Kamao Maya, Tatematsu Syuichiro, Sawada Natsumi, Sakaki Toshiyuki, Hatakeyama Susumi, Kubodera Noboru, Okano Toshio
Department of Hygienic Sciences, Kobe Pharmaceutical University, 4-19-1 Motoyamakita-machi, Higashinada-ku, Kobe 658-8558, Japan.
J Steroid Biochem Mol Biol. 2004 May;89-90(1-5):39-42. doi: 10.1016/j.jsbmb.2004.03.048.
It is well documented that Vitamin D3 metabolites and synthetic analogs are metabolized to their epimers of the hydroxyl group at C-3 of the A-ring. We investigated the C-3 epimerization of Vitamin D3 metabolites in various cultured cells and basic properties of the enzyme responsible for the C-3 epimerization. 1alpha,25-Dihydroxyvitamin D3 [1alpha,25(OH)2D3], 25-hydroxyvitamin D3 [25(OH)D3] and 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] were metabolized to the respective C-3 epimers in UMR-106 (rat osteosarcoma), MG-63 (human osteosarcoma), Caco-2 (human colon adenocarcinoma), LLC-PK1 (porcine kidney) and HepG2 (human hepatoblastoma)] cells, although the differences existed in the amount of each C-3 epimer formed with different cell types. In terms of maximum velocity (Vmax) and Michaelis constant (Km) values for the C-3 epimerization in microsome fraction of UMR-106 cells, 25(OH)D3 exhibited the highest specificity for the C-3 epimerization among 1alpha,25(OH)2D3, 25(OH)D3 and 24,25(OH)2D3. C-3 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 C-3 epimerization in vitro. Based on these results, the enzyme responsible for the C-3 epimerization of Vitamin D3 are thought to be different from already-known cytochrome P450-related Vitamin D metabolic enzymes and HSE.
有充分文献记载，维生素D3代谢产物和合成类似物在A环C-3位会代谢为其羟基差向异构体。我们研究了维生素D3代谢产物在各种培养细胞中的C-3差向异构化以及负责C-3差向异构化的酶的基本特性。1α,25-二羟基维生素D3 [1α,25(OH)2D3]、25-羟基维生素D3 [25(OH)D3] 和24,25-二羟基维生素D3 [24,25(OH)2D3] 在UMR-106（大鼠骨肉瘤）、MG-63（人骨肉瘤）、Caco-2（人结肠腺癌）、LLC-PK1（猪肾）和HepG2（人肝母细胞瘤）细胞中均代谢为各自的C-3差向异构体，尽管不同细胞类型形成的每种C-3差向异构体的量存在差异。就UMR-106细胞微粒体部分中C-3差向异构化的最大速度（Vmax）和米氏常数（Km）值而言，25(OH)D3在1α,25(OH)2D3、25(OH)D3和24,25(OH)2D3中对C-3差向异构化表现出最高的特异性。C-3差向异构化活性不受各种细胞色素P450抑制剂和抗NADPH细胞色素P450还原酶抗血清的抑制。CYP24、CYP27A1、CYP27B1和3(α→β)-羟基类固醇差向异构酶（HSE）在体外均不催化C-3差向异构化。基于这些结果，负责维生素D3 C-3差向异构化的酶被认为不同于已知的细胞色素P450相关维生素D代谢酶和HSE。
