Wang R W, Newton D J, Scheri T D, Lu A Y
Department of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA.
Drug Metab Dispos. 1997 Apr;25(4):502-7.
Cytochrome P450 3A4 is known to catalyze the metabolism of both endogenous substrates (such as the 6 beta-hydroxylation of testosterone) and many important therapeutic agents, including the N-demethylation of erythromycin. However, erythromycin and testosterone have been reported to have little or no effect on the metabolism of each other by recombinant CYP3A4. In an effort to understand the basis of these observations, we studied the N-demethylation of erythromycin and the 6 beta-hydroxylation of testosterone in human liver microsomes and in microsomes from cells containing recombinant human CYP3A4 and P450 reductase under a variety of experimental conditions. In both human liver microsomal and recombinant CYP3A4 systems, erythromycin inhibited testosterone 6 beta-hydroxylation in a concentration dependent manner, and vice versa. However, the inhibition mechanism was complex. At low substrate concentrations, testosterone and erythromycin acted as competitive inhibitors to each other. Under these experimental conditions, an apparent competitive inhibition of testosterone 6 beta-hydroxylation by erythromycin was observed, with Ki values similar to that of the K(m) values for erythromycin. When the rates of testosterone 6 beta-hydroxylation and erythromycin N-demethylation were determined in microsomal incubations containing both substrates at lower concentrations, the observed rates for each reaction were in good agreement with the calculated rates based on the rate equation describing simultaneous metabolism of two substrates by a single enzyme. However, at high substrate concentrations, the kinetic results could be best explained by a mechanism involving partial competitive inhibition. We conclude from these studies that testosterone and erythromycin mutually inhibit the metabolism of each other, consistent with the fact that CYP 3A4 catalyzes the metabolism of both substrates.
细胞色素P450 3A4已知可催化内源性底物（如睾酮的6β-羟基化）以及许多重要治疗药物的代谢，包括红霉素的N-去甲基化。然而，据报道，红霉素和睾酮对重组CYP3A4介导的彼此代谢几乎没有影响。为了理解这些观察结果的基础，我们在多种实验条件下，研究了人肝微粒体以及含有重组人CYP3A4和P450还原酶的细胞微粒体中红霉素的N-去甲基化和睾酮的6β-羟基化。在人肝微粒体和重组CYP3A4系统中，红霉素均以浓度依赖性方式抑制睾酮的6β-羟基化，反之亦然。然而，抑制机制很复杂。在低底物浓度下，睾酮和红霉素彼此作为竞争性抑制剂起作用。在这些实验条件下，观察到红霉素对睾酮6β-羟基化有明显的竞争性抑制作用，其Ki值与红霉素的K(m)值相似。当在含有较低浓度两种底物的微粒体孵育中测定睾酮6β-羟基化和红霉素N-去甲基化的速率时，每个反应观察到的速率与基于描述单一酶同时代谢两种底物的速率方程计算出的速率高度一致。然而，在高底物浓度下，动力学结果最好用涉及部分竞争性抑制的机制来解释。我们从这些研究中得出结论，睾酮和红霉素相互抑制彼此的代谢，这与CYP 3A4催化两种底物代谢的事实一致。
