Vitamin D Research Today is a free monthly online journal that collates and summarizes the latest research about Vitamin D, including details on sources, melanin, benefits, deficiency, supplements, calcium absorption. | ||||||||
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Measurement and characterization of C-3 epimerization activity toward vitamin D3.Kamao M, Hatakeyama S, Sakaki T, Sawada N, Inouye K, Kubodera N, Reddy GS, Okano T Department of Hygienic Sciences, Kobe Pharmaceutical University, Kobe 658-8558, Japan. Recently, epimerization of the hydroxyl group at C-3 has been identified as a unique metabolic pathway of vitamin D compounds. We measured C-3 epimerization activity in subcellular fractions prepared from cultured cells and investigated the basic properties of the enzyme responsible for the epimerization. C-3 epimerization activity was detected using a NADPH-generating system containing glucose-6-phosphate, NADP, glucose-6-phosphate dehydrogenase, and Mg(2+). The highest level of activity was observed in a microsomal fraction prepared from rat osteoblastic UMR-106 cells but activity was also observed in microsomal fractions prepared from MG-63, Caco-2, Hep G2, and HUH-7 cells. In terms of maximum velocity (V(max)) and the Michaelis constant (K(m)), 25-hydroxyvitamin D(3) [25(OH)D(3)] exhibited the highest specificity for the epimerization at C-3 among 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], 25(OH)D(3), 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)], and 22-oxacalcitriol (OCT). The epimerization activity was not inhibited by various cytochrome P450 inhibitors and antiserum against NADPH cytochrome P450 reductase. Neither CYP24, CYP27A1, CYP27B1 nor 3(alpha-->beta)hydroxysteroid epimerase (HSE) catalyzed the epimerization in vitro. Based on these results, the enzyme(s) responsible for the epimerization of vitamin D(3) at C-3 are thought to be located in microsomes and different from cytochrome P450 and HSE. Published 8 March 2005 in Arch Biochem Biophys, 436(1): 196-205.
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