Structural Stability of Streptococcal Serum Opacity Factor

Streptococcal serum opacity factor (SOF) is a protein that clouds the plasma of multiple mammalian species by disrupting high density lipoprotein (HDL) structure. Intravenous infusion of low dose SOF (4 µg) into mice reduces their plasma cholesterol concentrations ~ 40% in 3 h. Here we investigated the effects of pH, ionic strength, temperature, and denaturation with guanidinium chloride (GdmCl) on SOF stability and its reaction vs HDL. SOF stability was tested by pre-incubation of SOF at various temperatures, pH’s, and GdmCl concentrations and measuring the SOF reaction rate at pH 7.4 and 37 °C. SOF retained activity at temperatures up to 58 °C, at pH 4 to 10, and in 8.5 M GdmCl after being returned to standard buffer conditions. The effects of GdmCl, pH, and ionic strength on the SOF reaction rates were also measured. SOF was inactive at GdmCl ≥ 1 M; SOF was most active at pH 5, near its isoelectric point and at an ionic strength of 3 (in NaCl). These data reveal that SOF is a stable protein and suggest that its activity is determined, in part, by the effects of pH and ionic strength on its overall charge relative to that of its reaction target, HDL.