Vitamin D-binding protein (DBP) is an abundant serum protein, secreted by the liver, which transports vitamin D sterols and is part of an actin scavenging system. In this study, DBP was isolated from horse plasma in a highly reproducible, four step procedure: Affi-gel Blue affinity chromatography, gel filtration, hydroxy1apatite chromatography and anion exchange HPLC. 6-7 mg of DBP were obtained from 80 ml of plasma with a yield of 21-25%. The secondary structure of DBP was calculated from circular dichroism measurements to be 39% α-helix, 42% β-sheet and 19% random coil. A molecular mass of 53,000 ± 3,000 daltons was calculated from electrophoretic gels. Circular dichroism and fluorescence studies revealed that the disulphide bonds of DBP contribute substantial structural stabilization to the molecule with respect to thermal denaturation. Finally, acrylodan-labeled DBP was prepared. The fluorescence of this adduct was sensitive to the binding of actin and to the presence of dithiothreitol. / Science, Faculty of / Chemistry, Department of / Graduate