Stimulation of p-nitroanisole O-demethylation in perfused livers by xylitol and sorbitol

Increased catalytic activity of cytochrome P-450IIE1 in pericentral hepatocytes compared to periportal hepatocytes isolated from pyrazole-treated rats

Cytochrome P-450IIE1 is induced by a variety of agents, including acetone, ethanol and pyrazole. Recent studies employing immunohistochemical methods have shown that P-450IIE1 was expressed primarily in the pericentral zone of the liver. In order to evaluate whether catalytic activity of P-450IIE1 is preferentially localized in the pericentral zone of the liver acinus, the oxidation of aniline and p-nitrophenol, two effective substrates for P-450IIE1, by periportal and pericentral hepatocytes isolated from pyrazole-treated rats was determined. Periportal and pericentral hepatocytes were prepared by a digitonin-collagenase procedure; the marker enzymes glutamine synthetase and gamma-glutamyl transpeptidase indicated reasonable separation of the two cell populations. Viability, yield and total cytochrome P-450 content were similar for the periportal and pericentral hepatocytes. Pericentral hepatocytes oxidized aniline and p-nitrophenol at rates that were 2-4-fold greater than periportal hepatocytes under a variety of conditions. Carbon monoxide inhibited the oxidation of the substrates with both preparations and abolished the increased oxidation found with the pericentral hepatocytes. Pyrazole or 4-methylpyrazole, added in vitro, effectively inhibited the oxidation of aniline and p-nitrophenol and prevented the augmented rate of oxidation by the pericentral hepatocytes. Western blots carried out using isolated microsomes revealed a more than 2-fold increase in immunochemical staining with microsomes isolated from the pericentral hepatocytes, which correlated to the 2-4-fold increase in the rate of oxidation of aniline or p-nitrophenol by the pericentral hepatocytes. These results suggest that functional catalytic activity of cytochrome P-450IIE1 is preferentially localized in the pericentral zone of the liver acinus, and that most of the induction by pyrazole of P-450IIE1 appears to occur within the pericentral zone.

Increased oxidation of p-nitrophenol and aniline by intact hepatocytes isolated from pyrazole-treated rats

Induction of cytochrome P-450 IIE1 by pyrazole has been shown in a variety of studies with isolated microsomes or reconstituted systems containing the purified P-450 isozyme. Experiments were conducted to document induction by pyrazole in intact hepatocytes by studying the oxidation of p-nitrophenol to 4-nitrocatechol or of aniline to p-aminophenol. Hepatocytes prepared from rats treated with pyrazole for 2 days oxidized p-nitrophenol or aniline at rates which were 3- to 4-fold higher than saline controls. To observe maximal induction in hepatocytes, it was necessary to add metabolic substrates such as pyruvate, sorbitol or xylitol, which suggests that availability of the NADPH cofactor may be rate-limiting in the hepatocytes from the pyrazole-treated rats. Carbon monoxide inhibited the oxidation of p-nitrophenol and aniline by hepatocytes from the pyrazole-treated rats and controls, demonstrating the requirement for cytochrome P-450. The oxidation of both substrates by the hepatocyte preparations was inhibited by a variety of agents that interact with and are effective substrates for oxidation by P-450 IIE1 such as ethanol, dimethylnitrosamine, pyrazole and 4-methylpyrazole. Microsomes isolated from pyrazole-treated rats oxidized aniline and p-nitrophenol at elevated rats compared to saline controls. These results indicate that induction by pyrazole of the oxidation of drugs which are effective substrates for P-450 IIE1 can be observed in intact hepatocytes. The extent of induction and many of the characteristics of aniline or p-nitrophenol oxidation observed with isolated microsomes from pyrazole-treated rats can also be found in the intact hepatocytes.

Acetaminophen hepatotoxicity: is there a role for prostaglandin synthesis?

The hepatotoxicity of acetaminophen (APAP) overdose depends on metabolic activation to a toxic reactive metabolite via hepatic mixed function oxidase. In vitro studies have indicated that APAP may also be cooxidized by prostaglandin H synthetase. The present experiments were designed to assess the possible contribution of hepatic prostaglandin synthesis to APAP toxicity. Adult fed male mice were overdosed with 400 mg APAP/kg. Liver toxicity was estimated by measurement of serum transaminases. Hypertonic xylitol or sodium chloride (2250 mOsm/l), administered intragastrically to stimulate prostaglandin synthesis, increased APAP toxicity. By contrast, the cyclooxygenase inhibiting drugs aspirin (at 25 mg/kg) and indomethacin (at 10 mg/kg) protected against APAP-induced toxicity. APAP kinetics were not affected by hypertonic xylitol or indomethacin, nor were hepatic glutathione levels in overdosed mice. Imidazole, a nonspecific thromboxane synthetase inhibitor, also protected overdosed mice. This drug prolonged hexobarbital sleeping time and prevented the depletion of hepatic glutathione that followed APAP intoxication. Thus, the data support the conclusion that APAP-induced hepatoxicity may be modulated not only by inhibition of cytochrome P450 mediated oxidation, but also by controlling hepatic cyclooxygenase activity.

Oxidation of pyrazole to 4-hydroxypyrazole by intact rat hepatocytes

4-Hydroxypyrazole has been identified as a major metabolite found in the urine of rats and mice after in vivo administration of pyrazole, a potent inhibitor of alcohol dehydrogenase and of ethanol metabolism. The locus and the enzyme systems responsible for the oxidation of pyrazole have not been identified. In the current report, isolated hepatocytes from fed rats were shown to oxidize pyrazole to 4-hydroxypyrazole. An HPLC procedure employing UV and electrochemical detection was utilized to separate and quantify the 4-hydroxypyrazole. The apparent Km for pyrazole by intact hepatocytes was about 2 mM, whereas the apparent Vmax was about 0.06 nmol 4-hydroxypyrazole per min per mg liver cell protein. The production of 4-hydroxypyrazole was inhibited by carbon monoxide and metyrapone, as well as by competitive drug substrates such as aniline or aminopyrine. These results implicate a role for cytochrome P-450 in the oxidation of pyrazole by the hepatocytes. Ethanol was an effective inhibitor of pyrazole oxidation. Hepatocytes were also isolated from rats treated with acetone and 4-methylpyrazole, to attempt to evaluate whether pyrazole oxidation is induced. The rate of 4-hydroxypyrazole production by hepatocytes after acetone and 4-methylpyrazole treatment was actually lower than that of controls. Kinetic assays suggested the presence of an endogenous inhibitor (perhaps the inducer itself) in the induced hepatocytes. In contrast, hepatocytes isolated from rats fasted for 48 hr showed a 2-fold increase in the oxidation of pyrazole to 4-hydroxypyrazole. The Km for pyrazole was the same in hepatocytes from fasted and fed rats, whereas Vmax was increased after fasting. The locus and enzyme system responsible for the oxidation of pyrazole to 4-hydroxypyrazole, and the site of sensitivity to ethanol, appears to be the cytochrome P-450 system of the hepatocyte.

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.

Stimulation of mixed-function oxidation by NADPH in perfused mouse livers. Studies with saponin-permeabilized tissue

In perfused livers from fed and fasted beta-naphthoflavone-treated C57BL/6J mice, maximal rates of p-nitroanisole O-demethylation were 30-40 mu moles/g/hr and 15-20 mu moles/g/hr respectively. The detergent saponin, at concentrations ranging from 0.001 to 0.005%, was infused between 2 and 30 min to establish optimal conditions to permeabilize plasma membranes. Permeabilization was assessed by release of lactate dehydrogenase and stimulation of p-nitroanisole O-demethylation by citrate. Saponin (0.005% for 5 min) alone had little effect on the rates of p-nitroanisole O-demethylation or conjugation of p-nitrophenol by perfused livers. Further, dicarboxylates or NADPH had no effect on rates of monooxygenation by perfused mouse liver in the absence of saponin. In saponin-treated livers from fasted mice, however, rates of monooxygenation were increased rapidly by infusion of dicarboxylates (10 mM malate, citrate, or isocitrate) or an NADPH-generating system (60 and 110% respectively), over a 6-8 min period. During this time period, cellular energetics were not comprised as reflected by normal rates of glucuronidation of p-nitrophenol. Thus, non-permeable metabolites can enter saponin-permeabilized cells in the perfused liver. Rates of monooxygenation were increased 40-60% in livers from fed mice by citrate, NADPH (200 microM) or an NADPH-generating system. In contrast, saponin decreased mixed-function oxidation assayed in isolated microsomes incubated with an NADPH-generating system. Taken together, these data support the hypothesis that maximal rates of monooxygenation in intact hepatocytes from fed as well as fasted mice is limited by the availability of NADPH.

Inhibition of CO2 production from aminopyrine or methanol by cyanamide or crotonaldehyde and the role of mitochondrial aldehyde dehydrogenase in formaldehyde oxidation

Previous results have shown that cyanamide or crotonaldehyde are effective inhibitors of the oxidation of formaldehyde by the low-Km mitochondrial aldehyde dehydrogenase, but do not affect the activity of the glutathione-dependent formaldehyde dehydrogenase. These compounds were used to evaluate the enzyme pathways responsible for the oxidation of formaldehyde generated during the metabolism of aminopyrine or methanol by isolated hepatocytes. Both cyanamide and crotonaldehyde inhibited the production of 14CO2 from 14C-labeled aminopyrine by 30-40%. These agents caused an accumulation of formaldehyde which was identical to the loss in CO2 production, indicating that the inhibition of CO2 production reflected an inhibition of formaldehyde oxidation. The oxidation of methanol was stimulated by the addition of glyoxylic acid, which increases the rate of H2O2 generation. Crotonaldehyde inhibited CO2 production from methanol, but caused a corresponding increase in formaldehyde accumulation. The partial sensitivity of CO2 production to inhibition by cyanamide or crotonaldehyde suggests that both the mitochondrial aldehyde dehydrogenase and formaldehyde dehydrogenase contribute towards the metabolism of formaldehyde which is generated from mixed-function oxidase activity or from methanol, just as both enzyme systems contribute towards the metabolism of exogenously added formaldehyde.

Inhibition of mixed-function oxidation in perfused rat liver by fluoroacetate treatment

The effect of fluoroacetate, an inhibitor of the citric acid cycle, on the mixed-function oxidation of p-nitroanisole in isolated perfused livers from fed rats was studied. The citric acid cycle was inhibited by injection of 5 mg/kg sodium fluoroacetate into rats 3 hr prior to liver perfusion experiments. Inhibition of the citric acid cycle was marked by accumulation of citrate (5-fold) and decreases in rates of glycolysis and glycogenolysis by 50-90%. Fluoroacetate treatment inhibited mixed function oxidation in the perfused liver by about 50% without affecting p-nitroanisole O-demethylation by isolated microsomes. Fluorocitrate, at concentrations up to 50 microM, did not inhibit microsomal p-nitroanisole O-demethylation in vitro. These data support the hypothesis that mixed-function oxidation in intact hepatocytes is dependent upon reducing equivalents generated via the citric acid cycle.

Stimulation of mixed-function oxidation of 7-ethoxycoumarin in periportal and pericentral regions of the perfused rat liver by xylitol

Rates of O-deethylation of 7-ethoxycoumarin by perfused livers from fasted, phenobarbital-treated rats were 3.7 mumol X g-1 X h-1. Approximately 50% of the product was conjugated. When rates of 7-ethoxycoumarin O-deethylation were varied by infusing different concentrations of substrate, a good correlation (r = 0.91) was found between rates of O-deethylation of 7-ethoxycoumarin and fluorescence of 7-hydroxycoumarin detected from the liver surface. Micro-light guides (tip diameter 170 microns) placed on periportal and pericentral regions on the liver surface were used to monitor the conversion of nonfluorescent 7-ethoxycoumarin to fluorescent 7-hydroxycoumarin. The O-deethylation of 7-ethoxycoumarin to 7-hydroxycoumarin increased fluorescence 64% and 28% in pericentral and periportal regions of the liver lobule, respectively. Rates of 7-ethoxycoumarin O-deethylation estimated from these increases in fluorescence were 5.2 mumol X g-1 X h-1 in pericentral and 2.2 mumol X g-1 X h-1 in periportal regions of the liver. During mixed-function oxidation of 7-ethoxycoumarin, the oxidation:reduction state of NADP(H) was similar in both regions of the liver lobule. Xylitol (2 mM) decreased the NADP+/NADPH ratio and stimulated rates of drug metabolism in both regions of the liver lobule. This indicates that conditions exist where the supply of NADPH is an important rate-determining factor for 7-ethoxycoumarin metabolism in both periportal and pericentral regions of the liver lobule.

Effects of ethanol and sorbitol on mixed-function oxidation in perfused rat livers

Ethanol (20 mM) caused 50-90% inhibition of rates of mixed-function oxidation of p-nitroanisole, 7-ethoxycoumarin and benzo(a)pyrene in perfused rat livers; however, the microsomal metabolism of these substrates was unaltered by low concentrations of ethanol. The metabolism of ethanol was required for this inhibition in the perfused liver. In contrast to ethanol, sorbitol stimulated rates of p-nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats. Both sorbitol and ethanol infusion decreased the hepatic NAD+/NADH ratio; however, the NADP+/NADPH ratio was decreased by sorbitol but increased by ethanol. Stimulation of drug metabolism by sorbitol was abolished by pretreatment of fasted rats with 6-aminonicotinamide, an inhibitor of the pentose phosphate shunt. These data indicated that sorbitol stimulated p-nitroanisole metabolism by providing NADPH via the pentose phosphate shunt. The changes in intracellular concentrations of NADPH produced by ethanol and sorbitol correlated directly with changes in hepatic content of citrate and aspartate. These data suggest that inhibition of the citric acid cycle by ethanol decreases the movement of mitochondrial reducing equivalents into the cytosol via substrate shuttle mechanisms.

Regulation of NADPH-dependent mixed-function oxidation in perfused livers. Comparative studies with sorbitol and ethanol

Sorbitol and ethanol were shown to have opposite effects on p-nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats. Sorbitol (2 mM) stimulated drug metabolism by 50% while ethanol (20 mM) caused 80% inhibition. Both sorbitol and ethanol infusion decreased the NAD+/NADH ratio and increased fluorescence of pyridine nucleotides monitored from the liver surface; however, the NADP+/NADPH ratio was decreased by sorbitol but tended to be increased by ethanol. Stimulation of drug metabolism by sorbitol was abolished by pretreatment of fasted rats with 6-aminonicotinamide, an inhibitor of the pentose phosphate shunt, but was not affected by aminooxyacetate, a transaminase inhibitor. These results indicate that sorbitol stimulated p-nitroanisole metabolism by providing NADPH via the pentose phosphate shunt. Ethanol and sorbitol caused changes in intracellular concentrations of NADPH in livers from fasted rats which correlated directly with changes in hepatic levels of citrate and aspartate. Furthermore, aspartate infusion reduced the inhibition of p-nitroanisole O-demethylation by ethanol. This inhibition was also reversed partially by sorbitol in livers from 6-aminonicotinamide-treated rats. It is concluded that ethanol inhibits mixed-function oxidation primarily by decreasing the concentrations of citric acid cycle intermediates which leads to depletion of cytosolic NADPH.

Inhibition of paranitroanisole and antipyrine monooxygenation in isolated rat hepatocytes by compounds interacting with mitochondrially related carbohydrate metabolism

The monooxygenation of paranitroanisole (PNA) and antipyrine (AP) were measured in isolated rat hepatocytes incubated with compounds interacting with mitochondrially related carbohydrate metabolism. Phenylpyruvate, an inhibitor of pyruvate carboxylase, reduced the rate of PNA and AP metabolism to about 60 and 20%, respectively, in hepatocytes both from fasted and fed rats. Inhibition of amino acid transaminase with aminooxyacetate, decreased the metabolism of both PNA and AP to 60-70% of control values in hepatocytes from fasted rats, whereas this effect was not seen in fed rats. n-Butylmalonate, an inhibitor or mitochondrial malate/phosphate exchange, had only minimal effects on PNA and AP monooxygenation in both the nutritional states. The simultaneous presence of glyoxylate and pyruvate, known to inhibit the NADPH specific isocitrate dehydrogenase, reduced the metabolism of both PNA and AP in hepatocytes from fasted rats to about 60 and 35% of control values respectively, while the effect was not so marked in hepatocytes from fed rats. The metabolism both of PNA and of AP in hepatocytes from fasted rats was reduced to 50-60% of control values with the addition of NH4Cl. This effect could be blocked either by incubating the hepatocytes with pyruvate or by using hepatocytes isolated from fed rats. The addition of various carbon intermediates generally reduced the effect of the inhibitors used. Phenobarbital-treatment did not change the effects observed with cells from uninduced animals. The inhibitors did not alter PNA or AP metabolism in microsomal incubations, and therefore most likely reduced the monooxygenation in intact cells by affecting NADPH generation pathways.