One-electron reduction of mitomycin c by rat liver: Role of cytochrome P-450 and NADPH-cytochrome P-450 reductase

1. The role of cytochrome P-450 in the one-electron reduction of mitomycin c was studied in rat hepatic microsomal systems and in reconstituted systems of purified cytochrome P-450. Formation of H₂O₂ from redox cycling of the reduced mitomycin c in the presence of O₂ and the alkylation of ρ-nitrobenzylpyridine (NBP) in the absence of O₂ were taken as parameters.

2. With liver microsomes from both 3-methylcholanthrene (MC)- and phenobarbital (PB)-pretreated rats, reverse type I difference spectra were observed, indicative of a weak interaction between mitomycin c and the substrate binding site of cytochrome P-450. Mitomycin c inhibited the oxidative dealkylation of aminopyrine and ethoxyresorufin in both microsomal systems.

3. Under aerobic conditions the H₂O₂ production in the microsomal systems was dependent on NADPH, O₂ and mitomycin c, and was inhibited by the cytochrome P-450 inhibitors, metyrapone and SKF-525A.

4. Although purified NADPH-cytochrome P-450 reductase was also effective in reduction of mitomycin c and the concomitant reduction of O₂, complete microsomal systems and fully reconstituted systems of cytochrome P-450b or P-450c and the reductase were much more efficient.

5. Under anaerobic conditions in the microsomal systems both reduction of mitomycin c (measured as the rate of substrate disappearance) and the reductive alkylation of NBP were dependent on cytochrome P-450.

6. The relative rate of reduction of mitomycin c by purified NADPH-cytochrome P-450 reductase was lower than that by a complete microsomal system containing both cytochrome P-450 and a similar amount of NADPH-cytochrome P-450 reductase.

7. It is concluded that although NADPH-cytochrome P-450 reductase is active in the one-electron reduction of mitomycin c, the actual metabolic locus for the reduction of this compound in liver microsomes under a relatively low O₂ tension is more likely the haem site of cytochrome P-450.     

Biochemistry of misonidazole reduction by NADPH-cytochrome c (P-450) reductase

The biochemical mechanism for the reduction of misonidazole [1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol] by purified rabbit liver NADPH-cytochrome c (P-450) reductase, the primary nitroreductase of liver, has been studied. Neither the anaerobic nor the futile aerobic reduction velocities exhibited signs of Michaelis-Menten saturation at concentrations less than 5 and 10 mM, respectively. The anaerobic reduction of misonidazole resulted in the formation of glyoxal from fragmentation of the imidazole ring in 25% yield. The rate of glyoxal formation was linear with time and paralleled the reduction of misonidazole, suggesting that it was derived from the partitioning of a reactive intermediate between at least two alternative pathways. Negligible amounts of the 2-amino derivative of misonidazole were formed, however, indicating the existence of alternative reduction/fragmentation pathways.     

Purification and properties of cytochrome P-450 and NADPH-cytochrome c (P-450) reductase from human liver microsomes.

Cytochrome P-450 and NADPH-cytochrome c (P-450) reductase were purified to 10.6 nmoles per mg of protein and 19.9 units per mg of protein, respectively, from human liver microsomes. The purified cytochrome was assumed to be in a low spin state as judged by the absolute spectrum. n-Octylamine and aniline produced type II difference spectra and SKF 525-A and benzphetamine type I spectra when bound to the purified cytochrome P-450. The purified human cytochrome P-450 catalyzed laurate oxidation as determined by NADPH oxidation but not aniline hydroxylation, benzphetamine N-demethylation and 7-ethoxycoumarin O-deethylation when reconstituted with the reductases purified from human and rat liver microsomes. The human cytochrome P-450, however, catalyzed drug oxidations when cumene hydroperoxide was used as the oxygen source. The purified human NADPH-cytochrome c (P-450) reductase contained FAD and FMN at a ratio of 1:0.76. The reductase was capable of supporting 7-ethoxycoumarin O-deethylation activity of cytochrome P-448 purified from 3-methylcholanthrene-treated rat liver microsomes.     

Induction of cytochrome P-450, cytochrome b-5, NADPH-cytochrome c reductase and change of cytochrome P-450 isozymes with long-term trichloroethylene treatment

Several reports have described the effects of trichloroethylene (TCE) on the microsomal mixed function oxidase system (MFOS). These studies suggest that repeated TCE administration induces MFOS, especially cytochrome P-450 and NADPH-cytochrome c reductase. However, it is uncertain what isozymes are induced by TCE treatment, and it is not clear how microsomal enzymes or cytochrome P-450 isozymes are altered when TCE is administered for a duration longer than 28 days. We investigated the changes of MFOS by long-term TCE treatment. Male Wistar rats were injected with TCE, 1.0 g/kg body weight once a day for 5 continuous days or 2.0 g/kg body weight twice a week for 15 days. The mean body weight of the rats treated with TCE for 15 weeks was slightly, but not significantly, less than that of the control rats. Relative liver weights (liver wt/body wt) of the TCE-treated group were however significantly larger (21%) than those of the control group. The weights of the other organs were not changed by long-term TCE treatment. Trichloroethylene treatments for 5 days and 15 weeks caused significant increases in microsomal protein, cytochrome P-450, cytochrome b-5 and NADPH-cytochrome c reductase. TCE treatments produced an increase in a polypeptide band at 52,000 molecular weight range observed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). This increase in similar to, but less pronounced than that induced by phenobarbital (PB) treatment. There were no remarkable changes at 56,000 molecular weight range where a band appeared after the treatment with 3-methylcholanthrene (MC). It is likely that the induction of cytochrome P-450 by TCE is relatively similar to that by PB.     

Modification of cytochrome P-450, NADPH-cytochrome c reductase and aryl hydrocarbon hydroxylase activities by schistosomicidal drugs

This study was planned to investigate the modification of the mouse microsomal monooxygenase enzymes using various schistosomicidal drugs. Enzymes investigated were cytochrome P-450, NADPH-cytochrome c reductase and aryl hydrocarbon hydroxylase (AHH). Administration of oxamniquine and niridazole increased, whereas praziquantel and hycanthone lowered the cytochrome P-450 content. An apparent increase in the activity of NADPH-cytochrome c reductase was only observed with oxamniquine. The in vivo and in vitro effects of schistosomicidal drugs on the activity of AHH were investigated using benzo(a)pyrene (BP) as substrate. Oxamniquine and niridazole significantly increased the AHH activity in vivo and in vitro, while the antimonial drugs enhanced the enzyme activity only in vivo. On the other hand, praziquantel and hycanthone lowered the AHH activity only in vivo. Metrifonate did not show any effect either in vivo or in vitro. The mechanisms by which these drugs modify the AHH activity are discussed in the text.     

Role of lipid in the electron transfer between NADPH-cytochrome P-450 reductase and cytochrome P-450 from mammalian liver cells

1. The anaerobic NADPH-reduction of the isozymes cytochrome P-450 LM2 and LM4 was used as a functional tool to study the component interaction in reconstituted monooxygenase systems in dependence on different phospholipids. 2. The isozymes were shown to exhibit similar lipid interaction. The lipids generally favour a catalytically active 1:1 complex formation between reductase and cytochrome P-450 as the rate-determining unit in electron transfer. 3. The cytochrome P-450 reduction proceeds in a biphasic reaction. In dilauroyl phosphatidylcholine (DLPC)-reconstituted systems the amount of the fast reduction psi 1 is stoichiometrically limited by the reductase in deficit: psi 1 corresponds to the 1:1 complex formation capability of the reductase. 4. In vesicle-reconstituted systems an 'overstoichiometric' reductase cycling is observed which gives rise to a significantly increased amount of fast reduction psi 1. Reductase cycling is proposed to occur in protein clusters of cytochrome P-450 and reductase in deficit. 5. The dissociation constant KRP of the functionally active reductase-cytochrome P-450 complex has been determined by means of the amount of psi 1 (DPLC) and the rate constant kapp 1 (vesicles) of the fast reduction as a measure of the complex formation in dependence on the protein molar ratio. Taking into account the actual protein concentration in the vesicular lipid phase, KRP in vesicles has been calculated to be about 3 orders of magnitude increased in comparison to DLPC-reconstituted systems. 6. Vmax data reveal almost the same catalytic activity of both reconstitution modes, which justifies DLPC-reconstitution in model investigations. The vesicle-specific increased accumulation of reduced cytochrome P-450 in the steady state as originated by reductase cycling may offer the physiological advantage of an increased capacity of cytochrome P-450 for synergistic substrate conversion via cytochrome b5.     

Distribution of cytochromes P-450, cytochrome b5, and NADPH-cytochrome P-450 reductase in an entire human liver

In rat liver there appear to be significant differences between lobes in the concentration of individual cytochrome P-450 isozymes (Sumner and Lodola, Biochem Pharmacol 36: 391-393, 1987). Because studies in patients often rely on small pieces of liver obtained from diverse anatomical locations, it seemed important to determine if the cytochromes P-450 were also heterogeneously distributed in human liver. Accordingly, tissue was obtained from ten different locations in a single human liver including those most commonly biopsied by percutaneous needles, and by surgeons during laparotomy. The differences observed between locations in the microsomal concentrations of carbon monoxide-binding protein (total cytochrome P-450), cytochrome b5, and NADPH-cytochrome P-450 reductase appeared to be small and were not statistically significant. Likewise, no significant differences were observed between locations in the specific content of HLp, HLp3, HLj, HLx or P450MP. However, the specific concentrations of HLd varied almost 2-fold between the microsomes and this was statistically significant in some cases (P less than 0.05). Our results suggest that, in human livers, regional differences in the content of cytochromes P-450 are generally small but may be significant for some isozymes. With the exception of HLd, tissue obtained by percutaneous or surgical liver biopsies is probably representative of the entire organ with regard to the enzymes assayed.     

Role of lipid in the electron transfer between NADPH-cytochrome P-450 reductase and cytochrome P-450 from mammalian liver cells

1. The anaerobic NADPH-reduction of the isozymes cytochrome P-450 LM2 and LM4 was used as a functional tool to study the component interaction in reconstituted monooxygenase systems in dependence on different phospholipids.

2. The isozymes were shown to exhibit similar lipid interaction. The lipids generally favour a catalytically active 1 :1 complex formation between reductase and cytochrome P-450 as the rate-determining unit in electron transfer.

3. The cytochrome P-450 reduction proceeds in a biphasic reaction. In dilauroyl phosphatidylcholine (DLPC)-reconstituted systems the amount of the fast reduction ψ₁, is stoichiometrically limited by the reductase in deficit: ψ₁, corresponds to the 1:1 complex formation capability of the reductase.

4. In vesicle-reconstituted systems an ‘overstoichiometric’ reductase cycling is observed which gives rise to a significantly increased amount of fast reduction ψ₁, Reductase cycling is proposed to occur in protein clusters of cytochrome P-450 and reductase in deficit.

5. The dissociation constant K_RP of the functionally active reductase—cytochrome P-450 complex has been determined by means of the amount of ψ₁, (DPLC) and the rate constant K^app₁ (vesicles) of the fast reduction as a measure of the complex formation in dependence on the protein molar ratio. Taking into account the actual protein concentration in the vesicular lipid phase, K_RP in vesicles has been calculated to be about 3 orders of magnitude increased in comparison to DLPC-reconstituted systems.

6. V_max data reveal almost the same catalytic activity of both reconstitution modes, which justifies DLPC-reconstitution in model investigations. The vesicle-specific increased accumulation of reduced cytochrome P-450 in the steady state as originated by reductase cycling may offer the physiological advantage of an increased capacity of cytochrome P-450 for synergistic substrate conversion via cytochrome b₅.     

Mechanism of azoreduction of dimethylaminoazobenzene by rat liver NADPH-cytochrome P-450 reductase and partially purified cytochrome P-450. Oxygen and carbon monoxide sensitivity and stimulation by FAD and FMN

We have reported that the hepatocarcinogen dimethylaminoazobenzene (DAB) is reduced by rat liver microsomes in an oxygen- and carbon monoxide-insensitive manner and that activity is induced by clofibrate but no other recognized inducers of cytochrome P-450 activity. In the present study we have shown that the reaction proceeds in a partially purified reconstituted cytochrome P-450 system as well as with purified NADPH-cytochrome P-450 reductase alone. In the latter system, activity is totally inhibited in air whereas the former system is active in air as well as in a carbon monoxide atmosphere. Although clofibrate induces both DAB azoreductase and laurate hydroxylase activities, the suicide substrate 10-undecynoic acid blocks the latter but not the former, implying catalysis by distinct enzymes. FAD and FMN stimulate DAB azoreduction 40-50-fold by both NADPH-cytochrome P-450 reductase alone and by the reconstituted cytochrome P-450 system. However, it was shown that these flavins facilitate electron flow to DAB only from reductase and not from cytochrome P-450. The fact that the reconstituted system, which contains NADPH-cytochrome P-450 reductase, is oxygen insensitive suggests that there is an obligatory electron flow through cytochrome P-450 to DAB, bypassing the oxygen-sensitive step.     

Involvement of cytochrome P-450c in alpha-naphthoflavone metabolism by rat liver microsomes

Metabolism of alpha-naphthoflavone (ANF) is increased markedly in rat liver microsomes by 3-methylcholanthrene (3-MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), two inducers of cytochromes P-450c and P-450d (P-450c and P-450d). Although several indirect lines of evidence in the literature suggest that ANF is metabolized by P-450c, Vyas et al. [J. Biol. Chem. 258:5649-5659 (1983)] reported that ANF metabolism by 3-MC-induced rat liver microsomes was only partially inhibited by antibodies against P-450c. Our laboratory has previously reported clastogenic effects of metabolites of ANF, and in the present study we reexamined the role of P-450c in ANF metabolism by both uninduced and TCDD-induced rat liver microsomes, using monospecific polyclonal antibodies to P-450c and P-450d. ANF metabolism was inhibited to different extents in TCDD-induced microsomes by different preparations of anti-P-450c. One lot of anti-P-450c produced only 50% inhibition of ANF metabolism in TCDD-induced microsomes, whereas another lot of anti-P-450c inhibited ANF metabolism by 80%. Anti-P-450d had no effect on ANF metabolism. Neither anti-P-450c nor anti-P-450d inhibited ANF metabolism in uninduced rat liver microsomes. In a reconstituted enzyme system, purified P-450c metabolized ANF 47 and 510 times more rapidly than P-450d and P-450b, respectively. Metabolites resulting from oxidation at 7,8- or 5,6-positions (7,8-dihydro-7,8-dihydroxy-ANF, 5,6-dihydro-5,6-dihydroxy-ANF, 5,6-oxide-ANF, and 6-hydroxy-ANF) were formed by all preparations of microsomes. An unknown toxic ANF metabolite was formed only with a reconstituted P-450c system and with 3-MC- or TCDD-induced microsomes. Our results indicate that P-450c is responsible for the majority of the metabolism of ANF in TCDD-induced microsomes, whereas other constitutive isozymes are responsible for the metabolism seen in uninduced liver microsomes. The variable inhibition of ANF metabolism with different lots of anti-P-450c probably reflects the differences in the proportion of antibodies to different epitopes important in the binding or metabolism of this substrate.     

Immunohistochemical localization and quantitation of NADPH-cytochrome P-450 reductase in human liver

An antibody raised in a goat against the human liver NADPH-cytochrome P-450 reductase (EC 1.6.2.4.) enzyme has been used to: 1) immunoquantify the level of this enzyme in human liver microsomes, and 2) study the distribution of the reductase across the human liver acinus. Employing the Western blot procedure, anti-human reductase IgG recognized a single band in human liver microsomes which corresponded in molecular weight to the purified reductase. The content of the NADPH-cytochrome P-450 reductase in six normal human livers varied from 87 to 121 pmol/mg of microsomal protein. NADPH-cytochrome P-450 reductase activity of the same microsomes ranged from 107 to 222 nmol of cytochrome c reduced per min per mg of protein. The correlation between reductase content and activity (r = 0.54) was not statistically significant (p greater than 0.1). The total cytochrome P-450 content (cytochrome P-450 and P-420) of the same microsomes varied from 423 to 1413 pmol/mg of microsomal protein. The average ratio of cytochrome P-450 to NADPH-cytochrome P-450 reductase was 7.1:1 +/- 3.1 (mean +/- SD) in the human liver microsomal preparations studied. The reductase was found to be nonuniformly distributed across the human liver acinus. Although all hepatocytes stained positively for NADPH-cytochrome P-450 reductase, the staining intensity was highest in zone 3 and in some cases also in zone 1 hepatocytes. These results show that human liver contains a gross excess of cytochrome P-450 molecules to NADPH-cytochrome P-450 reductase molecules. Furthermore, the differential distribution of the reductase within the human liver acinus may lead to a better understanding of the mechanism underlining site-specific drug hepatotoxicity.     

Purification of cytochrome P-450 and NADPH cytochrome p-450 reductase from human liver

Two methods for the purification of cytochromes-P450 from microsomes of human liver are described. Method A: Cyt-P450 were solubilized from microsomes using a non ionic detergent, the Lubrol. The Cyt-P450 were purified by affinity, hydrophobicity followed by ion-exchange chromatography on DEAE-5PW column (HPLC) with an overall yield of 18% and a specific activity of 10 nmole/mg of protein. The recovery of NADPH Cyt-P450 reductase by method A (affinity) is about 60% with a specific activity of 16.2 U.I./mg of protein. Method B: Cyt-P450 were solubilized from microsomes using a zwitterionic detergent, the CHAPS. Cyt-P450 were filtered and separated by chromatofocusing on Mono-P column (HPLC). By this method it was possible to increase strongly the specific activity keeping a yield of 50% of Cyt-P450. Also it was possible to apply this method to small samples of human liver like biopsies (0.5 to 2.5 g).     

Effect of diabetes on rat liver cytochrome P-450: Evidence for a unique diabetes-dependent rat liver cytochrome P-450

Detergent-solubilized hepatic microsomal fractions from alloxan diabetic rats exhibited a 52,000 molecular weight hemeprotein band that was not present in the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) protein profiles of identically solubilized hepatic microsomal fractions from normal, 3-methylcholanthrene- or phenobarbital-treated rats. This 52,000 mol. wt hemeprotein band disappeared from the protein profile of insulin-treated diabetic rat liver to yield the SDS-PAGE profile of normal rat liver. When P-450 hemeproteins were purified by lauric acid affinity and hydroxylapatite chromatography from solubilized microsomes, only the diabetic rat had a 52,000 mol. wt P-450. This distinct 52,000 mol. wt diabetes-induced P-450 interacted with type II compounds to yield a 2-fold greater absorbance change than was observed with the purified P-450s from either the normal or the chemically induced rats. The properties of this unique 52,000 mol. wt P-450 suggest that it may be the catalytic component responsible for the increased rate of type II substrate (aniline) metabolism observed in the diabetic rat.     

Activation of misonidazole by rat liver microsomes and purified NADPH-cytochrome c reductase

Rat liver microsomes and purified NADPH-cytochrome c reductase metabolized [14C]misonidazole anaerobically to a reactive intermediate that covalently binds to tissue macromolecules. Air strongly inhibited the binding whereas carbon monoxide had no effect, indicating that misonidazole is activated via reduction and not by cytochrome P-450-dependent oxidation. Both systems showed an absolute requirement for NADPH and were stimulated by flavine (FAD) and paraquat. The apparent Km for misonidazole binding to microsomal protein was 0.74 mM the apparent Vmax was 0.64 nmole 14C bound . mg-1 . min-1. At a single substrate concentration, nitrofurantoin, nitrofurazone and desmethylmisonidazole inhibited the covalent binding of misonidazole to microsomal protein by 47, 26, and 38% respectively. The effect of nitrofurantoin on the kinetics of misonidazole binding gave a complex interaction indicative of uncompetitive inhibition. Glutathione reduced the binding of misonidazole to microsomal protein below the level observed for boiled microsomes while ascorbic acid had no effect. Compared to nitrofurantoin and paraquat, misonidazole was a poor stimulator of superoxide production as measured by adrenochrome formation.