Binding affinities of retinol and related compounds to retinol binding proteins

Fluorimetric titrations were used to determine apparent dissociation constants of the all-trans isomers of retinol, retinoic acid, retinyl acetate and retinyl palmitate to human-retinol binding protein and chicken-retinol binding protein. Enhancement of the fluorescence of retinol and retinyl acetate when bound to the protein was utilized to establish the binding affinity of these compounds. With retinoic acid which is essentially a non-fluorescent compound, quenching of protein fluorescence due to energy transfer to the bound ligand from tryptophanyl residues served to determine the binding affinity. The various ligands display 1:1 molecular complexes with both types of retinol binding proteins. Retinol, retinoic acid and retinyl acetate were found to have similar binding affinities to both species of carrier proteins: For retinol K'd=1.9 X 10(-7) M with human-retinol binding protein and K'd=1.5 X 10(-7) M with chicken-retinol binding protein; for retinoic acid K'd-2.1 X 10(-7) M with human-retinol binding protein; for retinyl acetate and K'd=2.2 X 10(-7) M with chicken-retinol binding protein; for retinyl acetate K'd=2.2 X 10(-7) M with human-retinol binding protein and K'd=1.7 X 10(-7) M with chicken-retinol binding protein. Retinyl palmitate appeared to have weak association with either of the two retinol binding proteins, if at all. The above results suggest that both human and chicken retinol binding proteins behave similar with respect to the binding of the ligands. Non-polar interactions probably play a primary role in the binding and effects of functional groups and charges are of secondary importance.