Double bond in the side chain of 1alpha,25-dihydroxy-22-ene-vitamin D(3) is reduced during its metabolism: studies in chronic myeloid leukemia (RWLeu-4) cells and rat kidney

1alpha,25-Dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is mainly metabolized via the C-24 oxidation pathway and undergoes several side chain modifications which include C-24 hydroxylation, C-24 ketonization, C-23 hydroxylation and side chain cleavage between C-23 and C-24 to form the final product, calcitroic acid. In a recent study we reported that 1alpha,25-dihydroxyvitamin D(2) [1alpha,25(OH)(2)D(2)] like 1alpha,25(OH)(2)D(3), is also converted into the same final product, calcitroic acid. This finding indicated that 1alpha,25(OH)(2)D(2) also undergoes side chain cleavage between C-23 and C-24. As the side chain of 1alpha,25(OH)(2)D(2) when compared to the side chain of 1alpha,25(OH)(2)D(3), has a double bond between C-22 and C-23 and an extra methyl group at C-24 position, it opens the possibility for both (a) double bond reduction and (b) demethylation to occur during the metabolism of 1alpha,25(OH)(2)D(2). We undertook the present study to establish firmly the possibility of double bond reduction in the metabolism of vitamin D(2) related compounds. We compared the metabolism of 1alpha,25-dihydroxy-22-ene-vitamin D(3) [1alpha,25(OH)(2)-22-ene-D(3)], a synthetic vitamin D analog whose side chain differs from that of 1alpha,25(OH)(2)D(3) only through a single modification namely the presence of a double bond between C-22 and C-23. Metabolism studies were performed in the chronic myeloid leukemic cell line (RWLeu-4) and in the isolated perfused rat kidney. Our results indicate that both 1alpha,25(OH)(2)-22-ene-D(3) and 1alpha,25(OH)(2)D(3) are converted into common metabolites namely, 1alpha,24(R),25-trihydroxyvitamin D(3) [1alpha,24(R),25(OH)(3)D(3)], 1alpha,25-dihydroxy-24-oxovitamin D(3) [1alpha,25(OH)(2)-24-oxo-D(3)], 1alpha,23(S),25-trihydroxy-24-oxovitamin D(3) and 1alpha,23-dihydroxy-24,25,26,27-tetranorvitamin D(3). This finding indicates that the double bond in the side chain of 1alpha,25(OH)(2)-22-ene-D(3) is reduced during its metabolism. Along with the aforementioned metabolites, 1alpha,25(OH)(2)-22-ene-D(3) is also converted into two additional metabolites namely, 1alpha,24,25(OH)(3)-22-ene-D(3) and 1alpha,25(OH)(2)-24-oxo-22-ene-D(3). Furthermore, we did not observe direct conversion of 1alpha,25(OH)(2)-22-ene-D(3) into 1alpha,25(OH)(2)D(3). These findings indicate that 1alpha,25(OH)(2)-22-ene-D(3) is first converted into 1alpha,24,25(OH)(3)-22-ene-D(3) and 1alpha,25(OH)(2)-24-oxo-22-ene-D(3). Then the double bonds in the side chains of 1alpha,24,25(OH)(3)-22-ene-D(3) and 1alpha,25(OH)(2)-24-oxo-22-ene-D(3) undergo reduction to form 1alpha,24(R),25(OH)(3)D(3) and 1alpha,25(OH)(2)-24-oxo-D(3), respectively. Thus, our study indicates that the double bond in 1alpha,25(OH)(2)-22-ene-D(3) is reduced during its metabolism. Furthermore, it appears that the double bond reduction occurs only during the second or the third step of 1alpha,25(OH)(2)-22-ene-D(3) metabolism indicating that prior C-24 hydroxylation of 1alpha,25(OH)(2)-22-ene-D(3) is required for the double bond reduction to occur.