Examination of phenothiazine-albumin interaction by ultraviolet difference spectrophotometry

Abstract

The binding properties of several phenothiazines to both human and bovine serum albumin have been systematically examined by ultraviolet difference spectrophotometric methods. The difference spectra of their interaction at pH 7·4 showed characteristically two positive and two negative absorption peaks at the 330, 260 and 290, 250 nm regions respectively. The difference spectra, which were derived mostly from perturbation of the phenothiazine chromophores in a hydrophobic environment, were quite specific, as structurally similar chlorpromazine sulfoxide, imipramine and chlorprothixene gave entirely different patterns of much weaker absorption. This may be taken as evidence that the latter compounds were bound at sites on the protein surface quite different from those of the phenothiazines. Contrary to the conclusion reached in recent publications which advocated that only one of the phenothiazine benzene rings was involved in the binding, the present studies with substituted derivatives on various positions show that this is not the case, but rather the whole phenothiazine nucleus takes part in the binding process. Furthermore, the effect of acetylsalicylic acid on the binding of phenothiazines demonstrated that the interaction is a noncompetitive interference which does not involve the same binding sites. The significance of these results is discussed in relation to various types of binding forces.