Binding of Vitamin D and Cholesterol to β-Lactoglobulin

Abstract β -Lactoglobulin was isolated directly from acidic whey by bioselective adsorption on N-retinyl-Celite TM , yielding preparations of ≥96% purity. Interactions of these preparations with vitamin D 2 , vitamin D 3 , ergosterol, cholesterol, and 7-dehydrocholesterol were examined by following changes in the fluorescence spectra. Both the excitation and emission spectra indicated that energy was transferred between the tryptophanyl residues of the protein and the chromophore of the ligand. Analyses of the fluorescence changes that occurred upon titration of β -LG with the various ligands allowed determination of the dissociation constant for the complex and the number of moles bound per mole of protein. The affinity for vitamin D 2 (dissociation constant of 4.91 n M ) was 10-fold higher than that of the other compounds, except for ergosterol, which was 5-fold larger than the others. Also, the affinity was 10-fold higher than that typically reported for the retinoids. Furthermore, the value obtained for the number of moles bound per mole of protein was 2 mol·mol –1 for each of the ligands examined in this study; it has been well established that all of the retinoids are bound with a stoichiometry of 1.0. These results suggest that β -LG may be a better carrier of vitamin D than of vitamin A.