Analysis of C-3 epimerization in (24R)-24,25-dihydroxyvitamin D3 catalyzed by hydroxysteroid dehydrogenase.

Studies on the C-3 epimerization in (24R)-24,25-dihydroxyvitamin D(3) [24R,25(OH)(2)D(3)] were performed using hydroxysteroid dehydrogenases (HSDs). 3-Epi-24R,25(OH)(2)D(3) was formed from 24R,25(OH)(2)D(3) by the catalysis of 3alpha- or beta-HSD. These HSDs also catalyzed the C-3 epimerization in 3-epi-24R,25(OH)(2)D(3) to form 24R,25(OH)(2)D(3). 24R,25(OH)(2)D(3) and its C-3 epimer were separated by inclusion high-performance liquid chromatography using gamma-cyclodextrin (gamma-CD) as the mobile phase additive or a gamma-CD bonded chiral column. The product derived from the intermediate during the C-3 epimerization was isolated from the incubation specimens and identified as (7Z)-(24R)-24,25-dihydroxy-9,10-secocholesta-4,7,10(19)-trien-3-one by several instrumental analyses including (1)H-nuclear magnetic resonance spectrometry. The occurrence of this compound strongly proves that the formation of the C-3 epimer by HSD involves a dehydrogenation process. The present study suggests that HSDs may catalyze the C-3 epimerization of vitamin D compounds and modulate their concentrations and biological activities in animals and humans.