Substrate and position specificity of hematin-activated monooxygenation reactions

Non-specific inhibition of human cytochrome P450-catalyzed reactions by hemin

Hemin, a stable form of heme, is known to have an antimutagenic effect. Inhibitory effects of hemin on the cytochrome P450 (CYP)-catalyzed reactions of human liver microsomes and reconstituted systems containing purified CYP and NADPH-cytochrome P450 reductase (NPR) were seen. Hemin non-specifically inhibited all of the microsomal CYP activities examined. Hemin also inhibited 7-ethoxyresorufin O-deethylation, 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin O-demethylation, and testosterone 6beta-hydroxylation catalyzed by purified CYPs 1A2, 2D6, and 3A4, with IC50 values of 27, 19, and 2.4 microM, respectively. Hemin also inhibited reduction of cytochrome c and ferricyanide by NPR, as much as 47%. Spectrally detectable CYP was destroyed in human liver microsomes and in a reconstituted system in the presence of hemin and an NADPH-generating system. We propose that the antimutagenic effect of hemin might be due to inhibition of CYP and NPR enzymes involved in the bioactivation of mutagens.

Cytosolic activation of hematin-dependent microsomal monooxygenase activity in the lung

Additions of micromolar concentrations of hematin to washed rat pulmonary microsomal preparations resulted in marked (5-7-fold) increases in the NADPH-dependent generation of phenolic metabolites of benzo[a]pyrene (BaP). 9-Hydroxy-BaP was identified as the major reaction product. Additions of pulmonary cytosolic fractions to microsomes produced no measurable effect but cytosol and hematin added together elicited 25-30-fold increases in total phenolic products. Cytosolic fractions from other tissues, including rat kidneys and perfused rat livers, were also highly effective in enhancing the hematin-mediated increases in monooxygenase activity. However, cytosol from human placental tissues was only minimally effective when either pulmonary or placental microsomes were utilized as enzyme source. Superoxide dismutase and catalase (alone or in combination) had no measurable effect on hematin-mediated increases. Horseradish peroxidase effectively inhibited the hematin-dependent reactions but hematin-independent reactions were inhibited with equal effectiveness. Carbon monoxide profoundly inhibited all hematin-mediated increases in metabolite formation. The activating cytosolic component was non-dialyzable, inactivated by trypsin and heat, and eluted in the void volume from Sephadex G-150 columns. This suggested that the cytosolic factor(s) responsible for the increased hematin-dependent oxidation was a protein(s) with a high molecular weight or perhaps an aggregate or oligomer of proteinaceous material. HPLC profiles indicated a major effect on the generation of phenolics; quinones were also increased but only minimal increases in diols were observed. Results were consistent with the hypothesis that hematin-mediated increases in pulmonary monooxygenase activity result from an increased association of a small pool of pulmonary P-450-apoprotein(s) with the hematin prosthetic group to result in increased levels of an unidentified holocytochrome(s) with a relatively high substrate turnover number. The current data suggest a quaternary interaction among P-450 apoprotein(s), heme prosthetic group, reaction products (particularly 3-hydroxy-BaP) and a cytosolic protein(s). We postulate that the mechanism of action of the cytosolic factor is to facilitate the interaction of hematin with the apocytochrome.

Variation in hepatic microsomal cytochrome P-450 1 concentration among untreated rabbits alters the efficiency of estradiol hydroxylation

The metabolism of 17 beta-estradiol was examined using both rabbit liver microsomes and highly purified forms of rabbit liver microsomal cytochrome P-450. The predominant microsomal metabolite of 17 beta-estradiol is the 2-hydroxylated product. 2-Hydroxyestradiol is also the principal metabolite in reconstitution experiments in which P-450 1 exhibits the greatest Vmax, ca. 6 mol min-1 mol P-450 1(-1), vs less than 0.6 mol min-1 mol P-450(-1) for forms 2, 3b-, 3b+, 3c, 4, and 6. In addition P-450 1 has the lowest Km, ca. 2 microM. This suggested that microsomes which differ in their content of P-450 1 would also differ in the kinetic parameters characterizing the 2-hydroxylation of 17 beta-estradiol. Microsomes containing low amounts of P-450 1, less than 0.1 nmol/mg protein, exhibit a low-efficiency (Vmax/Km) 2-hydroxylase activity. Microsomes containing elevated concentrations of P-450 1, greater than 0.3 nmol/mg protein, exhibit a substrate dependence suggestive of an additional high-efficiency enzyme. The latter is specifically inhibited by a monoclonal antibody that recognizes P-450 1. These results indicate that the elevated expression of P-450 1 in microsomes leads to a marked increase in the apparent first-order rate constant for the 2-hydroxylation of 17 beta-estradiol, as it does for the 21-hydroxylation of progesterone. This should have a marked effect on the metabolism of these two steroid hormones at concentrations that are likely to occur in vivo.