Double bond in the side chain of 1α,25-dihydroxy-22-ene-vitamin D3 is reduced during its metabolism:: studies in chronic myeloid leukemia (RWLeu-4) cells and rat kidney

1 alpha ,25-Dihydroxyvitamin D-3 [1 alpha ,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 1 alpha ,25-dihydroxyvitamin D-2 [1 alpha ,25(OH)(2)D-2] like 1 alpha ,25(OH)(2)D-3, is also converted into the same final product, calcitroic acid. This finding indicated that 1 alpha ,25(OH)(2)D-2 also undergoes side chain cleavage between C-23 and C-24. As the side chain of 1 alpha ,25(OH)(2)D-2, when compared to the side chain of 1 alpha ,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 1 alpha ,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 1 alpha ,25-dihydroxy-22-ene-vitamin D-3 [1 alpha ,25(OH)(2)-22-ene-D-3], a synthetic vitamin D analog whose side chain differs from that of lot,25(OH)2D3 only through a sing-le 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 1 alpha ,25(OH)(2)-22-ene-D-3 and 1 alpha ,25(OH)(2)D-3 are converted into common metabolites namely, 1 alpha ,24(R),25-trihydroxyvitamin D-3 [1 alpha ,24(R),25(OH)(3)D-3], 1 alpha ,25-dihydroxy-24-oxovitamin D-3 [1 alpha ,25(OH)(2)-24-oxo-D-3], 1 alpha ,23(S),25-trihydroxy-24-oxovitamin D-3 and 1 alpha ,23-dihydroxy-24,25,26,27-tetranorvitamin D-3. This finding indicates that the double bond in the side chain of 1 alpha ,25(OH)(2)-22-ene-D-3 is reduced during its metabolism. Along with the aforementioned metabolites, 1 alpha ,25(OH)(2)-22-ene-D-3 is also converted into two additional metabolites namely, 1 alpha ,24,25(OH)(3)-22-ene-D-3 and 1 alpha ,25(OH)(2)-24-oxo-22-ene-D-3. Furthermore, we did not observe direct conversion of 1 alpha ,25(OH)(2)-22-ene-D-3 into 1 alpha ,25(OH)(2)D-3. These findings indicate that 1 alpha ,25(OH)(2)-22-ene-D-3 is first converted into 1 alpha ,24,25(OH)(3)-22-ene-D-3 and 1 alpha ,25(OH)(2)-24-oxo-22-ene-D-3. Then the double bonds in the side chains of 1 alpha ,24,25(OH)(3)-22-ene-D-3 and 1 alpha ,25(OH)(2)-24-oxo-22-ene-D-3 undergo reduction to form 1 alpha ,24(R),25(OH)(3)D-3 and 1 alpha ,25(OH)(2)-24-oxo-D-3, respectively. Thus, our study indicates that the double bond in 1 alpha ,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 1 alpha, 25(OH)(2)-22-ene-D-3 metabolism indicating that prior C-24 hydroxylation of 1 alpha ,25(OH)(2)- 22-ene-D-3 is required for the double bond reduction to occur. (C) 2001 Elsevier Science Ltd. All rights reserved.