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Volume Effects Produced by Protein-Ion Interactions

THE BINDING OF BOVINE PLASMA ALBUMIN WITH THIOCYANATE AND TRICHLOROACETATE IONS

From the ¹ From the Biochemistry Department, West Virginia University, School of Medicine, Morgantown, West Virginia 26506

The volume changes produced by the reaction of bovine plasma albumin with thiocyanate and trichloroacetate ions differed substantially from each other and from those produced by sodium dodecyl sulfate (Katz, S., Shaw, M. E., Chillag, S., and Miller, J. E. (1972) J. Biol. Chem., 247, 5528–5233). The volume effects attributable to protein-anion interaction, V, resulting from the interaction of thiocyanate anion (SCN^-) with 2% albumin, in water, exhibited a minimum of -30 ml per 10⁵ g of protein at 0.05 m SCN^-. At higher anion concentrations there was a linear increase of this parameter reaching a value of 110 ml per 10⁵ g of protein at 0.5 m SCN^-. The corresponding reactions employing trichloroacetate ions generated a positive biphasic V isotherm with the value for V of 275 ml per 10⁵ g of protein at 0.5 m anion concentration.

Medium effects were investigated by comparing these reactions in water, 0.1 m acetate, pH 5.0, and 0.1 m EDTA, pH 5.0. The V for dilution of these anions by water and acetate was similar; however, the use of 0.1 m EDTA as solvent caused a diminution of this type of volume effect. On the other hand, the V isotherms for albumin-ligand interaction in these buffers differed; e.g. the V isotherms for albumin-SCN^- reaction in water and in EDTA were approximately of the same magnitude, whereas the V values in acetate were more negative. The isotherms produced by the reaction of trichloroacetate ion with albumin in water and 0.1 m acetate were virtually superimposable, but in 0.1 m EDTA the V values were more positive.

The distinctive volume effects produced by the respective systems establish that the association mechanisms are determined primarily by the nature of the protein and anion involved. Evidence supporting this hypothesis is provided by the relatively small effect of buffer ions on these volume parameters. Apparently several types of electrostriction phenomena contribute to the volume effects produced by protein-ion interaction. The Drude-Nernst phenomenon which is of paramount importance in determining the volume decrease produced by albumin-dodecyl sulfate binding plays a secondary role in thiocyanate and trichloroacetate interaction.

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