Interactions of β-Lactoglobulin Variants A and B with Vitamin A. Competitive Binding of Retinoids and Carotenoids

beta-Lactoglobulin (beta-Lg) is the major whey protein -of bovine milk present at a concentration of 2-3 g L-1. Its biological role is still not well-known. However, many studies have suggested that beta-Lg may play either nutritional or specific transporter role. The high affinity of beta-Lg for retinol and other retinoids was reported. The results of interaction studies of beta-Lg with carotenoids, that is, beta-carotene, beta-cryptoxanthin, and alpha-carotene, which display similar structures are reported in this study. The affinities of beta-Lg for binding of retinoids and carotenoids were compared, providing more information about the binding site(s) of these molecules by beta-Lg. Interactions were followed by the measurements of quenching of beta-Lg tryptophan fluorescence and retinol fluorescence. The obtained results indicate that carotenoids are bound by beta-Lg with high affinity of the order of 10(-8) M. Measurement of retinol competition with carotenoids for binding by beta-Lg suggests that the binding of these two ligands occurs at two different sites of beta-Lg.