Effect of Ligand Binding on the Conformation of Human Plasma Vitamin D Binding Protein (group-specific Component)

Several techniques have been used to demonstrate that the binding of specific ligands to human plasma vitamin D binding protein induces a change in protein conformation. Apoprotein and holoprotein show circular dichroism spectra of similar form in the peptide region with double minima at 207 and 218 nm. The minimum mean residue ellipticity of apoprotein (20.6 X 10(3) degrees.cm2.dmol-1) is decreased by about 8% after vitamin D3 binding, suggesting a small change in the backbone conformation. Spectrofluorimetric studies showed that 25-hydroxycholecalciferol causes a saturable enhancement of intrinsic fluorescence of human vitamin D binding protein and alters the pH profile of protein fluorescence, suggesting that there are alterations in the local environment of tryptophan residue(s) after ligand binding. Furthermore, in the presence of 25-hydroxycholecalciferol, the rate of chemical modification of the amino groups in human vitamin D binding protein is decreased and the susceptibility of intact vitamin D binding protein to proteolytic degradation is reduced, suggesting that some surface sites in the vitamin D binding protein molecule are less accessible to external agents. In addition, although the absorbance of vitamin made if difficult to interpret the ultraviolet spectra of holoprotein and apoprotein, the presence of vitamin D binding protein appears to stabilize the vitamin in an aqueous environment, a phenomenon that may be of physiological importance.