Metabolism of 1α,25-dihydroxyvitamin D3 in vitamin D receptor-ablated mice in vivo

The metabolism of 1 alpha,25-dihydroxyvitamin D-3 was studied in vitamin D receptor-ablated mice following the administration of a physiological dose of 1 alpha,25-dihydroxy-[26,27-H-3] vitamin D-3. The degradation of 1 alpha,25-dihydroxy-[26,27-H-3]vitamin D-3 in the vitamin D receptor null mutant mice was substantially reduced compared to the wild-type control mice. At 24 h postadministration of radiolabeled 1 alpha,25-dihydroxyvitamin D-3 more than 50% of the radioactivity was recovered unmetabolized, whereas in wild-type mice nearly all of the 1 alpha,25-dihydroxy-[26,27-H-3]vitamin D-3 was degraded. In wild-type mice three polar metabolites other than 1 alpha,25-dihydroxyvitamin D-3 were detected and identified on straight-phase and reverse-phase high-performance liquid chromatography as 1 alpha,24(R),25-trihydroxy-[26,27-H-3]-vitamin D-3, 1 alpha,25(S),26-trihydroxy-[26,27-H-3]vitamin D-3, and (23S,25R)-1 alpha,25-dihydroxy-[H-3]vitamin D-3-26,23-lactone. Only one metabolite was detected in the plasma and kidneys of vitamin D receptor null mutant mice at 3 h following an intrajugular dose of 1 alpha,25-dihydroxy-[26,27-H-3]vitamin D-3. This metabolite was clearly identified as 1 alpha,25(S),26-trihydroxy-[26,27-H-3]vitamin D-3 by comigration on two HPLC systems and periodate cleavage reaction. At 6, 12, and 24 h postinjection 1 alpha,24(R),25-trihydroxy-[26,27-H-3]vitamin D-3 was also detected at low levels in plasma, kidneys, and liver of some but not all mutant mice. The presence of 25-hydroxyvitamin D-3-24-hydroxylase mRNA in the kidneys of these vitamin D receptor null mutant mice was confirmed by ribonuclease protection assay.