Influence of hydrogen and chloride ions on the interaction between sulfaethidole and bovine serum albumin studied by microcalorimetric and acid-base titrimetric methods.

From earlier studies it is known that bovine serum albumin has one high affinity binding site and several lower affinity sites for the sulfa drug N1-(5-ethyl-1,3,4-thiadiazol-2-yl)sulfanilamide (sulfaethidole) (Kostenbauder, H.B., Jawad, M.J., Perrin, J.H., and Averhart, V. (1971) J. Pharm. Sci. 60, 1658-1660). This binding has been further studied using equilibrium dialysis, microcalorimetry, and pH titration technique. Results of these studies show that the binding of sulfaethidole to the first (high affinity) site may be accompanied by an uptake of protons. Proton uptake is found to be zero at pH 7.4 and approximately 0.6 at pH 8.5 for each sulfaethidole molecule bound. The other binding sites for sulfaethidole are not proton linked. The first, and probably the other binding sites, are also Cl- ion linked; for example, the binding of sulfaethidole to the first binding site is accompanied by the displacement of (on average) one Cl- ion at pH 7.4 in 0.1 M NaCl. This explains the observation that the heat of binding of sulfaethidole to the high affinity site is -33.0 kJ.mol-1 in the absence of chloride ions, but only -22.8 kJ.mol-1 in the presence of 0.1 M Cl- (at pH 7.4).