Synthesis, stereochemistry, and biological activity of 1 alpha,23,25-trihydroxy-24-oxovitamin D-3, a major natural metabolite of 1 alpha,25-dihydroxyvitamin D-3

The C(23) epimers of 1 alpha,23,25(OH)(3)-24-oxovitamin D-3, a major natural metabolite of the secosteroid hormone, 1 alpha,25(OH)(2)D-3, were chemically synthesized for the first time. The metabolite was synthesized by palladium coupling of the appropriate CD ring analog with an A ring enyne. Various approaches from quinic acid to the A ring precursors were explored, and a new route to the A ring enyne from quinic acid was developed. The C(23) stereochemistry of the natural 1 alpha,23,25(OH)(3)-24-oxovitamin D-3 produced in neonatal human keratinocytes was determined to be S on the basis of the H-1 NMR and the HPLC data. The biological activity of 1 alpha,23(S),25(OH)(3)-24-oxovitamin D-3 in primary cultures of bovine parathyroid cells was determined by comparing the potency of this metabolite to that of 1 alpha,25(OH)(2)D-3 in suppression of parathyroid hormone (PTH) secretion. The results indicate that 1 alpha,23(S),25(OH)(3)-24-oxovitamin D-3 potently suppressed PTH secretion even at concentrations as low as 10(-12) M and is equipotent with 1 alpha,25(OH)(2)D-3. The high activity of 1 alpha,23(S),25(OH)(3)-24-oxovitamin D-3 cannot be explained on the basis of its affinity for the vitamin D receptor as this metabolite was found to be 10 times less effective than radioinert 1 alpha,25(OH)(2)D-3 in blocking the uptake and receptor binding of [H-3]- 1 alpha,25(OH)(2)D-3 in intact parathyroid cells. Further studies are required to explain the molecular basis for the activity of 1 alpha,23(S),25(OH)(3)-24-oxovitamin D-3 in its ability to suppress PTH secretion. In summary, our present study indicates that the C(23) stereochemistry of the natural 1 alpha,23,25(OH)(3)-24-oxovitamin D-3 is S and this metabolite is equipotent to 1 alpha,25(OH)(2)D-3 in suppressing PTH secretion.