Involvement of CYP2E in 8-hydroxylation of theophylline in mouse hepatic microsomes-difference from its N-demethylations

Anti-tumor promoting effects of sesquiterpenes from Maytenus cuzcoina (Celastraceae)

Ten sesquiterpenoids (1-10), with a dihydro-beta-agarofuran skeleton, were isolated from Maytenus cuzcoina (Celastraceae). Their structures were elucidated on the basis of spectral analysis, including homo- and heteronuclear correlations NMR experiments (COSY, ROESY, HMQC and HMBC), and chemical correlations. The compounds have been tested for their inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), as a test for potential cancer chemopreventive agents. Compounds 1-3, 6 and 7 showed strong inhibitory effects on EBV-EA induction (100% inhibition at 1000 mol ratio/TPA). Their structure-activity relationship is discussed.

Preferential inhibition of CYP1A enzymes in hepatic microsomes by mexiletine

We examined the inhibitory behavior of theophylline oxidations and a variety of cytochrome P450 (P450)-dependent metabolism in the presence of mexiletine (MEX), using hepatic microsomes from both control mice and mice exposed to beta-naphthoflavone (beta-NF). Theophylline metabolism, which is mainly catalyzed by CYP1A2, was susceptible to competitive inhibition by MEX. The calculated inhibition constants (Ki) for theophylline 3-demethylation and its 8-hydroxylation were 4.3 microM and 8.3 microM, respectively, which are comparable to the recommended therapeutic serum range for MEX. The inhibitory potency of MEX on cytochrome P450-dependent enzyme activities diverged among the several metabolic reactions, which were probes for CYP1A, 2A, 2C, 2D, 2E and 3A subfamilies. The Ki value (6.7 microM) for methoxyresorufin O-demethylation mediated by CYP1A2 agreed with those from theophylline oxidations. These metabolic reactions exhibited the smallest Ki values, 1-3 orders of magnitude lower than activities of other constitutive cytochrome P450 species. Similar degrees of inhibition were observed in CYP1A1, a beta-NF-inducible isoform with a relatively high conformity to CYP1A2. These results indicate that MEX acts as a selective and potent inhibitor of the CYP1A enzymes responsible for oxidative biotransformation of chemicals such as theophylline. This evidence provides a fundamental explanation for the pharmacokinetic interactions experienced in clinical practice.

Preculturing of Chinese hamster V79 cells with sublethal concentration of theophylline sensitizes cells to cytotoxic effects of MNNG

In our previous work concerning the biologic effects of theophylline, we found that cells incubated during 48 h at low concentrations of theophylline (0.3 mg/ml of medium) manifested short-term deviations in the rate of DNA replication; however, this short-term inhibition of DNA replication did not reduce either the growth rate or the colony-forming ability of cells. In the present study, we concentrated on cytotoxic and DNA-damaging effects of MNNG on V79 cells precultured with sublethal concentration of methylxanthine theophylline. Cytotoxicity was evaluated on the basis of growth rate of treated cells as well as by colony-forming ability (plating efficiency) test and by trypan blue exclusion test. The level of DNA lesions (strand breaks) induced by MNNG was measured by alkaline DNA unwinding and by the comet assay. In an effort to explain higher cytotoxic effects of MNNG on precultured cells, we studied rejoining of damaged parental DNA after 4 h incubation post-MNNG-treatment as well. We found differences as against the controls in theophylline-precultured cells after treatment with the mutagen and carcinogen MNNG. The higher cytotoxic effect of MNNG in precultured cells was accompanied by a higher level of ss breaks of DNA and by more unrepaired lesions which remained after 4 h in parental DNA. Our results demonstrate that theophylline belongs to the group of agents inhibiting repair of potentially lethal DNA lesions.