pH-dependent interaction of halothane upon thiopental binding to human serum albumin.

At pH 7.4 the binding of thiopental to human serum albumin (HSA) was increased in the presence of halothane. In order to obtain information about the mechanism of this interaction, in vitro binding experiments by means of equilibrium dialysis were carried out at different pH values. At pH 4.97 the binding of thiopental to HSA 1% was low (23% bound) and not influenced by halothane. An increase of thiopental binding caused by halothane could be seen at pH 7.4 (55% bound vs. 41% in the control = without halothane) and at pH 8.23 (62% vs. 54%). At pH 10.15 an opposite interaction was found: in the presence of halothane thiopental binding was considerably decreased (36.2% vs. 47.0% in the control). Evaluation of the binding parameters of experiments using increasing substrate concentrations (Scatchard plot) revealed quite different changes of the two classes of binding sites of HSA for thiopental. It is assumed that halothane causes reversible conformational changes of the albumin molecule resulting in altered binding characteristics for thiopental.