Specific cytosol-binding protein for 1,25-dihydroxyvitamin D3 in rat intestine.

Cytosol prepared from intestinal homogenates of vitamin D3-deficient rats demonstrate a 3.2 S protein having high affinity and low capacity for 1,25-dihydroxyvitamin D3. Appearance of the protein is dependent upon the presence of 0.3 M potassium chloride and dithiothreitol in the homogenization buffer. A 6 S protein having greater affinity for 25-hydroxyvitamin D3 than 1,25-dihydroxyvitamin D3 is observed using all conditions and is increased by the addition of rat serum to cytosol. The 3.2 S protein is neither a serum contaminant nor a component of the 6 S protein and is probably not a consituent of tissues which which are not responsive to vitamin D3.