Lamellar bodies and tubular myelin isolated from the mitochondrial fraction of rat lung homogenates

This study was aimed at the identification of those components that account for the observed surface activity of lung homogenates. Two components were isolated from a rat lung mitochondrial fraction by recentrifugation in discontinuous sucrose gradients, purified and identified by electron microscopy as lamellar bodies (d = 1.059), an organelle of type II cells and tubular myelin (d = 1.089), a component of the extracellular lining. Both fractions contain lipid and protein and are surface active. In each fraction 92-95% of the lipid was present as polar components and 1.5-2.5% as cholesterol. However, 73% of the polar lipid of the lamellar bodies consisted of lecithin as compared to 58% for the tubular myelin. Regardless of its location 85% of the lecithin fatty acids were saturated. The proteins present in the two fractions exhibited major differences in composition as shown by polyacrylamide gel electrophoresis. It is concluded that tubular myelin is a liquid crystalline form of surface active lipoprotein and that the lamellar bodies supply the lipid of the tubular myelin whereas most protein of the latter is derived from another source. This suggests that the specific surface active lipoproteins are assembled in the alveolar spaces.