Studies on the pulmonary surface-active lipoprotein. II. Biophysical characterization of its peptide moiety.

Abstract

5tudies on the lung surface-active lipoprotein complex were carried out in order to establish, primarily, the physicoehemieal eharaeteristies of its peptide moiety. It has been undertaken in the hope that new findings may extend present knowledge on the nature of the surfactant agent and of lipoproteins in general. After removal of lipids from the SA-lipoprotein, the peptide moiety of the cow lung SA-lipoprotein was subjeeted to an exhaustive characterization study. It was observed that the lipid-free materials are insoluble in distilled water, in acid, and in neutral physiological saline solutions. They are slightly soluble in 0.1 molar sodium ehloride solutions at pH 1 0.0, and soluble in 0.1 normal sodium hydroxide solutions. The delipidized 5A-peptide is not dialyzable. The soluble lipid-free materials were eoncentrated by ultrafiltration through Diaflo Membranes and purified by gel filtration in columns of Sephadex. It was observed that the properties of the purified SA-peptide material were markedly influenced by solvent pH. In the pH range 4-6, where the peptide was nearly isoelectric, it formed a gel, which could be dispersed by 6 molar urea. At pH values higher than 7.0 or less than 4.0, traces of insoluble, perhaps aggregated material were observed in the solutions. It is apparent that the solution properties of this peptide material are controlled, at least in part, by its net charge. Ultracentrifugal and electrophoretical data indicate that the SA-peptide preparations are heterogeneous. Two bands appeared after electrophoresis of SA-peptide on ceHulose acetate strips: one major fast-moving component; and a minor one, close to the origin, which constituted about 5 per cent of the total peptide. Sedimentation patterns in the analytical ultracentrifuge showed two main components, with sedimentation constants (S20 W ) of 1.53S and 3.10S, with minimal anhydrous molecular weights of 8,600 and 24,800 (assummg spherical shape), respectively. Calculations have shown that the SA-peptide contains more negative than positive groups, in accord with its acidic properties. Molecular weight determinations with SA-peptide materials have led to values of 100,000 and 230,000, on the basis of chemical composition and sedimentation equlibrium data, respectively. The discrepancy between these two values may be explained by assuming that polymerization of SA-peptide occurs in solution, leading to a high value for the weight-average molecular weight determined by sedimentation equilibrium.