Differences in the side-chain metabolism of vitamin D3 between chickens and rats

In vitro metabolism of 25-hydroxy-24-oxovitamin D3 was studied in kidney homogenates from vitamin D-supplemented chickens and rats. In chicken homogenates, 25-hydroxy-24-oxovitamin D3 was converted predominantly to 23,25-dihydroxy-24-oxovitamin D3, 24,25-dihydroxyvitamin D3, and 23,24,25-trihydroxyvitamin D3. In rat homogenates, 25-hydroxy-24-oxovitamin D3 was not converted to either 24,25-dihydroxyvitamin D3 or 23,24,25-trihydroxyvitamin D3, but it was converted to 23,25-dihydroxy-24-oxovitamin D3 and 23-hydroxy-24,25,26,27-tetranorvitamin D3. The latter metabolite was not produced by the chicken preparations. The stereochemical configuration at C-24 of the 24,25-dihydroxyvitamin D3 produced by chicken homogenates was determined to be S. This contrasts with the R configuration of 24,25-dihydroxyvitamin D3 produced by 24-hydroxylation of 25-hydroxyvitamin D3. These results suggest that chickens have an enzyme that can reduce the 24-oxo group to 24S-hydroxyl group, whereas rats do not.