Binding of p-Nitrophenyl Phosphate and Other Aromatic Compounds by β-Lactoglobulin

Results obtained from gel filtration showed that beta-lactoglobulin binds p-nitrophenyl phosphate with a stoichiometry of 1 mol of ligand per 18,360 monomer. Circular dichroic spectra confirmed the binding and implicated tryptophan and phenylalanine residues in the interaction. Fluorescence of the protein was quenched on binding also supporting complex formation; analysis of these data indicates that p-nitrophenyl phosphate binds to beta-lactoglobulin A with a dissociation constant of 31 microM. The B and C genetic variants of beta-lactoglobulin bind p-nitrophenyl phosphate with dissociation constants of 63 and 70 microM, respectively. In addition, a series of other nitrophenyl compounds and pyridoxal phosphate were also investigated by fluorescence analysis and found to bind to the protein. These results are discussed with respect to a recent hypothesis that beta-lactoglobulin binds retinol and is structurally related to serum retinol binding protein.