Lung concentric laminar organelle. Hydrolase activity and compositional analysis.

An analytical study was made of the concentric lamellar organelles (CLO) isolated from rabbit lung. These subcellular structures, which are normally located in the type II cells of mammalian lungs, were purified by flotation using a discontinuous sucrose density gradient. Electron microscopy of the isolated CL0 revealed many intact structures about 1 pm in diameter with densely packed trilaminar membranes having, also, a granular core and an outer limiting membrane. The CL0 contained a preponderance of polar lipids, mostly as phosphatidylcholine and phosphatidylethanolamine, with trace amounts of neutral lipid. A phospholipid to protein ratio of approximately 12: 1, was repeatedly obtained and served as a criterion for organelle purification. Acrylamide gel electrophoresis of the detergent-solubilized CL0 revealed a prominent staining band migrating with albumin. These structures had numerous hydrolases similar to those observed in lysosomes. High specific activities were observed for such enzymes as acid phosphatase, aryl sulfatase and @-26acetylglucosaminidase. Multiple forms of the latter enzyme were observed in zonal electrophoresis studies. Purification of the CL0 resulted in a ZOto 40-fold increase in specific activity over homogenates for most of the hydrolases. Cathepsin D, acid DNase, acid RNase and neuraminidase, other lysosome-associated hydrolases, were not detected in the CL0 fraction. It is suggested from these and other studies that CL0 are secreted to the acellular lining of lung where they contribute surfactant phospholipids, and possibly, functional hydrolases. The lung CL0 also appear to have many properties in common with those subcellular structures associated with certain human genetic lipidoses.