Temperature-dependent alteration of 1,25-dihydroxy-vitamin D3 receptor macromolecules in MC3T3-E1 cells: affinity of hexafluoro analog of 1,25-dihydroxyvitamin D3, ST-630, for these forms.

The binding properties of [3H]1,25-dihydroxyvitamin D3 (1,25(OH)2D3) to 1,25(OH)2D3 receptor in mouse osteoblastic MC3T3-E1 cells and the affinity of 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 (ST-630) were examined. Sucrose density gradient experiments demonstrated that [3H]1,25(OH)2D3 bound to the 6S macromolecule in the cytosol of MC3T3-E1 cells at 4 degrees C. The inhibitory effect of 1,25(OH)2D3 was compared with that of ST-630. However, at 30 degrees C, only the 3.7S macromolecule was labeled, and 1,25(OH)2D3 and ST-630 demonstrated similar affinity for the 3.7S macromolecule. Under the lower temperature condition, the cytosol from MC3T3-E1 cells showed one binding site for [3H]1,25(OH)2D3 with Kd of 0.31 nM and Bmax of 13.0 fmol/mg protein, respectively. Furthermore, from the competitive binding experiments at 4 degrees C, the affinity of ST-630 was 6.4-fold lower than that of 1,25(OH)2D3. These results suggest that the 6S macromolecule formed at the lower temperature in MC3T3-E1 cells has a property of 1,25(OH)2D3 receptor, but ST-630 has a higher affinity for 3.7S macromolecule and it may be comparable to 1,25(OH)2D3.