Isolation and characterization of a 25-hydroxyvitamin D binding protein from rat enterocyte cytosol

In this study, we have purified and partially characterized a unique protein in rat enterocyte cytosol that is capable of binding 25- hydroxyvitamin D₃. The protein was purified using ammonium sulfate precipitation and gel permeation chromatography, followed by two anion exchange chromatography steps. The protein has an apparent molecular weight of 68,000 assessed by gel permeation chromatography, and 58,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino acid composition and N-terminal amino acid sequence were determined. Differences in chromatographic behavior, molecular weight, amino acid composition, and sequence suggest that the protein is distinct from serum vitamin D binding protein and serum albumin. It differs from the 1,25-dihydroxyvitamin D receptor as well in its stability during chromatography, cytosolic location, molecular weight, and sequence and differs from the putative basal-lateral membrane receptor in its cytosolic location. The protein isolated is a candidate cytosolic vitamin D-binding protein, which may be involved in absorption or in intracellular metabolism of vitamin D metabolites.