ON THE SURFACE-ACTIVITY OF SURFACTANT-ASSOCIATED PROTEIN (SP-C) - EFFECTS OF PALMITOYLATION AND PH

The effect of palmitoylation on the surface activity of bovine surfact ant-associated protein C (SP-C) in lipid mixtures was investigated. Na tive and chemically depalmitoylated SP-C were reconstituted with dipal mitoylphosphatidylcholine/egg phosphatidylglycerol (7:3) using two dif ferent procedures, one of which included lyophilization and sonication . When tested using a pulsating bubble surfactometer, no significant c hanges in the surface activity of these mixtures were observed upon th e hydrolysis of the palmitates. Since the purification and deacylation procedures of SP-C included the use of acid and alkali, the effect of pH was examined. The surface activity of the mixtures was found to va ry with pH. At low pH values (approx. 2.5), surface tensions between 3 and 10 mN/m at minimum bubble radius were reached within 5 pulsations , while at neutral and slightly alkaline pH, surface tension reduction was much slower and near zero (< 5 mN/m) values at minimum bubble rad ius were not reached by the fiftieth pulsation. Protein-free lipid sam ples that were exposed to acid exhibited enhanced surface activity ove r similar non-treated samples. It is therefore concluded that low surf ace tension measurements recorded for acidic samples are secondary to a pH effect and do not reflect the surface activity at physiological c onditions.