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1 Southeast Dairy Foods Research Center, Department of Food Science, North Carolina State University, Raleigh 27695-7624
ß-Lactoglobulin was isolated directly from acidic whey by bioselective adsorption on N-retinyl-CeliteTM, yielding preparations of
96% purity. Interactions of these preparations with vitamin D2, vitamin D3, 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 ß-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 D2 (dissociation constant of 4.91 nM) 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·mol1 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 ß-LG may be a better carrier of vitamin D than of vitamin A.
Key Words: ßbeta;-lactoglobulin vitamin D binding cholesterol binding bioselective adsorption
Submitted on June 5, 1996
Accepted on December 2, 1996
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