On the surface activity of surfactant-associated protein C (SP-C): effects of palmitoylation and pH

The effect of palmitoylation on the surface activity of bovine surfactant-associated protein C (SP-C) in lipid mixtures was investigated. Native and chemically depalmitoylated SP-C were reconstituted with dipalmitoylphosphatidylcholine/egg phosphatidylglycerol (7:3) using two different procedures, one of which included lyophilization and sonication. When tested using a pulsating bubble surfactometer, no significant changes in the surface activity of these mixtures were observed upon the 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 vary 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 radius were not reached by the fiftieth pulsation. Protein-free lipid samples that were exposed to acid exhibited enhanced surface activity over similar non-treated samples. It is therefore concluded that low surface tension measurements recorded for acidic samples are secondary to a pH effect and do not reflect the surface activity at physiological conditions.