Isolation and partial characterization of plasma membranes from rat lung alveoli

Plasma membranes from rat lung alveoli were isolated under hypotonic conditions (1 mM NaHCO3, 5 mM dithiothreitol, pH 7.5) using differential centrifugation and nonlinear sucrose gradient centrifugation techniques. Plasma membranes possessed densities of 1.13 and 1.16 g/cm3. The low density (1.13 g/cm3) fraction represented 2.2 and 2.0% of the total initial homogenate 5'-nucleotidase, EC 3.1.3.5 (a plasma membrane marker enzyme for rat lung, B.L. Riemer and C.C. Widnell, Arch. Biochem. Biophys. 171, 343-347, 1975) and adenylate cyclase, EC 4.6.1.1, respectively. The purification of 5'-nucleotidase and adenylate cyclase in the low density fraction relative to their specific activities in the homogenate was 11.2 and 5.7, respectively. The cholesterol to phospholipid ratio of this fraction was 0.73. The electron microscopic study of this fraction revealed the presence of small vesicles exhibiting a typical unit membrane structure. Plasma membrane vesicles isolated by this procedure could provide a source of material to study this adenylate cyclase system.