Selective inactivation of rat lung and liver microsomal NADPH-cytochrome c reductase by acrolein

Addition of acrolein to rat lung or liver microsomal suspensions resulted in total inactivation of NADPH-cytochrome c reductase and partial conversion of cytochrome P-450 to P-420 in a concentration- and time-dependent fashion. Acrolein also caused total loss of nonprotein sulfhydryl content in both preparations, whereas protein sulfhydryl content was decreased by 40% and 28% in lung and liver preparations, respectively. Maxima of about 60% of the total lung cytochrome P-450 and 50% of the liver cytochrome P-450 in acrolein-treated microsomes did not support the N-demethylation of benzphetamine or ethylmorphine or hydroxylation of aniline because of the total loss of NADPH-cytochrome c reductase. Addition of purified NADPH-cytochrome c reductase to the acrolein-treated lung or liver microsomal suspension largely restored these monooxygenase activities. Addition of glutathione or dithiothreitol to the lung or liver microsomal suspension prior to the addition of acrolein significantly protected cytochrome P-450 from conversion to cytochrome P-420 as well as NADPH-cytochrome c reductase from inactivation. Thus, selective conjugation of acrolein with lung and liver NADPH-cytochrome c reductase but not cytochrome P-450 was responsible for total loss of these lung and liver monooxygenase activities.     

Effect of malotilate (diisopropyl 1,3-dithiol-2-ylidenemalonate) on drug metabolizing activity in rat liver microsomes

The effect of malotilate (diisopropyl 1,3-dithiol-2-ylidenemalonate) on drug metabolizing activity in rat liver microsomes was examined. Malotilate (500 mg/kg/day) was administered orally to rats for 3 days. The contents of cytochrome P-450 (P-450) and cytochrome b5 (b5), the activity of NADPH-cytochrome c reductase, and the metabolization of aniline, aminopyrine, benzo(a)pyrene (B(a)P) and 7-ethoxycoumarin (7-EC) in the microsomal fraction were examined 24 hr after the final administration of malotilate. The content of b5 and the activity of NADPH-cytochrome c reductase were increased by the malotilate treatment, but the content of P-450 was not significantly affected. 7-EC O-deethylation was markedly and aminopyrine N-demethylation was moderately enhanced; in contrast, aniline hydroxylation was significantly and B(a)P hydroxylation was slightly reduced. Such different effects of malotilate among the four substrate-metabolizing activities may be due mainly to the increase in the content of b5, which participates in the transport of the second electron required for P-450 function to various extents. It is also possible that malotilate affects the population of P-450 subtypes, each having a different substrate specificity and a different affinity for b5.     

Inhibition by cyanide of drug oxidations in rat liver microsomes.

Cyanide inhibited microsomal activities of aniline hydroxylation and aminopyrine, ethylmorphine and codeine demethylations and produced a modified type II difference spectrum of cytochrome P-450 to give two spectral dissociation constants, 0.21mM and 1.05 mM. The binding of cyanide to cytochrome P-450 resulted in innhibition of NADPH-cytochrome P-450 reductase activity. The cyanide inhibition of drug oxidations was partially avoided by increasing oxygen tension. A possible mechanism for the inhibition of drug oxidations by cyanide is discussed.     

The effect of galactosamine on rat liver cytochrome P-450 activities

The effect of galactosamine on hepatic drug metabolizing activities was examined in rats. In the microsomal fraction, the contents of cytochrome P-450 (P-450) and cytochrome b5 (b5) and the activity of NADPH-cytochrome c reductase (reductase) were examined for 7 days after galactosamine administration. In addition, substrate metabolizing activities in damaged microsomes were examined using four substrates: aminopyrine, aniline, benzo(a)pyrene (B(a)P) and 7-ethoxycoumarine (7-EC). The contents of P-450 and b5 and the activity of reductase showed a minimal value after 3 days of galactosamine administration and then gradually increased, reaching to the control level after 7 days. All four substrate metabolizing activities showed a similar response as the content of P-450, but the decrement among the four activities was not uniform. The activities of B(a)P hydroxylation and 7-EC deethylation were more impared than those of aminopyrine demethylation and aniline hydroxylation. This nonuniformity was clear on the activity based on P-450. This result suggested that galactosamine disturbed the population of multiple P-450 subtypes, and each P-450 subtype was damaged to the various extent by galactosamine administration.     

A novel form of cytochrome P-450 in beagle dogs. P-450-D3 is a low spin form of cytochrome P-450 but with catalytic and structural properties similar to P-450d

A form of cytochrome P-450, P-450-D3, cross reactive with antibodies to rat P-450d was purified from liver microsomes of polychlorinated biphenyl (PCB)-treated female Beagle dogs to an electrophoretic homogeneity. Judging from the result of sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the molecular weight of P-450-D3 was estimated to be 54,000. The oxidized form of P-450-D3 showed a peak at 416 nm indicating that the cytochrome is mostly in a low spin state. The carbon monoxide bound reduced form of P-450-D3 showed a peak at 448 nm. In a reconstituted system, P-450-D3 catalyzed drug oxidations including benzphetamine and aminopyrine N-demethylations, 7-ethoxycoumarin and p-propoxyaniline O-dealkylations, and aniline and benzo(a)pyrene hydroxylations. The rate of aniline hydroxylation catalyzed by P-450-D3 was similar to that catalyzed by P-450c which is a low spin form of cytochrome P-450 purified from liver microsomes of PCB-treated rats, whereas the catalytic activities of P-450-D3 for 7-ethoxycoumarin O-deethylation and benzo(a)pyrene hydroxylation were considerably lower than those of P-450c. The amino terminal portion of P-450-D3 was found to be highly similar to those of P-450d, human P3-450 and P3-450 when four amino acid deletions were tentatively inserted between fifth and sixth amino acids from the N-terminal, but not that of P-450c which is a low spin form of cytochrome P-448 purified from rat liver microsomes. These results indicate that Beagle dogs possess a low spin form of cytochrome P-450 with spectral properties similar to P-450c but with catalytic and structural properties similar to P-450d.     

Effect of successive administration of zopiclone, a new class of minor tranquillizer, on mouse liver microsomal mono-oxygenase system

Oral administration of zopiclone to mice (10 mg/kg daily) for 14 days had no significant effect on body weight, liver weight, hepatic microsomal protein or cytochrome P-450 contents but produced significant increases in the activities of NADPH-cytochrome c reductase, NADH-ferricyanide reductase, aniline hydroxylation and aminopyrine N-demethylation.     

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 p-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.     

Lack of in vitro and in vitro effects of fenbendazole on phase I and phase II biotransformation enzymes in rats, mice and chickens.

Intraperitoneal administration of 10 mg fenbendazole/kg bw daily for 5 d caused no significant alterations in the activities of hepatic microsomal drug-metabolizing enzymes viz aminopyrine N-demethylase, aniline hydroxylase and cytosolic glutathione S-transferase in rats, mice and chickens. Similarly no significant difference in the amount of microsomal cytochrome P-450 and NADPH-cytochrome c reductase was found between control and treated animals. In vitro incubation of fenbendazole with rat, mouse and chicken microsomes suggests that the drug neither binds to microsomal protein cytochrome P-450 nor inhibits the activities of aminopyrine N-demethylase and aniline hydroxylase. Similarly in vitro addition of fenbendazole to cytosolic glutathione S-transferase from the above species did not alter the activity of this enzyme. The results indicate that fenbendazole does not alter the activity of hepatic microsomal monooxygenase system significantly in rats, mice and chickens at a dosage level of 10 mg/kg body weight. In vitro studies also indicate that fenbendazole does not interact with the hepatic microsomal monooxygenase system, indicating it is not a substrate for cytochrome P-450-dependent monooxygenase system.     

Inhibition of hepatic mixed function oxidase activity by propyl gallate

Propyl gallate was found to inhibit microsomal benzpyrene hydroxylase activity and demethylase activity with ethylmorphine, aminopyrine or benzphetamine as a substrate. The extent of inhibition with different substrates varied with the age and diet of the animals. The benzpyrene hydroxylase activity of the microsomes of the 3-methylcholanthrene-treated rats was shown to be less susceptible to propyl gallate inhibition. Propyl gallate does not inhibit the NADPH-dependent reduction of cytochrome P-450; therefore, the site of inhibition is not on NADPH-cytochrome c reductase as suggested previously. Propyl gallate interacts with cytochrome P-450 to produce a positive absorption peak around 420 nm, and it may also interfere with the binding of a type I substrate, benzphetamine. It inhibits ethylmorphine demethylation by a noncompetitive mechanism and aminopyrine demethylation by a mixed mechanism. The mode of propyl gallate inhibition and the implications of these observations are discussed.     

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.     

Interaction of the substrate analogue of cytochrome P-450 and mixed function oxidases

The interaction of a spin labeled compound carrying an alkylating group 4-(3-iodo-2-oxopropylidene)-2,2,3,5,5-pentamethylimidozolydene-1-oxyl (RJ) and capable of binding covalently to mixed function oxidases (MFO) was studied. Measurements of the difference spectrum of cytochrome P-450 demonstrated that RJ induces spectral changes characteristic of type I substrates (lambda max = 403 nm; lambda min = 418 nm). The spectral binding constant (Ks) was 66 microM as determined from the difference spectrum. RJ inhibited the microsomal oxidation of substrates of cytochrome P-450 (aniline, aminopyrine and benzo [a]pyrene). This inhibition was shown not to be associated with the conversion of cytochrome P-450 to cytochrome P-420, or with the suppression of the activities of NADPh-cytochrome c reductase and NADPH-cytochrome P-450 reductase. Thus, evidence was obtained for the possible interaction of RJ with cytochrome P-450. RJ injected to rats (5 mg/100 g body wt, i.p.), inhibited the hydroxylation of benzo[a]pyrene, a type I substrate, (21%) and aniline, a type II substrate, (40%) in the microsomes from their livers. The presence of a paramagnetic center in RJ made it possible to study its interaction with microsomes. The electron paramagnetic resonance (EPR) spectrum of RJ was recorded in the rat liver microsomal fraction after in vivo administration of RJ. In rats treated with RJ (5 mg/100 g), hexobarbital sleeping time was prolonged 1.5-fold. Alkylating analogs of substrates of cytochrome P-450 are suggested as agents for structural studies of the active center of cytochrome P-450 and the development of efficient inhibitors of reactions catalyzed by this enzyme.     

Effect of the suicide substrate 3,5-diethoxycarbonyl-2,6-dimethyl-4-ethyl-1,4-dihydropyridine on the metabolism of xenobiotics and on cytochrome P-450 apoproteins

Treatment of rats with the cytochrome P-450 suicide substrate, 3,5-diethoxycarbonyl-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP), produced a 95% inhibition of the in vivo demethylation of either aminopyrine or morphine within 2 hr. One-carbon metabolism of formaldehyde or formate to carbon dioxide was not altered. DDEP also produced a time-dependent decrease in total hepatic microsomal cytochrome P-450 but had no effect on either NADPH-cytochrome c reductase or p-nitrophenol glucuronyl-transferase activities up to 24 hr after administration. A rapid decrease in rat liver microsomal aniline hydroxylation and ethoxyresorufin deethylation was observed in vitro following DDEP administration. Although in vitro testosterone metabolism to 16 alpha-, 16 beta-, and 2 alpha-hydroxy metabolites was depressed profoundly by DDEP in microsomes from untreated and 3-methylcholanthrene-treated animals, 7 alpha-hydroxylation of testosterone was much less affected. Immunochemical quantification of various microsomal cytochrome P-450 protein moieties showed that cytochromes P-450 beta NF-B, P-450UT-A, P-450PCN-E, and P-450PB-C were decreased in hepatic microsomes from DDEP-treated rats. However, the protein moiety of cytochrome P-450UT-H was not diminished and the immunoreactive protein for cytochromes P-450UT-F, P-450PB-B, and P-450ISF-G was only slightly decreased. These results show that DDEP treatment leads to marked decreases in holoprotein and apoproteins of many but not all hepatic microsomal cytochrome P-450 isozymes.     

The interaction between two forms of cytochrome P-450 during drug oxidation in the reconstituted system containing limited amount of NADPH-cytochrome P-450 reductase

The activities of drug oxidation in a reconstituted system which contains two forms of cytochrome P-450 and a limiting amount of NADPH-cytochrome P-450 reductase were determined. Cytochrome P-450 (termed MC P-4481 and MC P-4482) purified from liver microsomes of 3-methyl-cholanthrene-treated rats was active in both 2- and 4-hydroxylation of biphenyl but cytochrome P-450 (termed PB P-450) purified from liver microsomes of phenobarbital-treated rats was active in 4-hydroxylation of biphenyl only. PB P-450, MC P-4481 and MC P-4482 were most active toward benzphetamine N-demethylation, aniline hydroxylation and 7-ethoxycoumarin O-deethylation, respectively. PB P-450 inhibited the activity of biphenyl 2-hydroxylation supported by MC P-4481 or MC P-4482. On the contrary, no inhibition of PB P-450 supported benzphetamine N-demethylation was observed when MC P-4481 or MC P-4482 was added to the system containing PB P-450 and limited amount of the reductase. The apparent Km of PB P-450 for the reductase obtained from double reciprocal plot of the reductase concentration and the activity of biphenyl hydroxylase or benzphetamine N-demethylation was lower than that of MC P-4481 or MC P-4482. These and other results suggest that there is a certain hierarchy among the cytochrome P-450 species for receiving electrons from reductase.     

EFFECTS OF MOTORCYCLE EXHAUST ON CYTOCHROME P-450-DEPENDENT MONOOXYGENASES AND GLUTATHIONE S-TRANSFERASE IN RAT TISSUES

The effects of motorcycle exhaust (ME) on cytochrome P-450 (P-450) -dependent monooxygenases were determined using rats exposed to the exhaust by either inhalation, intratracheal, or intraperitoneal administration. A 4-wk ME inhalation significantly increased benzo[a]pyrene hydroxylation, 7-ethoxyresorufin O-deethylation, and NADPH-cytochrome c reductase activities in liver, kidney, and lung microsomes. Intratracheal instillation of organic extracts of ME particulate (MEP) caused a dose- and time-dependent significant increase of monooxygenase activity. Intratracheal treatment with 0.1 g MEP extract/ kg markedly elevated benzo[a]pyrene hydroxylation and 7- ethoxyresorufin O-deethylation activities in the rat tissues 24 h following treatment. Intraperitoneal treatment with 0.5 g MEP extract/ kg/d for 4d resulted in significant increases of P-450 and cytochrome b contents and NADPH-cytochrome c reductase 5 activity in liver microsomes. The intraperitoneal treatment also markedly increased monooxygenases activities toward methoxyresorufin, aniline, benzphetamine, and erythromycin in liver and benzo[a]pyrene and 7-ethoxyresorufin in liver, kidney, and lung. Immunoblotting analyses of microsomal proteins using a mouse monoclonal antibody (Mab) 1-12-3 against rat P-450 1A1 revealed that ME inhalation, MEP intratracheal, or MEP intraperitoneal treatment increased a P-450 1A protein in the hepatic and extrahepatic tissues. Protein blots analyzed using antibodies to P-450 enzymes showed that MEP intraperitoneal treatment caused increases of P-450 2B, 2E, and 3A subfamily proteins in the liver. The ME inhalation, MEP intratracheal, or MEP intraperitoneal treatment resulted in significant increases in glutathione S -transferase activity in liver cytosols. The present study shows that ME and MEP extract contain substances that can induce multiple forms of P-450 and glutathione S-transferase activity in the rat.     

Effects of chronic acetone administration on ethanol-inducible monooxygenase activities in the rat

Liver microsomal monooxygenase activities known to be ethanol-inducible were determined in female Sprague-Dawley rats after 2-week treatment with 1% (v/v) acetone. Daily acetone intake was in the order of 1.2 g/kg. The final body weight, liver weight and microsomal protein content of acetone-treated rats were identical to those of untreated controls. Microsomal NADPH-cytochrome c reductase activity was also unaffected, while cytochrome P-450 content was only increased 12-18%. Ethanol-inducible p-nitrophenol hydroxylation, aniline hydroxylation and 7-ethoxycoumarin O-deethylation activities were enhanced 5.3-, 4.4- and 2.6-fold, respectively, by chronic acetone treatment. The sex-dependent inducing effect of ethanol on benzphetamine N-demethylation activity in female rats was not observed however, after acetone. Addition of acetone in vitro had a stimulatory effect on aniline hydroxylation by microsomes from control and acetone-induced rats. Acetone, however, was found to be a competitive inhibitor of p-nitrophenol hydroxylation activity (apparent Ki = 1.8 mM), an observation suggesting that p-nitrophenol is a more selective substrate than aniline for rat liver ethanol- and acetone-inducible cytochrome P-450j. Interruption of the chronic acetone treatment for 24 hr resulted in the almost complete disappearance of its inducing affects, this treatment apparently reproducing only the rapidly reversible preferential inducing effects of chronic ethanol administration. This experimental model of induction by acetone in the rat, when compared to chronic ethanol administration, would thus permit a more selective look at the consequences of these common inducing effects in particular, with respect to drug metabolism and toxicity in vivo, and this, in the absence of the hepatotoxic effects of ethanol itself.     

Endotoxin- and inflammation-induced depression of the hepatic drug metabolism in rats

Carrageenan-induced inflammation and exposure to endotoxin considerably decreased the content of cytochrome P-450 and activities of ethylmorphine N-demethylase and meperidine N-demethylase, but did not decrease the activities of aniline hydroxylase or NADPH-cytochrome c reductase, compared with the respective activities in rats treated with carrageenan alone. These results suggest that under these experimental conditions, the two host-related environmental factors interact and enhance a decrease in rat hepatic microsomal drug metabolizing enzymes depending on the substrate used.     

Monooxygenase Activities of Human Liver,Lung, and Kidney Microsomes – A Study of 42 post mortem Cases

Abstract: The cytochrome P-450-dependent monooxygenase system was examined in microsomal fractions prepared from 42 post mortem human livers and 9 lungs and kidneys. Electron microscopy studies indicated that the human liver samples were relatively free of mitochondrial and plasma membrane contamination, but samples of kidney and lung were less pure. The microsomal fractions from all organs were judged to be relatively free of haemoglobin and methaemoglobin. The specific enzyme activities for several drug substrates for the monooxygenase, NADPH-cytochrome c reductase activity and the content of the microsomal cytochromes were measured. The values of the biochemical parameters studied were found to be quite variable and the values for the human liver were appreciably lower than those obtained with liver microsomes from laboratory rodents. The enzyme activities of the human kidney and lung microsomal fractions were 1–10% of those seen for human liver samples, except for NADPH-cytochrome c (P-450) reductase activity. In order to evaluate any post mortem changes in human liver, correlations between drug metabolism activities and either cytochrome P-450 or NADPH-cytochrome c (P-450) reductase content were examined. Strong correlations (r>0.91) were seen only between aminopyrine or ethylmorphine demethylase activity and cytochrome P-450 content in samples obtained within 4 hours of death. Longer post mortem times gave poorer correlation between activity and cytochrome content. These studies document several conditions required in order to obtain human microsomal fractions representative of the activities in fresh, viable tissue.     

Inhibition by SKF 525-A of 7-ethoxycoumarin O-deethylation in microsomal and the reconstituted monooxygenase systems from PCB-treated rat livers

Addition of diethylaminoethyl 2,2-diphenylvalerate-HCl (SKF 525-A) to the incubation mixture containing liver microsomes or purified cytochrome P-450 (PCB P-450) from PCB (KC-500)-treated rats resulted in non-competitive inhibition of 7-ethoxycoumarin O-deethylation activity whereas the addition to the incubation mixture containing purified cytochrome P-448 (PCB P-448) showed a competitive inhibition. Fortification of PCB-induced microsomes with purified NADPH-cytochrome P-450 reductase enhanced the O-deethylation activity. With the reductase-fortified microsomes, SKF 525-A inhibited the O-deethylation in a competitive manner. Based on these results, we confirmed that SKF 525-A inhibits non-competitively and competitively, depending on the species of cytochrome P-450. Our results also support the view that in microsomes from PCB-treated rats, PCB P-450 rather than PCB P-448 is mainly involved in the O-deethylation reaction, presumably due to the presence of a limited amount of NADPH-cytochrome P-450 reductase in microsomes.     

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.     

Isopropanol enhancement of cytochrome P-450-dependent monooxygenase activities and its effects on carbon tetrachloride intoxication

Acute or chronic treatment of rats with isopropanol caused a significant increase in hepatic cytochrome P-450 content and a two- to threefold increase in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but no significant change in ethylmorphine N-demethylase or benzo(a)pyrene hydroxylase activity. In rats treated with isopropanol and challenged with CCl4, liver toxicity of CCl4 was characteristically potentiated, as assessed by elevation of serum glutamic-pyruvic transaminase (SGPT) levels. Isopropanol pretreatment also potentiated CCl4-induced damage to the hepatic monooxygenase system. In addition to a decrease in cytochrome P-450, rats treated with isopropanol and challenged with CCl4 showed a nonspecific decrease not only in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but also in ethylmorphine N-demethylase, benzo(a)pyrene hydroxylase, and NADPH-cytochrome c reductase activities. These results were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized microsomes. The electrophoretic results showed that isopropanol pretreatment markedly potentiated the CCl4-caused destruction of cytochrome P-450 hemeproteins. The data strongly suggest that isopropanol increases one or more forms of cytochrome P-450 which selectively enhance the metabolism of CCl4 to an active metabolite. This active metabolite then causes a nonselective damage to the microsomal mixed-function oxidase system.