In vivo metabolism of 24R,25-dihydroxyvitamin D-3: Structure of its major bile metabolite

In vivo metabolism of 24R,25-dihydroxyvitamin D-3 (24,25-(OH)(2)D-3) in female dogs has been studied thoroughly, and its major bile metabolite identified. After single oral administration of 24,25-(OH)(2) [6,19,19-H-3]D-3 the plasma concentrations of radioactive metabolites were monitored for 504 h, and the metabolites in the bile collected and analyzed. The concentration of 24,25-(OH)(2)D-3 in plasma reached a maximum after 6 h and decayed in two distinct phases; a fast-phase with a half-life of 17 h, followed by a slow-phase with a 17-day half-life. The area under the concentration/time curve (AUC) was 78-84% (0-504 h). The only detectable metabolite in the plasma was 25-hydroxy-24-oxovitamin D-3 whose AUC was less than 5%. At 504 h, about 50% of administered radioactivity has been excreted, of which about 90% was found in the feces, indicating most of the administered 24,25-(OH)(2)D-3 to be excreted in bile. A major metabolite, which constituted 23% of the total bile radioactivity at 504 h, was found in the bile. This metabolite was efficiently deconjugated by beta-glucuronidase to afford an aglycone which was identified as 23S,25-dihydroxy-24-oxovitamin D-3 (23S,25-(OH)(2)-24-oxo-D-3), by co-chromatography on HPLC with synthetic standards. The glucuronide was isolated from the bile of dogs given large doses of 24,25-(OH)(2)D-3, and the structure determined being 23-(beta-glucuronide) of 23S,25-(OH)(2)-24-oxo-D-3, by analyzing its negative ion mass spectrum and the positive ion mass spectrum of its derivatives. Thus it was concluded that, in dogs, 24,25-(OH)(2)D-3 is a long lasting vitamin D metabolite, is mainly excreted in bile when metabolized to 23S,25-(OH)(2)-24-oxo-D-3 and is conjugated at 23-OH as glucuronide.