J. Biol. Chem., Vol. 258, Issue 15, 9100-9107, 08, 1983
Intestinal synthesis of 24-keto-1,25-dihydroxyvitamin D3. A metabolite formed in vivo with high affinity for the vitamin D cytosolic receptor
JL Napoli, BC Pramanik, PM Royal, TA Reinhardt and RL Horst
24-Keto-1,25-dihydroxyvitamin D3 has been identified as an intestinal
metabolite of 1,25-dihydroxyvitamin D3 by ultraviolet absorbance, mass
spectroscopy, and chemical reactivity. The metabolite was produced from
1,25-dihydroxyvitamin D3 and 1,24R,25-trihydroxyvitamin D3 in rat
intestinal mucosa homogenates. 24-Keto-1,25-dihydroxyvitamin D3 is present
in vivo in the plasma and small intestinal mucosa of rats fed a stock diet,
receiving no exogenous 1,25-dihydroxyvitamin D3, and in the plasma and
small intestinal mucosa of rats dosed chronically with 1,25-
dihydroxyvitamin D3. 24-Keto-1,25-dihydroxyvitamin D3 has affinity
equivalent to 1,24R,25-trihydroxyvitamin D3 for the 3.7 S cytosolic
receptor specific for 1,25-dihydroxyvitamin D3 in the intestine and thymus.
In cytosolic preparations contaminated with the 5 S vitamin D- binding
protein, both metabolites are about 7-fold less potent than
1,25-dihydroxyvitamin D3. In contrast, in cytosolic preparations largely
free of the 5 S binding protein, both metabolites are equipotent with the
parent compound. No evidence was obtained supporting a substantial presence
of 23-keto-1,25-dihydroxyvitamin D3 in vivo; nor was the latter compound
generated in detectable amounts from 1,25-dihydroxyvitamin D3 by intestinal
homogenates. Thus, C-24 oxidation is a significant pathway of intestinal
1,25-dihydroxyvitamin D3 metabolism that produces metabolites with high
affinity for the cytosolic receptor which mediates vitamin D action.