Binding of cholecalciferol metabolites to rat duodenal mucosa cytosol.

In vitro binding of 25-hydroxycholecalciferol (25-(OH)D3) and 1alpha,25-dihydroxycholecalciferol (1,25-(OH)2D3) was studied in the duodenal mucosa cytosol of rachitic rats: both 25-(OH)D3 and 1,25-(OH)2D3 bind to a macromolecule (sedimentation coefficient 5.5 to 6 S in low or high ionic strength) with a high affinity (KD at 4 degrees C = 1.2 X 10-9M and 2 x 10-9M, respectively). In addition, it is concluded from competition and chase experiments that 25-(OH)D3 binding sites differ from that for 1,25-(OH)2D3.