Nadin L, Murray M
School of Physiology and Pharmacology, University of New South Wales, Sydney, NSW 2052, Australia.
Br J Pharmacol. 2000 Oct;131(4):851-7. doi: 10.1038/sj.bjp.0703579.
The quantitative importance of prostaglandin H synthase (PGHS)-mediated cooxidation of all-trans-retinoic acid (ATRA) was evaluated in human liver microsomes (n=17) in relation to CYP-dependent ATRA 4-hydroxylation. Observed rates of ATRA cooxidation (4.6 - 20 pmol mg protein(-1) min(-1)) and 4-hydroxylation (8.7 - 45 pmol mg protein(-1) min(-1)) were quantitatively similar and exhibited similar individual variation (4 and 5 fold, respectively). From kinetic studies cooxidation was an efficient process in human hepatic microsomes (V(max) K(m)(-1)=0.25) compared with NADPH- and NADH-mediated 4-hydroxylation by CYP (V(max) K(m)(-1)=0.14 and 0.02, respectively). The capacity of lipid hydroperoxide metabolites of arachidonic acid to mediate ATRA oxidation was established directly, but downstream products (D, E, F and I-series prostaglandins) were inactive. cDNA-expressed CYPs supported ATRA oxidation by lipid hydroperoxides. Whereas CYPs 2C8, 2C9 and 3A4, but not CYPs 1A2 or 2E1, were effective catalysts of the NADPH-mediated reaction, cooxidation supported by 15(S)-hydroperoxyeicosatetraenoic acid was mediated by all five CYPs. The cooxidation reaction in human hepatic microsomes was inhibited by the CYP inhibitor miconazole. These findings indicate that ATRA oxidation is quantitatively significant in human liver. Lipid hydroperoxides generated by intracellular enzymes such as prostaglandin synthase and lipoxygenases are sources of activated oxygen for CYP-mediated deactivation of ATRA to polar products.
在人肝微粒体（n = 17）中，评估了前列腺素H合酶（PGHS）介导的全反式维甲酸（ATRA）共氧化相对于细胞色素P450（CYP）依赖性ATRA 4-羟基化的定量重要性。观察到的ATRA共氧化速率（4.6 - 20 pmol mg蛋白⁻¹ min⁻¹）和4-羟基化速率（8.7 - 45 pmol mg蛋白⁻¹ min⁻¹）在数量上相似，并且表现出相似的个体差异（分别为4倍和5倍）。动力学研究表明，与人肝微粒体中CYP介导的NADPH和NADH依赖性4-羟基化（Vmax/Km⁻¹分别为0.14和0.02）相比，共氧化是一个高效的过程（Vmax/Km⁻¹ = 0.25）。直接证实了花生四烯酸的脂质氢过氧化物代谢产物介导ATRA氧化的能力，但下游产物（D、E、F和I系列前列腺素）无活性。cDNA表达的CYP支持脂质氢过氧化物介导的ATRA氧化。虽然CYP 2C8、2C9和3A4是NADPH介导反应的有效催化剂，而CYP 1A2或2E1不是，但由15（S）-氢过氧化二十碳四烯酸支持的共氧化由所有五种CYP介导。人肝微粒体中的共氧化反应受到CYP抑制剂咪康唑的抑制。这些发现表明，ATRA氧化在人肝脏中具有重要的定量意义。细胞内酶如前列腺素合酶和脂氧合酶产生的脂质氢过氧化物是CYP介导的ATRA失活形成极性产物的活性氧来源。
