Interaction of pyrazole and 4-methylpyrazole with hepatic microsomes: effect on cytochrome P-450 content, microsomal oxidation of alcohols, and binding spectra

Distinct effects of form selective cytochrome P450 inhibitors on cytochrome P450-mediated monooxygenase and hydrogen peroxide generating NADPH oxidase

A characteristic of cytochrome P450 (CYP) enzymes is their ability to generate H2O2, either directly or indirectly via superoxide anion, a reaction referred to as "NADPH oxidase" activity. H2O2 production by CYPs can lead to the accumulation of cytotoxic reactive oxygen species which can compromise cellular functioning and contribute to tissue injury. Herein we determined if form selective CYP inhibitors could distinguish between the activities of the monooxygenase and NADPH oxidase activities of rat recombinant CYP1A2, CYP2E1, CYP3A1 and CYP3A2 and CYP1A1/2-enriched β-naphthoflavone-induced rat liver microsomes, CYP2E1-enriched isoniazide-induced rat liver microsomes and CYP3A subfamily-enriched dexamethasone-induced rat liver microsomes. In the presence of 7,8-benzoflavone (2.0 μM) for CYP1A2 and 4-methylpyrazole (32 μM) or DMSO (16 mM) for CYP2E1, monooxygenase activity was blocked without affecting NADPH oxidase activity for both the recombinant enzymes and microsomal preparations. Ketoconazole (1.0 μM), a form selective inhibitor for CYP3A subfamily enzymes, completely inhibited monooxygenase activity of rat recombinant CYP3A1/3A2 and CYP3A subfamily in rat liver microsomes; it also partially inhibited NADPH oxidase activity. 7,8-benzoflavone is a type I ligand, which competes with substrate binding, while 4-methylpyrazole and DMSO are type II heme binding ligands. Interactions of heme with these type II ligands was not sufficient to interfere with oxygen activation, which is required for NADPH oxidase activity. Ketoconazole, a type II ligand known to bind multiple sites on CYP3A subfamily enzymes in close proximity to heme, also interfered, at least in part, with oxygen activation. These data indicate that form specific inhibitors can be used to distinguish between monooxygenase reactions and H2O2 generating NADPH oxidase of CYP1A2 and CYP2E1. Mechanisms by which ketoconazole inhibits CYP3A NADPH oxidase remain to be determined.

Effect of acute and prolonged alcohol administration on Mg(2+) homeostasis in cardiac cells

Alcoholic cardiomyopathy represents a major clinical complication in chronic alcoholics. Previous studies from our laboratory indicate that acute and chronic exposure of liver cells to ethanol results in a major loss of cellular Mg(2+) as a result of alcohol oxidation. We investigated whether exposure to ethanol induces a similar Mg(2+) loss in cardiac cells. The results indicate that chronic exposure to a 6% ethanol-containing diet depleted cardiac myocytes of >25% of their cellular Mg(2+) content. Acute ethanol exposure, instead, induced a time- and dose-dependent manner of Mg(2+) extrusion from perfused hearts and collagenase-dispersed cardiac ventricular myocytes. Pretreatment with chlormethiazole prevented ethanol-induced Mg(2+) loss to a large extent, suggesting a role of ethanol oxidation via cyP4502E1 in the process. Magnesium extrusion across the sarcolemma occurred via the amiloride-inhibited Na(+)/Mg(2+) exchanger. Taken together, our data indicate that Mg(2+) extrusion also occurs in cardiac cells exposed to ethanol as a result of alcohol metabolism by cyP4502E1. The extrusion, which is mediated by the Na(+)/Mg(2+) exchanger, only occurs at doses of ethanol ≥0.1%, and depends on ethanol-induced decline in cellular ATP. The significance of Mg(2+) extrusion for the onset of alcoholic cardiomyopathy remains to be elucidated.

Effect of Alcohol Administration on Mg2+ Homeostasis in H9C2 Cells

Alcoholic cardiomyopathy represents one of the main clinical complications in chronic alcoholics. This pathology contrasts the seemingly beneficial effect of small doses of alcohol on the cardiovascular system. Studies carried out in liver cells exposed acutely or chronically to varying doses of EtOH indicate that intrahepatic alcohol metabolism results in a major loss of cellular Mg²⁺. To investigate whether EtOH administration also induced Mg²⁺ extrusion in cardiac cells, H9C2 cells were exposed to varying doses of EtOH for short- or ling-term periods of time. The results indicate that H9C2 cells exposed to EtOH doses higher than 0.1% (v/v, or 15 mM) extruded Mg²⁺ into the extracellular medium on a time- and dose-dependent manner. Consistent with the involvement of cyP4502E1 in metabolizing EtOH, administration of chloro-methiazole (CMZ) as an inhibitor of the cytochrome prevented EtOH-induced Mg²⁺ loss to a large extent. EtOH-induced Mg²⁺ extrusion was also prevented by the administration of di-thio-treitol (DTT) and n-acetyl-cysteine (NAC), two agents that prevent the negative effects of ROS formation and free radicals generation associated with EtOH metabolism by cyP4502E1.

Taken together, our data indicate that Mg²⁺ extrusion also occur in cardiac cells exposed to EtOH as a result of alcohol metabolism by cyP4502E1 and associated free radical formation. Interestingly, Mg²⁺ extrusion only occurs at doses of EtOH higher than 0.1% administered for an extended period of time. The significance of Mg²⁺ extrusion for the onset of alcoholic cardiomyopathy remains to be elucidated.

Fomepizole: a critical assessment of current dosing recommendations

Fomepizole, 4-methylpyrazole (4-MP), is a competitive antagonist of alcohol dehydrogenase with a binding affinity >8000 times that of ethanol. The drug is currently labeled by the United States Food and Drug Administration for the treatment of adult patients with known or suspected ethylene glycol or methanol poisoning. Fomepizole's wide therapeutic dose range and safety profile confer several advantages over standard ethanol therapy for the treatment of toxic alcohol exposures, including the lack of ethanol-associated side effects. Published data and data obtained from the drug's manufacturer implies that the dose escalation after 48 hours is to compensate for fomepizole-induced increased body clearance resulting from autoinduction of the cytochrome P450 (CYP) drug metabolizing enzyme CYP2E1. However, we were unable to identify any evidence of fomepizole's metabolism occurring via CYP2E1 in humans while the data most frequently cited as evidence for induction do not appear to support this claim. Based on this data along with the apparent zero-order kinetics, the current dose increase recommendations may be unnecessary and considering the safety margin described for fomepizole, an extremely conservative constant higher dose administered every 12 hours would appear to assure efficacy and tolerability. Despite the evidence, dose changes should only be implemented after careful clinical trials.

Induction of a cytosolic 54 kDa protein in rat liver that is highly homologous to selenium-binding protein

We have previously shown that a 54 kDa protein in rat liver is highly homologous to selenium-binding protein (SeBP) or acetaminophen-binding protein (APBP) in mice and is highly inducible by treatment with 3,3',4,4',5-pentachlorobiphenyl or 3-methylcholanthrene. In this study, we examine the effect of six typical inducers, 3-methylcholanthrene (MC), isosafrole (ISO), phenobarbital (PB), dexamethasone (DEX), clofibrate (CLO), pyrazole (PYR) and butylated hydroxytoluene (BHT), on the expression level of this 54 kDa protein. Male Wistar rats were given each inducer following a predetermined schedule. Among these inducers, the 54 kDa protein was inducible by MC and BHT. The response to MC and BHT was compared with that of NAD(P)H: quinone oxidoreductase and ethoxyresorufin O-deethylase activities. The induction mechanisms and physiological role of the 54 kDa protein are discussed in the light of our results.

Ethanol cytotoxicity to a transfected HepG2 cell line expressing human cytochrome P4502E1

The effect of ethanol on the viability of a HepG2 cell model which was developed to constitutively express human CYP2E1 was studied in an attempt to establish a linkage between CYP2E1, reactive oxygen intermediates, and ethanol toxicity. Assays of toxicity included leakage of lactate dehydrogenase, trypan blue uptake, morphology, and formazan production. Ethanol was toxic to HepG2 E9 cells, which express CYP2E1, but not to HepG2 MV5 cells, which do not express CYP2E1. The ethanol toxicity was dependent on the concentration of ethanol, starting with 10 m ethanol, and on the time of incubation with ethanol. Phorbol 12-myristate 13-acetate, which increases the expression of CYP2E1 in this model, increased the toxicity by ethanol. Ethanol toxicity was prevented by 4-methylpyrazole and by diallyl sulfide, inhibitors of CYP2E1. The ethanol toxicity was also prevented by radical trapping agents such as N-acetylcysteine and N-t-butyl-alpha-phenylnitrone, antioxidative agents such as catalase, superoxide dismutase, thiourea, and uric acid, and inhibitors of lipid peroxidation, such as vitamin E phosphate, Trolox, and diphenylphenylenediamine. Besides ethanol, other substrates such as Me2SO, CCl4, isoniazid, and N,N-dimethylnitrosamine were cytotoxic to cells expressing CYP2E1 but not to control cells. These results indicate that ethanol was toxic to HepG2 cells which express human CYP2E1 by a pathway sensitive to inhibitors of CYP2E1 and to a variety of antioxidative agents. This model appears to be useful in efforts to establish a CYP2E1-dependent ethanol hepatotoxicity system and to evaluate the role of oxidative stress and reactive radical species in the toxicity by ethanol.

Kinetic interactions between 4-methylpyrazole and ethanol in healthy humans

4-Methylpyrazole (4-MP), a potent inhibitor of alcohol dehydrogenase activity, is a candidate to replace ethanol as the antidote for methanol and ethylene glycol intoxications, because it has a longer duration of action and apparently fewer adverse effects. To study a probable mutual inhibitory effect between ethanol and 4-MP on their elimination, two studies were performed in healthy human volunteers using double-blind crossover designs. In study A1 4-MP in the presumed therapeutic dose range of 10 to 20 mg/kg caused a 40% reduction in the rate of elimination of ethanol in 12 subjects given 0.5 to 0.7 g/kg of ethanol. These data suggest that such doses of 4-MP inhibit alcohol dehydrogenase activity in humans in vivo and would be effective at blocking methanol or ethylene glycol metabolism. In study B, ethanol (0.6 g/kg followed by 0.2 g/kg twice) significantly decreased the rate of elimination of 4-MP (5 mg/kg, given intravenously to four subjects). These moderate doses of ethanol also inhibited the rate of urinary excretion of 4-carboxypyrazole, the primary metabolite of 4-MP in humans. Data suggest that ethanol inhibits 4-MP metabolism, thereby increasing the duration of therapeutic blood levels of 4-MP in the body. This mutual interaction may have clinical implications, because most self-poisoned patients have also ingested ethanol. Theoretically, methanol and ethylene glycol might also show such interactions with 4-MP.

Inhibitory effect of 4-methylpyrazole on antipyrine clearance in rats

Pyrazole and 4-methylpyrazole (4-MP) are potent, effective inhibitors of alcohol dehydrogenase. Pyrazole and its derivatives also have been shown to affect the cytochrome P-450 dependent monooxygenase system. This study was performed to investigate the effect of 4-MP on the disposition kinetics of antipyrine (AP). Groups of male Fisher 344 rats were given an ip injection of 4-MP (100 mg/kg) or 4-MP HCl (equivalent to 4-MP 100 mg/kg) or an equivalent volume of saline. AP (20 mg/kg) was injected intravenously via the jugular vein catheter 30 minutes later. Blood samples were collected upto 24 hours and assayed by HPLC. 4-MP pretreatment significantly decreased AP clearance from 0.490 +/- 0.032 to 0.095 +/- 0.014 (4-MP HCl) and 0.076 +/- 0.008 (4-MP) L/hr.kg (p less than 0.01). The volume of distribution of AP decreased from 0.82 +/- 0.07 to 0.65 +/- 0.06 (4-MP HCl) and 0.56 +/- 0.04 (4-MP) L/kg (p less than 0.05). Mean residence time increased from 1.68 +/- 0.09 to 6.91 +/- 0.58 (4-MP HCl) and 7.39 +/- 0.56 (4-MP) hr (p less than 0.01). These results demonstrate a significant inhibitory effect of 4-MP on the cytochrome P-450 isozyme(s) which is responsible for AP metabolism in intact animals.

Oxidation of glycerol to formaldehyde by rat liver microsomes. Effects of cytochrome P-450 inducing agents

Glycerol was shown recently to be metabolized to formaldehyde by microsomes from chowfed control rats (Winters et al., Biochem Biophys Res Commun 153: 612-617, 1988). In the present study, experiments were carried out to evaluate the oxidation of glycerol by microsomes isolated from rats treated with inducers of different isozymes of cytochrome P-450. The oxidation of glycerol to formaldehyde was increased in microsomes from rats treated with pyrazole, ethanol or acetone relative to their respective controls, but not after treatment with phenobarbital or 3-methylcholanthrene. This reaction was sensitive to inhibition by carbon monoxide and was inhibited by compounds known to be effective substrates for P-450j, e.g. aniline, ethanol, pyrazole and 4-methylpyrazole. Treatment with pyrazole caused an increase in Vmax for glycerol oxidation but did not affect affect the Km (about 15 mM) for glycerol, as compared to saline controls. Evidence that the product of glycerol metabolism is formaldehyde was provided by the observation that this product served as a substrate for the glutathione-dependent formaldehyde dehydrogenase, and the amount of formaldehyde detected was identical to that detected by the Nash reaction. By utilizing [14C]glycerol, and coupling the formaldehyde dehydrogenase reaction to the formate dehydrogenase reaction, 14CO2 could be detected, indicating that the formaldehyde produced was derived from the added glycerol. These results suggest that that glycerol is not metabolically inert when added to microsomes but serves as an effective substrate for the cytochrome P-450j isozyme, extending the alcohol substrate specificity of this enzyme to poly-ols. The production of formaldehyde from glycerol may require caution since glycerol is often present in microsomal or reconstituted systems.

Pyrazole treatment of rats potentiates CCL4-but not CHCL3-hepatotoxicity

Rats were treated with pyrazole to increase the liver content of the "alcohol-inducible" form of cytochrome P-450. This treatment increased the sensitivity of these animals to CCl4-hepatotoxicity assessed by increases in SGPT and SGOT levels and decreases in microsomal cytochrome P-450 and aniline p-hydroxylase activity. However, the hepatotoxicity of CHCl3 was not increased by pyrazole-treatment. These data are consistent with the hypothesis that the "alcohol-inducible" form of cytochrome P-450 is capable of CCl4- but not CHCl3-activation.

4-Methylpyrazole: a controlled study of safety in healthy human subjects after single, ascending doses

4-Methylpyrazole (4-MP), an inhibitor of alcohol dehydrogenase, is a possible future drug for the treatment of methanol and ethylene glycol intoxications and the severe ethanol-disulfiram reaction. Therefore a placebo-controlled, double-blind, single-dose, randomized, sequential, ascending-dose "Phase I study" was performed in healthy volunteers in order to determine the tolerance of 4-MP at dose levels of 10 (n = 4), 20 (n = 4), 50 (n = 4), and 100 mg/kg (n = 3). Along with each dose group, there were two placebos except with the 100 mg/kg group where there was only one placebo. In the 10 and 20 mg/kg group there were no side-effects in any subject. At the 50 mg/kg level, three out of four subjects experienced slight to moderate nausea and dizziness from 0 to 2.5 h after dosing. In the 100 mg/kg group all three subjects reported side-effects like nausea, dizziness, and vertigo, that were short-lived in two subjects, but lasted up to 30 h in one subject. The study was stopped after evaluation of the latter subject, so fewer subjects were completed in this last group. Despite these subjective side-effects, there were no significant changes in objective clinical parameters like pulse, blood pressure, body temperature, or blood and urine chemistries. We conclude that at a single dose of 4-MP (10-20 mg/kg) producing plasma levels within a probable therapeutic range, no side-effects were attributed to 4-MP.

Distribution of oral 4-methylpyrazole in the rat: inhibition of elimination by ethanol

4-Methylpyrazole (4-MP), a potent competitive inhibitor of alcohol dehydrogenase activity, is being studied as a therapeutic agent for methanol and ethylene glycol poisoning. In order to evaluate the distribution of 4-MP using doses in the potentially therapeutic range, male Sprague-Dawley rats were administered 4-MP orally at zero time in doses of 5, 10, or 20 mg/kg. Half of the rats were also treated orally at 0, 1, 2, and 3 h with ethanol (1 g/kg each h) and half with glucose in isocaloric amounts. At doses of 10 and 20 mg/kg, 4-MP elimination appeared to be saturated, with an elimination rate of 10 mumol/L/h. Elimination at 5 mg/kg was non-conclusive as to the order. The rate of 4-MP elimination was decreased about 50% by concomitant administration of ethanol. Urinary excretion of unchanged 4-MP accounted for only about 1% of the dose; the amount excreted unchanged was significantly increased by ethanol administration. The results demonstrate the mutual inhibition of metabolism by ethanol and 4-methylpyrazole, which may explain why the inhibition of ADH by 4-MP can be longer than that predicted by the elimination rate of 4-MP alone.

Alteration of liver microsomal monooxygenases and substrate competition with aniline hydroxylase from rats chronically fed low-fat and high-fat-containing alcohol diets

Male Sprague-Dawley rats fed ethanol (EtOH) 36% of total calories for four weeks in a liquid diet containing either 34% (HF) or 12% (LF) of calories as fat were studied with respect to induction of microsomal monooxygenases (MFO) and substrate competition with EtOH-inducible aniline hydroxylase. The specific activity and turnover of aniline hydroxylase were induced to similar extents by HF-EtOH and LF-EtOH diets. Whereas, both LF-EtOH and HF-EtOH caused a decrease in the turnover of arylhydrocarbon (benzo[a]pyrene) hydroxylase (AHH) and aldrin epoxidase compared to pair-fed (PF) controls, LF-EtOH but not HF-EtOH increased the turnover of ethoxycoumarin and ethoxyresorufin O-deethylase (ECOD and EROD). The increase in ECOD and EROD and the decrease in AHH by EtOH is contrary to the parallel induction of these activities by 3-methylcholanthrene (3-MC) and Aroclor 1254 (Aroclor). Benzo(a)pyrene (BaP) stimulated aniline hydroxylase in the HF-EtOH and PF systems, whereas with LF diet, stimulation was seen only in the EtOH group. Ethoxycoumarin (EC) inhibited aniline hydroxylase by microsomes from EtOH- and pyrazole-treated rats, whereas it stimulated aniline hydroxylase by control microsomes, suggesting that the EC effects were associated with EtOH-inducible cytochrome P-450. Ethoxyresorufin (ER) inhibited aniline hydroxylase in EtOH and PF groups, thus the differential effects of EC were not nonspecific O-deethylase effects. The effects of EtOH feeding on ECOD, EROD, and AHH (ie, substrates for 3-MC-inducible cytochrome P-450) displayed a greater differential between the experimental and control group with the LF- than with the HF-containing diet. The findings suggest that the alteration of certain MFO activities by chronic EtOH ingestion can be modified by the content of dietary fat. Moreover, the competition dynamics of MFO substrates toward EtOH-inducible aniline hydroxylase are altered by EtOH feeding and, in turn, modified by dietary fat.

Inhibition of catalase-dependent ethanol metabolism in alcohol dehydrogenase-deficient deermice by fructose

Hepatic microsomal fractions from ADH (alcohol dehydrogenase)-negative deermice incubated with an NADPH-generating system metabolized butanol and ethanol at rates around 10 nmol/min per mg. In contrast, cytosolic catalase from ADH-negative deermouse liver oxidized ethanol, but not butanol, when incubated with an H2O2-generating system. Thus butanol is oxidized by cytochrome P-450 in microsomal fractions, but not by cytosolic catalase, in tissues from ADH-negative deermice. In perfused livers from ADH-negative deermice, rates of ethanol uptake at low concentrations of ethanol (1.5 mM) were about 60 mumol/h per g, yet butanol (1.5 mM) uptake was undetectable (less than 4 mumol/h per g). At higher concentrations of alcohol (25-30 mM), rates of ethanol uptake were about 80 mumol/h per g, whereas rates of butanol uptake were only about 9 mumol/h per g. Because rates of butanol metabolism via cytochrome P-450 in deermice were more than an order of magnitude lower than rates of ethanol uptake in livers from ADH-negative deermice, it is concluded that ethanol uptake by perfused livers from ADH-negative deermice is catalysed predominantly via catalase-H2O2. In support of this conclusion, rates of H2O2 generation, which are rate-limiting for the peroxidation of ethanol by catalase, were about 65 mumol/h per g in livers from ADH-negative deermice, values similar to rates of ethanol uptake of about 60 mumol/h per g measured under identical conditions. Rates of ethanol uptake by perfused livers from ADH-positive, but not from ADH-negative, deermice were increased by about 50% by infusion of fructose. Thus it is concluded that the stimulation of hepatic ethanol uptake by fructose is dependent on the presence of ADH. Unexpectedly, fructose decreased rates of ethanol metabolism and H2O2 generation by about 60% in perfused livers from ADH-negative deermice, probably by decreasing activation of fatty acids and thus diminishing rates of peroxisomal beta-oxidation.

Increased sensitivity of the microsomal oxidation of ethanol to inhibition by pyrazole and 4-methylpyrazole after chronic ethanol treatment

Pyrazole and 4-methylpyrazole, inhibitors of the oxidation of ethanol by alcohol dehydrogenase, also inhibit microsomal metabolism of ethanol. The inhibitory effectiveness of these agents was increased in microsomes isolated from rats treated chronically with ethanol as compared to microsomes from pair-fed controls or from rats treated with other cytochrome P-450 inducers such as phenobarbital or 3-methylcholanthrene. Pyrazole and 4-methylpyrazole produced type II binding spectra with all the microsomal preparations. However, there was an increased affinity (lower Ks value) for these agents by the microsomes from the ethanol-fed rats. A correlation between Ks values and inhibitory effectiveness against ethanol oxidation by the various microsomal preparations could be observed. This suggests that an increase in affinity, which may reflect the induction of an alcohol-preferring isozyme of cytochrome P-450, is responsible for the increased inhibitory effectiveness of pyrazole and 4-methylpyrazole towards ethanol oxidation by microsomes after chronic ethanol treatment. One difference between pyrazole and 4-methylpyrazole was the increased affinity and inhibitory effectiveness of the latter but not the former with microsomes from rats treated with 3-methylcholanthrene. This could be due to the ability of 4-methylpyrazole, compared to pyrazole, to interact with and induce several isozymes of cytochrome P-450. Pyrazole and 4-methylpyrazole are often utilized to evaluate ethanol metabolism by alcohol-dehydrogenase-dependent and -independent pathways. However, the sensitivity of microsomal ethanol oxidation to inhibition by these agents, especially after chronic ethanol treatment, would suggest that their use in this regard is complex and could tend to underestimate the contribution of the microsomal pathway towards the metabolic tolerance found after ethanol treatment.

Ethanol oxidation and toxicity: role of alcohol P-450 oxygenase

The isolation and characterization of ethanol-inducible rabbit liver microsomal cytochrome P-450, termed P-450 3a or P-450ALC, has provided definitive evidence for the role of this enzyme in alcohol oxidation. From findings on the distribution, substrate specificity, and mechanism of action of P-450ALC we have suggested "alcohol P-450 oxygenase" as a more biochemically accurate name than "microsomal ethanol-oxidizing system." The present review is concerned with studies in this and other laboratories on activities and inducers associated with this versatile enzyme. Numerous xenobiotics, including alcohols and ketones, nitrosamines, aromatic compounds, and halogenated alkanes, alkenes, and ethers, are known to undergo increased microsomal metabolism after chronic exposure of various species to ethanol. Diverse compounds and treatments may induce P-450ALC, including the administration of ten or more chemically different compounds, fasting, or the diabetic state. Whether a common mechanism of induction is involved is unknown at this time. As direct evidence that P-450ALC catalyzes numerous metabolic reactions, the purified rabbit enzyme has been used in a reconstituted system to demonstrate various metabolic transformations, including the oxidation of various alcohols, acetone, acetol, p-nitrophenol, and aniline, the dealkylation of substituted nitrosamines, the reductive dechlorination of carbon tetrachloride, carbon tetrachloride-induced lipid peroxidation, and acetaminophen activation to form the glutathione conjugate.

Inhibition of microsomal oxidation of ethanol by pyrazole and 4-methylpyrazole in vitro. Increased effectiveness after induction by pyrazole and 4-methylpyrazole

Pyrazole and 4-methylpyrazole, which are inhibitors of alcohol dehydrogenase, were also found to be effective inhibitors of the oxidation of ethanol by liver microsomes (microsomal fractions) in vitro. Ethanol oxidation by microsomes from rats previously treated for 2 or 3 days with either pyrazole or 4-methylpyrazole appeared to be especially sensitive to inhibition in vitro by pyrazole or 4-methylpyrazole. The kinetics of inhibition by pyrazole or 4-methylpyrazole in all microsomal preparations were mixed, as the Km for ethanol was elevated while Vmax was lowered. However, Ki values for pyrazole (about 0.35 mM) and especially 4-methylpyrazole (about 0.03-0.10 mM) were much lower than those found with the saline controls (about 0.7-1.1 mM). In contrast, Ki values for dimethyl sulphoxide as an inhibitor of microsomal ethanol oxidation were similar in all microsomal preparations. Pyrazole and 4-methylpyrazole reacted with microsomes to produce type II spectral changes whose magnitude increased after treatment with either pyrazole or 4-methylpyrazole. Thus the increased inhibitory effectiveness of pyrazole and 4-methylpyrazole appears to be associated with increased interactions with the cytochrome P-450 isoenzyme(s) induced by these compounds. These isoenzymes have properties similar to those of the isoenzyme induced by chronic ethanol treatment. Therefore, caution is needed in the use of pyrazole or 4-methylpyrazole to assess pathways of ethanol metabolism, especially after chronic ethanol treatment, since these agents, besides inhibiting alcohol dehydrogenase, are also effective inhibitors of microsomal ethanol oxidation.