Three metabolites were isolated after incubation of vitamin D3, vitamin D2, or 25-hydroxyvitamin D3 with bovine rumen microbes. They are identified as 5(E)-19-nor-10-ketovitamin D3, 5(E)-19-nor-10-ketovitamin D2, and 5(E)-19-nor-10-keto-25-hydroxyvitamin D3, respectively. The identifications were based on ultraviolet absorbance, mass spectroscopy, and chemical reactivity. All 5(E)-19-nor-10-ketovitamin D derivatives examined had an absorbance maximum at 312 nm and a characteristic fragment in their mass spectra corresponding to loss of 43 amu from their molecular ions. The vitamin D3 metabolite was identical in all essential spectral and chromatographic aspects with authentic synthetic 5(E)-19-nor-10-ketovitamin D3. These metabolites represent a unique pathway of vitamin D metabolism and the first characterized products of microbial vitamin D metabolism. The conversion of vitamin D and its metabolites to their 19-nor-10-keto forms likely represents a detoxification mechanism.
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