Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
J Biol Chem. 2011 May 27;286(21):18426-33. doi: 10.1074/jbc.M111.240457. Epub 2011 Apr 6.
If cholesterol is a substrate of P450 3A4, then it follows that it should also be an inhibitor, particularly in light of the high concentrations found in liver. Heme perturbation spectra indicated a K(d) value of 8 μM for the P450 3A4-cholesterol complex. Cholesterol inhibited the P450 3A4-catalyzed oxidations of nifedipine and quinidine, two prototypic substrates, in liver microsomes and a reconstituted enzyme system with K(i) ∼ 10 μM in an apparently non-competitive manner. The concentration of cholesterol could be elevated 4-6-fold in cultured human hepatocytes by incubation with cholesterol; the level of P450 3A4 and cell viability were not altered under the conditions used. Nifedipine oxidation was inhibited when the cholesterol level was increased. We conclude that cholesterol is both a substrate and an inhibitor of P450 3A4, and a model is presented to explain the kinetic behavior. We propose that the endogenous cholesterol in hepatocytes should be considered in models of prediction of metabolism of drugs and steroids, even in the absence of changes in the concentrations of free cholesterol.
如果胆固醇是 P450 3A4 的底物，那么它也应该是一种抑制剂，特别是考虑到在肝脏中发现的高浓度。血红素扰动光谱表明 P450 3A4-胆固醇复合物的 K(d)值为 8 μM。胆固醇以非竞争性方式抑制 P450 3A4 催化的硝苯地平和奎尼丁两种典型底物在肝微粒体和重组酶系统中的氧化，K(i)∼10 μM。在培养的人肝细胞中，胆固醇孵育可将胆固醇浓度升高 4-6 倍；在使用的条件下，P450 3A4 的水平和细胞活力没有改变。当胆固醇水平升高时，硝苯地平的氧化受到抑制。我们得出结论，胆固醇既是 P450 3A4 的底物又是抑制剂，并提出了一个模型来解释其动力学行为。我们建议，即使在游离胆固醇浓度没有变化的情况下，也应考虑肝细胞内源性胆固醇在药物和甾体代谢预测模型中的作用。
