Dinemorphan N-demethylation by mouse liver microsomes

Dimemorfan N-demethylation by mouse liver microsomal cytochrome P450 enzymes

Dimemorfan (d-3-methyl-N-methylmorphinan), an analogue of dextromethorphan, is commonly used as a non-opioid antitussive. To clarify the contribution of cytochrome P450 (P450) in dimemorfan N-demethylation, effects of selective inducers and inhibitors were studied in ICR mice. Phenobarbital (PB)- and dexamethasone (Dex)-treatments caused 5-fold increases of liver microsomal dimemorfan N-demethylation activity. In untreated mouse liver microsomes, demethylation activity was strongly inhibited by a CYP3A inhibitor, ketoconazole. In PB-and Dex-treated mouse liver microsomes, ketoconazole caused strong inhibition, whereas orphenadrine caused a decrease of less than 20%. Pretreatment of control mouse liver microsomes with anti-CYP3A inhibited demethylation activity, whereas pre-treatment with anti-CYP2B had no effect. In PB-and Dex-treated mouse liver microsomes, the demethylation activity was inhibited by both anti-CYP3A and anti-CYP2B. In control mice, the intrinsic clearance of dimemorfan from N-demethylation was 5.8 microl min(-1)mg protein(-1). In PB- and Dex-treated mice, the correlation coefficient of fitting using one-enzyme and two-enzyme models were similar. The intrinsic clearances of induced mouse liver microsomes were similar. These results revealed that CYP3A played a major role in hepatic demethylation in untreated mice. Both CYP3A and CYP2B were involved in this demethylation in PB- and Dex-treated mice.

Stability of microsomal monooxygenases in murine liver S9 fractions derived from phenobarbital and beta-naphthoflavone induced animals under various long-term conditions of storage

The aim of this study was to define the long-term stability of metabolizing enzymes in activating preparations for short-term genotoxicity bioassays under various storage conditions. Expressions of cytochrome P450 content, NADPH-cytochrome (P450) c-reductase activity, and of the several monooxygenases, such as aminopyrine N-demethylase (class IIIA P450), p-nitroanisole O-demethylase (mixed), dinemorphan N-demethylase (IIB1), ethoxyresorufin O-deethylase (IA1), ethoxycoumarin O-deethylase (mixed), and pentoxyresorufin O-dealkylase (IIB1), were examined in S9 fractions derived from Na-phenobarbital (PB) plus beta-naphthoflavone (beta-NF) induced male and female mice, stored at -80 degrees C, or lyophilized and stored at -20 degrees C. Lipid peroxidation was also determined. Cytochrome P450 and the associated activities were decreased by 30-82% within 9 months of storage. The pattern and degree of relative stabilities were different for the various isoforms. The IA1-like activity, for example, was much more stable (approximately 49% loss) than IIB1-like activities (up to 82% loss). In general, lyophilized enzymes were less stable than directly frozen preparations. In addition, immediately after freeze-drying (lyophilization), a marked decrease in activity of up to 35% was observed. On the contrary, demethylation of aminopyrine and p-nitroanisole remains almost constant over 6 months storage at -196 degrees C. The results obtained indicate that either fresh, daily made S9 fractions or, alternatively, fractions stored in liquid nitrogen (up to 6 months) are recommended for mutagenesis studies.

NADPH as rate-limiting factor for microsomal metabolism. An alternative and economic NADPH-generating system for microsomal mono-oxygenase in in vitro genotoxicity studies

The effect of NADPH supply on enzymatic activity and its stability were investigated with respect to the mono-oxygenase activities of 7-ethoxyresorufin O-deethylase (ERD), dinemorphan N-demethylase (DND), aminopyrine N-demethylase (APD), 7-ethoxycoumarin O-deethylase (ECD) and p-nitroanisole O-demethylase (p-NAD) under incubation conditions for the liver microsomal assay (LMA). Experiments with S9 liver fractions of mouse (induced with Na-phenobarbital and beta-naphthoflavone) and rat (induced with Aroclor 1254) were set out at different pre-incubation times with and without exogenous isocitrate dehydrogenase (IC-DH) in the LMA. Such LMA mixtures contain Mn2+, NADP+, DL-isocitrate (IC) and endogenous IC-DH as NADPH-generating machinery. No changes in mono-oxygenase stability and lipid peroxidation (LP) were observed in the presence of exogenous IC-DH. The metabolizing capability at the considered times was the maximal one, as shown by no stability changes after the direct addition of IC-DH to the enzymatic assays. Exogenous IC-DH in the incubation for LMA did not alter the mitotic crossing-over and the mitotic gene conversion of dimethylnitrosamine (DMNA) and AR2MNFN (a nitroimidazo[2,1-b]thiazole) in the tester D7 strain of Saccharomyces cerevisiae. It was concluded that endogenous IC-DH seems to be sufficient to provide a saturating level of NADPH for mono-oxygenase activities during incubations for LMA without additional external NADPH-generating enzyme activity.

Isoprene metabolism by liver microsomal mono-oxygenases

Mouse-liver microsomal mono-oxygenases metabolize isoprene to the corresponding mono-epoxides. The reaction was NADPH- and O2-dependent and was inhibited by CO, SKF525-A and metyrapone. 3,4-Epoxy-3-methyl-1-butene was the major metabolite of isoprene, and the kinetic constants (Km and Vmax) for this epoxidation were determined by analysing the corresponding diol by g.l.c. in incubations with microsomes from control or pretreated mice. 3,4-Epoxy-2-methyl-1-butene was a minor metabolite (approx. 20%). 3,4-Epoxy-2-methyl-1-butene was epoxidated further to the mutagenic isoprene dioxide by microsomes from control or pretreated mice. The Km and Vmax were determined and phenobarbital shown to be an inducer of this epoxidation.