Solute effects on the irreversible aggregation of serum albumin

Thermal stress on bovine serum albumin (BSA) promotes protein aggregation through the formation of intermolecular β-sheets. We have used light scattering and chromatography to study effects of (< 1 M) Na2SO4, NaSCN, sucrose, sorbitol and urea on the rate of the thermal aggregation. Both salts were strong inhibitors of BSA aggregation and they reduced both the size and number (concentration) of aggregate particles compared to non-ionic solutes (or pure buffer). Hence, the salts appear to suppress both nucleation- and growth rate. The non-electrolyte additives reduced the initial aggregation rate (compared to pure buffer), but did not significantly limit the extent of aggregation in samples quenched after 27 min. heat exposure (40-50% aggregation in all samples). The non-electrolytes did, however, modify the aggregation process as they consistently brought about smaller but more concentrated aggregates than pure buffer.The results are discussed along the lines of linkage- and transition state theories. In this framework, the rate of the aggregation process is governed by the equilibrium between a thermally denatured state (D) and the transition state D≠. Thus, the effect of a solute relies on its preferential interactions with respectively D and D≠. The current results do not show any correlation between the solutes' preferential interactions with native BSA and their effect on the rate of aggregation. This suggests that non-specific, “Hofmeister-type” interactions, which scale with the solvent accessible surface area, are of minor importance. Rather, salt induced suppression of aggregation is suggested to depend on the modulation of specific electrostatic forces in the D≠ state.