Reductase and oxidase activity of rat liver cytochrome P450 with 2,3,5,6-tetramethylbenzoquinone as substrate

Duroquinone reduction during passage through the pulmonary circulation

The lungs can substantially influence the redox status of redox-active plasma constituents. Our objective was to examine aspects of the kinetics and mechanisms that determine pulmonary disposition of redox-active compounds during passage through the pulmonary circulation. Experiments were carried out on rat and mouse lungs with 2,3,5,6-tetramethyl-1,4-benzoquinone [duroquinone (DQ)] as a model amphipathic quinone reductase substrate. We measured DQ and durohydroquinone (DQH2) concentrations in the lung venous effluent after injecting, or while infusing, DQ or DQH2 into the pulmonary arterial inflow. The maximum net rates of DQ reduction to DQH2 in the rat and mouse lungs were approximately 4.9 and 2.5 micromol. min(-1).g dry lung wt(-1), respectively. The net rate was apparently the result of freely permeating access of DQ and DQH2 to tissue sites of redox reactions, dominated by dicumarol-sensitive DQ reduction to DQH2 and cyanide-sensitive DQH2 reoxidation back to DQ. The dicumarol sensitivity along with immunodetectable expression of NAD(P)H-quinone oxidoreductase 1 (NQO1) in the rat lung tissue suggest cytoplasmic NQO1 as the dominant site of DQ reduction. The effect of cyanide on DQH2 oxidation suggests that the dominant site of oxidation is complex III of the mitochondrial electron transport chain. If one envisions DQ as a model compound for examining the disposition of amphipathic NQO1 substrates in the lungs, the results are consistent with a role for lung NQO1 in determining the redox status of such compounds in the circulation. For DQ, the effect is conversion of a redox-cycling, oxygen-activating quinone into a stable hydroquinone.

Spectrophotometric assay of yeast vitality using 2,3,5,6-tetramethyl-1,4-benzoquinone and tetrazolium salts

A method for the spectrophotometric assay of yeast vitality was developed using 2,3,5,6-tetramethyl-1,4-benzoquinone and tetrazolium salts. The metabolic efficiency of 2,3,5,6-tetramethyl-1,4-benzoquinone by yeast cells was used as an index of yeast vitality. 2,3,5,6-Tetramethyl-1,4-benzoquinone was reduced to 2,3,5,6-tetramethyl-1,4-hydroquinone by yeast cells. Then, the superoxide anion radicals generated from O2 by reduction with 2,3,5,6-tetramethyl-1,4-hydroquinone under alkaline conditions reduced tetrazolium salts to formazan, which exhibited absorbance maxima at 440 nm. A linear relationship between the absorbance and viable cell density was obtained in the range of 1.0 x 10(5)-2.0 x 10(7) cells/ml for a sample solution. During the cultivation of yeast cells, the absorbance showed almost an anti-parallel change with that of glucose in yeast growth and fermentation, suggesting that the absorbance change reflected the vitality of yeast cells.

Enzyme-induction dependent bioactivation of troglitazone and troglitazone quinone in vivo

Troglitazone (TGZ), a 2,4-thiazolidinedione antidiabetic, causes hepatotoxicity in 1.9% of patients. TGZ is an inducer of, and substrate for, hepatic P450 3A. Microsomal metabolism yields a benzoquinone (TGZQ) and reactive intermediates. Kassahun et al. [Kassahun et al. (2001) Chem. Res. Toxicol. 14, 62-70] have trapped the intermediates as thioester, thioether, and disulfide conjugates of glutathione and found five conjugates in rat bile. The thioether was substituted in the chromane moiety. We have investigated the effect of the P450 3A inducer, dexamethasone (DEX), on metabolism of TGZ and TGZQ in rats and assessed the compounds' cytotoxicity. TGZ-glucuronide and sulfonate were confirmed as principal biliary metabolites of TGZ (50 mg/kg, iv). Bile from noninduced animals also contained a TGZ-glutathione thioether adduct (ML3) but it was substituted in the thiazolidinedione moiety. Pretreatment with DEX (50 mg/kg/day for 3 days) resulted in a 2-5-fold increase in the biliary concentration of ML3 and a 2-fold increase in the concentration of TGZQ, which was commensurate with the induction of hepatic P450 3A. Three of the known glutathione-conjugated metabolites were also found. TGZQ (50 mg/kg, iv) was metabolized to an analogue of one of the TGZ-glutathione thioesters and a glutathione adduct of TGZQ hydroquinone after DEX pretreatment. TGZ quinol glucuronide was a biliary metabolite of TGZ and TGZQ. Its formation would represent deactivation of TGZQ. TGZ was toxic to rat hepatocytes and Hep-G2 cells at concentrations exceeding 50 and 25 microM, respectively, after 24 h. In contrast, TGZQ was nontoxic to rat hepatocytes and toxic to Hep G2 cells only at concentrations exceeding 100 microM. Our results show that TGZQ as well as TGZ yields reactive metabolites in vivo, and that bioactivation is enhanced by induction of P450 3A. However, hepatotoxicity is unlikely to be due to either TGZQ or its metabolites.

Lipid peroxidation induced by adriamycin in linolenic acid-loaded cultured hepatocytes

Addition of more than 10 microM of adriamycin to cultured rat hepatocytes loaded with alpha-linolenic acid (linolenic acid-loaded hepatocytes) caused marked lipid peroxidation as measured by an accumulation of malondialdehyde during a 9 hr incubation. After addition of 50 microM of adriamycin to linolenic acid-loaded hepatocytes, malondialdehyde accumulation significantly increased at 3 hr, followed by cellular reduced glutathione decrease and lactate dehydrogenase leakage after 6 hr. Inhibition of adriamycin-induced lipid peroxidation by addition of N,N'-diphenyl-p-phenylenediamine or alpha-tocopherol, both lipid radical scavengers, or deferoxamine, which is a Fe ion chelator, prevented both glutathione decrease and lactate dehydrogenase leakage, indicating that lipid peroxidation caused cellular damage to linolenic acid-loaded hepatocytes exposed to adriamycin. The effect of SKF 525-A, which is a cytochrome P450 inhibitor, on adriamycin-induced lipid peroxidation and on 7-ethoxycoumarin O-deethylase activity was determined by 6 hr incubation of linolenic acid-loaded cells. Addition of SKF 525-A suppressed adriamycin-induced lipid peroxidation comparably with its 7-ethoxy-coumarin 0-deethylase inhibitory activity. These results suggest that cytochrome P450 contributes to the one-electron bioreduction of adriamycin into its semiquinone radical in rat hepatocytes.

The cytotoxicity of mitomycin C and adriamycin in genetically engineered V79 cell lines and freshly isolated rat hepatocytes

The objective of the present study was to investigate the cytotoxicity of Adriamycin (ADR) and mitomycin C (MMC) in tumor and non-tumor cells with respect to the role of cytochrome P450 (P450). Therefore, genetically engineered V79 Chinese hamster fibroblasts expressing only single enzymes of P450 were used. SD1 and XEM2 cells expressed rat P450IIB1 and P450IA1, respectively, whereas the V79 parental cells contained no detectable P450 levels. The cytotoxicity of ADR and MMC in the V79 cell system was compared with that in freshly isolated hepatocytes from phenobarbital (PB-hepatocytes)- and beta-naphthoflavone (beta NF-hepatocytes)-induced rats. Following 24 h of exposure to ADR equal cytotoxicity was observed in V79, SD1 and XEM2 cells. Addition of metyrapone (MP, an inhibitor of P450IIB1) and alpha-naphthoflavone (alpha NF, an inhibitor of P450IA1) had no effect on the ADR-induced cytotoxicity in SD1 and XEM2 cells, respectively. Likewise, MMC was equitoxic in V79 and SD1 cells. Co-incubation of SD1 cells with MP did not alter MMC-induced cytotoxicity. MMC, however, showed a decreased cytotoxicity in XEM2 cells when compared to the parental V79 cells. Unexpectedly, the cytotoxicity of MMC in XEM2 cells was increased by alpha NF to the same level as observed in the parental V79 cells. In contrast to V79- and V79-derived cells, in freshly isolated hepatocytes from PB or beta NF-induced rats, MMC was cytotoxic (measured as lactate dehydrogenase leakage) within 3 h of incubation. ADR, however, was only cytotoxic to the hepatocytes when intracellular glutathione was first depleted by diethylmaleate. The MMC- and ADR-induced cytotoxicity was found to be more pronounced in PB-hepatocytes than in beta NF-hepatocytes. Contrary to the findings in the V79-derived cells, MP afforded complete protection against both MMC- and ADR-induced cytotoxicity in PB-hepatocytes, whereas alpha NF only partially inhibited the cytotoxicity of MMC in beta NF-hepatocytes. In conclusion, we have demonstrated that PB-inducible P450s play a role in the cytotoxicity of both MMC and ADR in freshly isolated PB-hepatocytes but that P450IIB1 does not in genetically reconstituted SD1 cells. P450IA1, however, decreased the cytotoxicity of MMC in the XEM2 cells. The ADR-induced cytotoxicity, which was observed in XEM2 cells, was not mediated by P450IA1. The present study underscores the complexity in the comparison of ADR- and MMC-induced cytotoxicities in normal and tumor cells.

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.