Probing the determinants of protein solubility with amino acid modification

Chemical modification was used as a probe to study the effect of structural features of serum albumin (charge, conformation, surface hydrophobicity, etc.) on its solubility behavior in concentrated ammonium sulfate solutions. Four different acetylated derivatives of goat serum albumin namely 18% acetylated, 40% acetylated, 53% acetylated, and 93% acetylated albumins were prepared. The homogeneity of these preparations was established by gel chromatography and polyacrylamide gel electrophoresis. Hydrodynamic data on the Stokes radius of native and acetylated albumins suggested gradual change in conformation on increasing modification. Solubility experiments performed in concentrated ammonium sulfate solutions at pH 7.0 and at 30 degrees C showed a slight decrease in salting-out parameter, Ks, up to 40% modification, whereas a significant decrease was obtained at higher modification. However, the salting-out parameter, beta, decreased monotonously. Similar decrease in these parameters was also observed with different modified albumins at other pH values viz. pH 5.5, 4.5, and 3.6. From these results we conclude that the decrease in solubility of serum albumin on increasing modification was primarily due to change in conformation.