Halothane-induced hepatic microsomal lipid peroxidation in guinea pigs and rats

Halothane-induced hepatic microsomal lipid peroxidation in guinea pigs and rats was examined with respect to the mixed function oxidase system, anaerobic dehalogenation activity of halothane, and the antioxidant system. The levels of cytochrome P-450 and NADPH-cytochrome P-450 reductase were significantly higher in guinea pigs than in rats. There was no difference between the two animals in anaerobic dehalogenation activity of halothane per cytochrome P-450 in microsomes. Microsomal alpha-tocopherol was significantly lower in guinea pigs than in rats, and was increased by multiple exposure to halothane in guinea pigs but remained lower than in rats. Microsomal alpha-tocopherol was decreased in rats by multiple exposure. The concentration of reduced glutathione and ascorbic acid was decreased significantly by multiple exposure to halothane in guinea pigs but not in rats. These results suggest that the higher level of halothane-induced hepatic microsomal lipid peroxidation in guinea pigs is due to the large production of radical metabolites resulting from the large amounts of cytochrome P-450, the high activity of NADPH-cytochrome P-450 reductase, and the low concentration of microsomal alpha-tocopherol.     

Comparison of effects of acetaminophen on liver microsomal drug metabolism and lipid peroxidation in rats and mice.

Studies were conducted to determine the in vivo effect of acetaminophen (AAP) on the lipid peroxidation, drug metabolizing enzyme activity and microsomal electron transfer system of rat and mouse liver. AAP was found to inhibit ethylmorphine N-demethylase activity in the presence of NADPH and this inhibition of the enzyme was due to decrease in cytochrome P-450 content, but not due to change in lipid peroxidation in liver microsomes. Kinetical data showed that AAP administration had no effect on Km values of ethylmorphine N-demethylase, however, a decrease in the Vmax values was seen in rats and mice. There was no significant effect of AAP on both NADPH-cytochrome c reductase and the content of cytochrome b5 3 hours after this administration to rats and mice. On the other hand, AAP induced a significant decrease in NADH-ferricyanide reductase in mice, but not in rats. The greatest decrease in cytochrome P-450 observed among the components of the liver microsomal electron transfer system of rats and mice.     

NADPH- and linoleic acid hydroperoxide-induced lipid peroxidation and destruction of cytochrome P-450 in hepatic microsomes

Temporal aspects of the effects of inhibitors on hepatic cytochrome P-450 destruction and lipid peroxidation induced by NADPH and linoleic acid hydroperoxide (LAHP) were compared. In the absence of added Fe2+, NADPH-induced lipid peroxidation in hepatic microsomes exhibited a slow phase followed by a fast phase. The addition of Fe2+ eliminated the slow phase, thus demonstrating that iron is a rate-limiting component in the reaction. EDTA, which complexes iron, and p-chloromercurobenzoate (pCMB), which inhibits NADPH-cytochrome P-450 reductase, inhibited both phases of the reaction. Catalase as well as scavengers of hydroxyl radical, inhibited NADPH-induced lipid peroxidation almost completely. GSH also inhibited the NADPH-dependent reaction but only when added at the beginning of the reaction. In contrast with NADPH-dependent lipid peroxidation, the autocatalytic reaction induced by LAHP was not biphasic, NADPH-dependent or iron-dependent, nor was it inhibited by hydroxyl radical scavengers, catalase or GSH. A synergistic effect on lipid peroxidation was observed when both NADPH and LAHP were added to microsomes. It is concluded that both the fast and slow phases of NADPH-dependent microsomal lipid peroxidation are catalyzed enzymatically and are dependent upon Fe2+, whereas LAHP-dependent lipid peroxidation is autocatalytic. Since the fast phase of enzymatic lipid peroxidation occurred during the fast phase of destruction of cytochrome P-450, it is postulated that iron made available from cytochrome P-450 is sufficient to promote optimal lipid peroxidation. Since catalase and hydroxyl radical scavengers inhibited NADPH-dependent but not LAHP-dependent lipid peroxidation, it is concluded that the hydroxyl radical derived from H2O2 is the initiating active-oxygen species in the enzymatic reaction but not in the autocatalytic reaction.     

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.     

Different effects of paraquat on microsomal lipid peroxidation in mouse brain, lung and liver

Paraquat stimulates NADPH-Fe(2+)-dependent microsomal lipid peroxidation in mouse brain and strongly inhibits it in the liver. In lung microsomes, the lipid peroxidation was stimulated by paraquat at 10(-4) M, but not at higher doses. An antioxidant action of paraquat seemed to account, at least in part, for the lack of stimulation in lung microsomes, but it was inappropriate to explain the result in hepatic microsomes. There was no apparent correlation between the effects of paraquat on the lipid peroxidation and on the activity of NADPH-cytochrome P-450 reductase, the enzyme which initiates redox cycling of paraquat, resulting in generation of active oxygen species. In fact, the effect of paraquat on the lipid peroxidation was independent of paraquat radical production, an intermediate in the cycle. However, the inhibitory potency of N-ethylmaleimide on NADPH-cytochrome P-450 reductase activity paralleled that on the lipid peroxidation stimulated by paraquat in brain and lung. These findings indicate that the effect of paraquat on microsomal lipid peroxidation differs among the organs and that other factors, besides NADPH-cytochrome P-450 reductase, might be involved in the stimulation of lipid peroxidation by paraquat.     

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.     

Effect of vitamin E and vitamin C combination on experimental influenza virus infection

Successful antioxidant treatment of the so-called "free radical diseases" has been reported in the literature. In this study we examined the preventive effect of vitamin E and vitamin C, alone and in combination, on the damage caused by influenza virus infection (IVI). Male mice (ICR), infected with influenza virus A/2/68/(H3N2) (1.5 of LD(50)), were administered single once-daily doses of vitamin E (60 mg/kg b.w.) and vitamin C (80 mg/kg b.w.) intraperitoneally (3 days before virus inoculation). On the 5th and 7th day, respectively, after virus inoculation, animals were decapitated. Monooxygenase enzyme activity (ethylmorphine N-demethylase, amidopyrin N-demethylase, analgin N-demethylase, aniline hydroxylase, cytochrome P-450 content and NADPH-cytochrome C reductase [CCR]) was determined in liver 9000 x g supernatant. Primary and secondary products of lipid peroxidation (LPO; conjugated dienes [CD] and TBA-reactive substances) were measured in blood plasma, lung and liver 9000 x g supernatant. Vitamin E effectively restored LPO-levels increased by IVI. The effect of vitamin C was similar, but slighter. The combination (vitamin E + C) had greater effect on LPO levels than their separate administration. P-450-dependent monooxygenase activity was significantly restored and more pronounced cytochrome P-450 content and NADPH-CCR activity was noted. The preventive effect of vitamin E was stronger than the effect of vitamin C, but the combination (vitamin E + C) had the strongest effect. The superior protective effect of the combination is probably due to vitamin C's repairing effect on vitamin E's tocopheroxyl radical.     

Rat liver microsomal NADPH-supported oxidase activity and lipid peroxidation dependent on ethanol-inducible cytochrome P-450 (P-450IIE1)

The liver microsomal ethanol-inducible cytochrome P-450 (P-450IIE1) form is known to exhibit a high rate of oxidase activity in the absence of substrate and it was therefore of interest to evaluate whether this form of P-450 could contribute to microsomal and liposomal NADPH-dependent oxidase activity and lipid peroxidation. The rate of microsomal NADPH-consumption, O2--formation, H2O2-production and generation of thiobarbituric acid (TBA) reactive substances correlated to the amount of P-450IIE1 in 28 microsomal samples from variously treated rats. Anti-P-450IIE1 IgG inhibited, compared to control IgG, microsomal H2O2-formation by 45% in microsomes from acetone-treated rats and by 22% in control microsomes. NADPH-dependent generation of TBA-reactive products was completely inhibited by these antibodies, whereas preimmune IgG was essentially without effect. Liposomes containing reductase and P-450IIE1 were peroxidized in a superoxide dismutase (SOD) sensitive reaction at a 5-10-fold higher rate than membranes containing 3 other forms of cytochrome P-450. Lipid peroxidation in reconstituted vesicles dependent on the presence of P-450IIB1 was by contrast not inhibited by SOD. Microsomal peroxidase activities, using 15-(S)-hydroperoxy-5-cis-8,11,13-trans-eicosatetraenoic acid as a substrate were high in microsomes from phenobarbital- or ethanol-treated rats but low in membranes from isoniazid-treated rats, having the highest relative level of P-450IIE1. It is suggested that the oxidase activity of P-450IIE1 contributes to microsomal NADPH-dependent lipid peroxidation. The combined action of the oxidase activity by P-450IIE1 and the peroxidase activities by P-450IIB1 and other forms of P-450 may be important for the high rate of lipid peroxidation observed in e.g. microsomes from ethanol- or acetone-treated rats. The possible importance of cytochrome P-450IIE1-dependent lipid peroxidation in vivo after ethanol abuse is discussed.     

Effects of multiple administration of halothane on the mixed function oxidase system in liver microsomes. Difference between guinea pigs and rats

The effects of multiple administration of halothane on the mixed function oxidase system in liver microsomes were investigated and compared between guinea pigs and rats to clarify the difference in sensitivity to halothane. The mixed function oxidase system, except for NADPH-cytochrome P-450 recductase activity was increased in both animals. The inhibition in guinea pigs may have resulted from the microsomal membrane damage induced by the acceleration of lipid peroxidation.     

Hepatic mixed-function oxidase activities of wild pigeons

Hepatic mixed-function oxidase activities of wild pigeons were determined and compared with those of rat to assess the apparent differences in avian and mammalian drug metabolism. Aminopyrine N-demethylase, benzphetamine N-demethylase, ethylmorphine N-demethylase, aniline hydroxylase, NADPH-cytochrome c reductase, glutathione S-transferase activities and cytochrome P-450 levels in pigeon liver were 30-80% lower than the corresponding activities in rat liver. p-Nitroanisole O-demethylase activity in pigeon liver was similar to that of rat liver. Wild pigeon-liver benzo[a]pyrene hydroxylase activity was approx. five times higher than that in the rat. Pigeons did not reveal any noticeable sex differences in mixed-function oxidase activities. Administration of 3-methylcholanthrene and phenobarbital to pigeons resulted in the induction of demethylase and benzo[a]pyrene hydroxylase activities and in cytochrome P-450 levels.     

Mechanisms responsible for the thermal sensitivity of adrenal microsomal monooxygenases.

Studies were done to determine the mechanism(s) responsible for the thermal lability of adrenal microsomal monooxygenases. Preincubation of guinea pig adrenal microsomal suspensions at 37 degrees C caused large time-dependent declines in benzo(a)pyrene (BP) hydroxylase and benzphetamine (BZ) demethylase activities. Similar preincubations with hepatic microsomes had little effect on enzyme activities. The decreases in adrenal enzyme activities were completely prevented by co-incubation of microsomes with cytosol, but were not diminished by reduced glutathione, ascorbic acid, or bovine serum albumin. Partial protection was afforded by EDTA, suggesting that lipid peroxidation might be involved, but malonaldehyde production was not demonstrable and MnCl2, a potent inhibitor of lipid peroxidation, did not affect the decline in enzyme activities. The decreases in the rates of BP and BZ metabolism were prevented by including NADPH or NADP+ in the preincubation medium. The preincubation conditions causing losses of adrenal enzyme activities did not affect cytochrome P-450 concentrations or substrate binding to cytochromes P-450, as indicated by type I difference spectra. NADH-cytochrome c reductase activity also was not affected, but there were decreases in NADPH-cytochrome c reductase activity that were proportionately similar to the declines in drug-metabolizing activities. Direct assessment of NADPH-cytochrome P-450 reductase revealed similarly large decreases in enzyme activity resulting from preincubation of adrenal microsomes. The results demonstrate a need for extra caution when doing preincubation experiments with adrenal microsomal preparations, and suggest that the thermal lability of adrenal monooxygenases is attributable to effects at the active site of NADPH-cytochrome P-450 reductase.     

1-(8-Methoxy-4,8-dimethylnonyl)-4-(1-methyl-ethyl)benzene (MV-678): a reversible inducer of rat hepatic microsomal drug metabolism

MV-678 [1-(8-methoxy-4,8-dimethynonyl)-4-(1-methylethyl)benzene], a recently developed insect growth regulator, increased the hepatic cytochrome P-450-dependent monooxygenase enzymes that metabolize endogenous and exogenous chemicals. In an initial set of experiments, male and female rats received 0, 50, or 800 mg/kg X d of MV-678 by gavage for 3 d, and in a second set of experiments, male rats received 0, 50, or 800 mg/kg X d of MV-678 by gavage for 30 d. A significant increase in both absolute and relative liver weight, microsomal protein content, cytochrome P-450 content, NADPH-cytochrome P-450 reductase activity, and ethylmorphine N-demethylase activity was observed in male and female rats at the high dose level at 3 d. Similar increases were observed in the 800-mg/kg X d males at 30 d. Hepatocellular hypertrophy and proliferation of endoplasmic reticulum observed at both 3 and 30 d correspond to and was consistent with microsomal enzyme induction. Reversibility of both induction and changes in morphology was determined by measuring the same parameters in animals treated for 30 d after a 15- or 30-d recovery period. At 15 d recovery, all biochemical parameters at the high dose level, except relative liver weight and microsomal ethylmorphine N-demethylase activity, had returned to control levels. No significant differences between the control and high dose group animals were noted at 30 d recovery. The hepatocellular changes observed in the high-dose group at 30 d were less apparent at 15 d recovery, and absent at 30 d recovery.     

Stimulation of microsomal drug oxidation activities by incorporation into microsomes of purified NADPH-cytochrome c (P-450) reductase.

The effects of addition of purified NADPH-cytochrome c (P-450) reductase on microsomal activities of aniline hydroxylation, p-phenetidine O-deethylation and ethylmorphine and aminopyrine N-demethylations were investigated utilizing microsomes from untreated, phenobarbital-treated and 3-methylcholanthrene-treated rats. The purified reductase was incorporated into microsomes. The drug oxidation activities were increased by the fortification of microsomes with the reductase while the extent of increase in the activities varied with the substrate and microsomes employed. The most pronounced enhancement was seen in p-phenetidine O-deethylation, followed by aniline hydroxylation and aminopyrine and ethylmorphine N-demethylations. The enhancement was more remarkable in microsomes from rats treated with 3-methylcholanthrene or phenobarbital. alpha-Naphthoflavone inhibited p-phenetidine O-deethylation activity markedly when the reductase was incorporated into microsomes, indicating that a larger amount of a species of cytochrome P-450 sensitive to the inhibitor was capable of participating in the oxidation of this substrate in the presence of the added reductase. One of the two Km values seen with higher concentrations of aniline or aminopyrine was altered by the fortification of microsomes with the purified NADPH cytochrome c (P-450) reductase. From these results, we propose that NADPH-cytochrome c (P-450) reductase transfers electrons to the selected one or two of multiple species of cytochrome P-450 more preferentially depending upon the substrate and the concentration of the substrate in microsomal membranes.     

Solubilisation, purification and reconstitution of hepatic microsomal azoreductase activity

Microsomal NADPH-cytochrome c (P-450) reductase and cytochrome P-450 were purified from the livers of phenobarbitone-treated rats. Purified NADPH-cytochrome c (P-450) reductase effected the NADPH-dependent reduction of FMN and FAD under anaerobic conditions in a non-enzymic manner, but was unable to reduce directly the azo dye, amaranth. In the presence of FMN, the purified reductase effected reduction of amaranth through the production of reduced FMN. Incorporation of NADPH-cytochrome c (P-450) reductase into the microsomal fraction increased the azoreductase activity of liver preparations from phenobarbitone-treated rats, but had no effect on azoreductase activity in preparations from control animals. Azoreductase activity was reconstituted into dilauroyl phosphatidylcholine vesicles containing purified cytochrome P-450 and purified NADPH-cytochrome c (P-450) reductase. In the absence of supplementary FMN, amaranth reduction was completely dependent upon all three components, but in the presence of FMN, the omission of any one component failed to abolish completely azoreductase activity.     

Inhibition of hepatic microsomal drug-metabolizing enzymes by habu snake (trimeresurus flavoviridis) venom fractions

An inhibitor of hepatic microsomal drug-metabolizing enzyme activity was isolated from the venom of the Habu snake (Trimeresurus flavoviridis) by gel filtration through Sephadex G-100 and Amberlite CG-50 column chromatography. The inhibitor, designated R-CG-50-2, gave one band on sodium dodecylsulfate (SDS) polyacrylamide gel electrophoresis and caused high hemorrhagic activity when administered i.c. to rabbits. R-CG-50-2 inhibited the drug-metabolizing enzyme system even after being heated at 70° for 5 min, in spite of a complete loss of hemorrhagic activity. Cytochrome P-450 content and NADPH-cytochrome c reductase activity of rat hepatic microsomes were decreased by administration in vivo either R-CG-50-2 or heated R-CG-50-2. The extent of the decrease was greater with unheated R-CG-50-2 than with heated R-CG-50-2. In both cases, cytochrome P-420, the inactive form of cytochrome P-450, was not detected. Lipid peroxidation in hepatic microsomes was also decreased by administration of unheated R-CG-50-2 but the decrease was not significant. In an in vitro experiment, both heated and unheated R-CG-50-2 decreased the cytochrome P-450 content and the NADPH-cytochrome c reductase activity of microsomes, but, unlike the results in vivo, cytochrome P-450 was converted to cytochrome P-420. Microsomal lipid peroxidation was greatly inhibited by either heated or unheated R-CG-50-2 in vitro. It was concluded that the inhibition of the hepatic microsomal drug-metabolizing enzyme system by either heated or unheated R-CG-50-2 may have been due to the decrease in the cytochrome P-450 content and the NADPH-cytochrome c reductase activity, and that lipid peroxidation may not have had an effect on the inhibition.     

Influence of norethindrone on drug-metabolizing enzymes of female rat liver in various B-vitamin deficiency states.

Ingestion of high levels of thiamin significantly decreased the activity of cytochrome P-450, NADPH cytochrome c reductase, and the metabolism of aniline and ethylmorphine. Apparent VmaxS for ethylmorphine N-demethylase and aniline hydroxylase were decreased by high levels of riboflavin even though NADPH cytochrome c reductase was elevated. High levels of dietary pyridoxine significantly decreased only the Vmax for aniline hydroxylase. Generally, norethindrone produces either no change or slight depression of cytochrome P-450 regardless or diet, whereas the administration of norethindrone produced no change or an increase in activity of c reductase and ethylmorphine N-demethylase. Norethindrone induces aniline hydroxylase in animals fed all diets except those deficient in thiamin and riboflavin. The activities of the four parameters of the drug metabolizing system measured in these studies as well as the effects of norethindrone are clearly affected by the dietary status of the animal.     

The effects of adriamycin in vitro and in vivo on hepatic microsomal drug-metabolizing enzymes: role of microsomal lipid peroxidation

The quinone-containing anticancer drug adriamycin augments the reduction of dioxygen to reactive oxygen species and thereby stimulates (sixfold) NADPH-dependent microsomal lipid peroxidation. In vitro the extensive adriamycin-promoted peroxidation depleted (85%) rat liver microsomal cytochrome P-450, severely inhibited cytochrome P-450-dependent monooxygenation (70%), and glucose-6-phosphatase activity (80%), and activated (450%) UDP-glucuronyltransferase activity. When lipid peroxidation was blocked by EDTA, adriamycin selectively decreased cytochrome P-450 and aminopyrine N-demethylase activity; NADPH-cytochrome c reductase, UDP-glucuronyltransferase, and glucose-6-phosphatase activities were unchanged. Washing and resedimenting peroxidized microsomes to remove adriamycin and soluble lipid peroxidation products failed to restore enzyme activities to control values. Adriamycin administered subacutely (5 mg/kg × three doses) to rats significantly descreased hepatic microsomal cytochrome P-450 content and reduced aminopyrine N-demethylase and NADPH-cytochrome c reductase activities compared to saline-treated controls. Microsomal lipid peroxidation was increased following the above adriamycin treatment. Thus, these data suggested that adriamycin was capable of impairing hepatic drug metabolism in vitro by stimulating membrane lipid peroxidation in a manner similar to carbon tetrachloride; the mechanism by which adriamycin treatment in vivo decreased the activity of the drug monooxygenase system remains unclear.     

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.     

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.     

Action of carbon tetrachloride in the reconstituted monooxygenase system using partially purified cytochrome P-450 and P-448

In the cytochrome P-450-reconstituted system, CCl4 stimulated NADPH-dependent lipid peroxidation of the system containing the P-450 form to a much greater extent than that of the system containing the P-448 form. When the P-450-reconstituted system was preincubated in the presence of both NADPH and CCl4, 7-ethoxycoumarin O-deethylase, aminopyrine N-demethylase and aniline hydroxylase activities were decreased by 40-60%, whereas, with P-448 form reconstituted system, no suppression was observed in these enzyme activities or in 7-ethoxyresorufin O-deethylase activity. These observations suggest that the P-450 form, but not the P-448 form, is active in metabolizing CCl4 to a reactive species that subsequently impairs the hemoprotein.