Binding of retinoids and beta-carotene to beta-lactoglobulin. Influence of protein modifications.

The binding of retinol, retinyl acetate, retinoic acid and beta-carotene to native, esterified and alkylated beta-lactoglobulin was followed by quenching of tryptophan fluorescence. Three studied retinoids bind to native or modified beta-lactoglobulin in 1:1 molar ratios, with apparent dissociation constants in the range of 10(-8) M. The maximum tryptophan fluorescence quenching of unmodified beta-lactoglobulin by beta-carotene is observed at the ligand/protein ratio of 1:2. Esterification and alkylation of beta-lactoglobulin shift the ratio of beta-carotene/protein to 1:1. In all the cases, except for retinoic acid binding to N-ethyllysyl-BLG, the performed chemical modifications of beta-lactoglobulin enhance protein binding affinity. Measured apparent dissociation constants of beta-carotene complexes with native and modified beta-lactoglobulin are an order of magnitude lower from binding constants of other studied retinoids.