C(24)- and C(23)-oxidation, converging pathways of intestinal 1,25-dihydroxyvitamin D3 metabolism: identification of 24-keto-1,23,25-trihydroxyvitamin D3

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Abstract
24-Keto-1,23,25-trihydroxyvitamin D3 was identified as a major 1,25-dihydroxyvitamin D3 metabolite, produced by intestinal mucosa cells isolated from rats dosed chronically with 1,25-dihydroxyvitamin D3. The identification was based on UV absorbance spectroscopy, mass spectroscopy and chemical derivatization. The pathway of biosynthesis proceeded through 1,24,25-trihydroxyvitamin D3 and 24-keto-1,25-dihydroxyvitamin D3, which are physiological metabolites of 1,25-dihydroxyvitamin D3. Previous work showed that the amount of 24-keto-1,23,25-trihydroxyvitamin D3 in intestine in vivo, relative to its C(24)-oxidized precursors, is enhanced by chronically dosing rats with 1,25-dihydroxyvitamin D3. These results establish the C(24)-oxidation pathway as a predominant route of intestinal 1,25-dihydroxyvitamin D3 metabolism under physiological conditions and indicate that treatment of the rat with exogenous 1,25-dihydoxyvitamin D3 causes expression of C(23)-hydroxylase activity, which uses C(24)-oxidized 1,25-dihydroxyvitamin D3 metabolites as substrates.