the region of the density gradient demonstrated to localize microsomes. Use of 3H-radiolabeled microsomes confirmed that all of the NADPH cytochrome c reductase activity present in the lamellar body preparations could be accounted for by microsomal contamination. When lamellar bodies or liposomal membranes synthesized from the total phospholipid fraction of lamellar bodies were analyzed by the electron paramagnetic resonance probe, 5-dioxyl-methylstearate, they exhibited a high degree of fluidity at physiological temperature. This was in contrast to the low fluidity of liposomal membranes composed of pure dipalmitoylphosphatidylcholine, the major component (50%) of rabbit lamellar body phospholipids. Furthermore, the major temperature-dependent phase transition in lamellar body membranes occurred at a different temperature (30.5 degrees C) from that of dipalmitoyl-phosphatidylcholine (41.0 degrees C). It is clear, therefore, that the membrane fluidity of lamellar bodies must be highly influenced by the minor lipid component.