Leo M A, Lasker J M, Raucy J L, Kim C I, Black M, Lieber C S
Alcohol Research and Treatment Center, Bronx Veterans Medical Center, New York.
Arch Biochem Biophys. 1989 Feb 15;269(1):305-12. doi: 10.1016/0003-9861(89)90112-4.
Liver microsomes obtained from nine subjects were found to metabolize retinol to polar metabolites, including 4-hydroxyretinol. In a reconstituted monooxygenase system containing human liver P450IIC8, retinol was converted to 4-hydroxyretinol and other polar metabolites, with a Km of 0.071 mM and a Vmax of 1.73 nmol/min/nmol P450. Neither P450IIC9 nor P450IIE1, two other purified human P450s, displayed significant retinol hydroxylase activity. Immunoblots performed with a monospecific antibody directed against human P450IIC8 revealed that appreciable amounts of this enzyme were present in human liver microsomes. The same antibody significantly inhibited retinol metabolism in liver microsomes and in the system reconstituted with P450IIC8. The system reconstituted with P450IIC8 also converted retinoic acid to polar metabolites. Thus, this study shows, for the first time, metabolism of two physiologic substrates by a human liver cytochrome P450 related to a group of "constitutive" rodent P450s believed to participate in the metabolism of endogenous compounds. Through its involvement in vitamin A metabolism, P450IIC8 may participate in maintaining the balance between those vitamin A concentrations that promote cellular integrity (and oppose the development of cancer) and those concentrations that cause cellular toxicity.
研究发现，从九名受试者获取的肝微粒体可将视黄醇代谢为极性代谢产物，包括4-羟基视黄醇。在含有人类肝脏P450IIC8的重组单加氧酶系统中，视黄醇被转化为4-羟基视黄醇和其他极性代谢产物，其米氏常数（Km）为0.071 mM，最大反应速度（Vmax）为1.73 nmol/分钟/每nmol P450。另外两种纯化的人类细胞色素P450，即P450IIC9和P450IIE1，均未表现出明显的视黄醇羟化酶活性。用针对人类P450IIC8的单特异性抗体进行的免疫印迹分析显示，人类肝脏微粒体中存在相当数量的这种酶。相同的抗体可显著抑制肝脏微粒体以及用P450IIC8重组的系统中的视黄醇代谢。用P450IIC8重组的系统还可将视黄酸转化为极性代谢产物。因此，本研究首次表明，一种与一组被认为参与内源性化合物代谢的“组成型”啮齿动物细胞色素P450相关的人类肝脏细胞色素P450可代谢两种生理底物。通过参与维生素A代谢，P450IIC8可能参与维持促进细胞完整性（并对抗癌症发展）的维生素A浓度与导致细胞毒性的浓度之间的平衡。
