Coupling between fatty acid binding and sulfhydryl oxidation in bovine serum albumin

Albumin with and without a free sulfhydryl group (mercapt-albumin and nonmercapt-albumin) has low and high amounts of fatty acids, respectively. It was found that this difference was brought about by the following two mechanisms: (a) the binding of fatty acids increases the rate of oxidation of the sulfhydryl group, (b) the oxidation of the sulfhydryl group of the protein enhances the binding of fatty acids. The incubation of the protein with fatty acids enhanced reversibly the reaction rate of the free sulfhydryl group with 5,5'-dithiobis(2-nitrobenzoic acid). This effect of fatty acids depends on the kind of fatty acid added. Among saturated long-chain fatty acids tested, the shortest fatty acid, lauric (dodecanoic) acid, had the strongest effect. Oleic (cis-9-octadecenoic) acid, which has a long chain but also one cis-double bond, however, was as effective as lauric acid in enhancing the reactivity of the sulfhydryl group on the protein. Analyses with palmityl-Sepharose and equilibrium partitioning of palmitic acid between heptane and aqueous protein solutions have shown that nonmercapt-albumin has higher affinity to the fatty acid than mercapt-albumin. These results imply that the binding of fatty acids and the oxidation of sulfhydryl groups of the protein are intimately coupled.