PROBING THE DETERMINANTS OF PROTEIN SOLUBILITY WITH AMINO-ACID MODIFICATION

Chemical modification was used as a probe to study the effect of struc tural features of serum albumin (charge, conformation, surface hydroph obicity, etc.) on its solubility behavior in concentrated ammonium sul fate solutions. Four different acetylated derivatives of goat serum al bumin namely 18% acetylated, 40% acetylated, 53% acetylated, and 93% a cetylated albumins were prepared. The homogeneity of these preparation s was established by gel chromatography and polyacrylamide gel electro phoresis. Hydrodynamic data on the Stokes radius of native and acetyla ted albumins suggested gradual change in conformation on increasing mo dification. Solubility experiments performed in concentrated ammonium sulfate solutions at pH 7.0 and at 30 degrees C showed a slight decrea se in salting-out parameter, K-s, up to 40% modification, whereas a si gnificant decrease was obtained at higher modification. However, the s alting-out parameter, beta, decreased monotonously. Similar decrease i n 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 co nclude that the decrease in solubility of serum albumin on increasing modification was primarily due to change in conformation.