A high-affinity cytosol binding protein for 1 alpha,25-dihydroxycholecalciferol in the uterus of Japanese quail.

Cytosol fractions prepared from the uterine mucosa of egg-laying Japanese Quail were analysed for binding of the metabolites of cholecalciferol. When the uterus was incubated at 37 degrees C with various radioactive metabolites of cholecalciferol, the nuclear fraction incorporated only 1 alpha,25-dihydroxy[3H]cholecalciferol. When the uterus was incubated at 0 degree C with 1 alpha,25-dihydroxy[3H]cholecalciferol, most of the radioactivity was found in the cytosol. Translocation of 1 alpha,25-dihydroxy[3H]cholecalciferol from the cytosol to the nucleus was temperature-dependent. The addition of 100-fold excess amounts of unlabelled 1 alpha-25-dihydroxycholecalciferol significantly diminished the nuclear binding of 1 alpha,25-dihydroxy[3H]cholecalciferol. The cytosol fraction contained a 3.5 S macromolecule that specifically binds 1 alpha,25-dihydroxy[3H]cholecalciferol. The dissociation constant was 0.39 nM and the maximal binding was 55 fmol/mg of protein. These results strongly suggest that the uterus in egg-laying birds is a target organ or 1 alpha,25-dihydroxycholecalciferol.