Observations on the binding of lanthanides and calcium to vitamin D-dependent chick intestinal calcium-binding...

The binding of calcium and terbium to purified chick vitamin D-dependent intestinal calcium-binding protein was studied by terbium fluorescence, circular dichroism, and intrinsic protein fluorescence techniques. Calcium-binding protein bound, with high affinity, at least 3 mol of terbium/mol of protein; numerous low affinity terbium-binding sites were also noted. The three highest affinity sites were resolved into one very high affinity site (site A) and two other sites (sites B and C) with slightly lower affinity. Resonance energy transfer from tryptophan residues to terbium occurred only with site A. This site was filled before sites B and C. Competition experiments in which calcium was used to displace terbium bound to the protein showed that larger amounts of calcium were needed to displace terbium from site A than from sites B and C. Energy transfer from terbium to holmium indicated that the terbium-binding sites (B and C) were located close to each other (about 7-12 A) but were distant (greater than 12 A) from site A. The addition of EDTA to calcium-binding protein resulted in a 25% decrease in intrinsic protein fluorescence, suggesting a conformational change in the protein. The titration of EDTA-treated calcium-binding protein with calcium resulted in recovery of intrinsic protein fluorescence. A reversible calcium-dependent change in the ellipticity of calcium-binding protein in circular dichroism experiments was also seen. These observed properties suggest that vitamin D-dependent chick intestinal calcium-binding protein behaves in a manner similar to other well-known calcium-binding regulatory proteins.