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E S Fiala

Publications and source records attributed to E S Fiala.

At least 37 records · Page 2Linked to original sources
PubMed1995

Secondary nitroalkanes: induction of DNA repair in rat hepatocytes, activation by aryl sulfotransferase and hepatocarcinogenicity of 2-nitrobutane and 3-nitropentane in male F344 rats.

The secondary nitroalkanes, 2-nitropropane, 2-nitrobutane, 3-nitropentane, 2-nitroheptane, nitrocyclopentane and nitrocyclohexane, as well as the primary nitroalkanes, 1-nitropropane, 1-nitrobutane, 1-nitropentane and 1-nitroheptane, were examined for their ability to induce DNA repair in rat hepatocytes and to serve as substrates for activation by partially purified rat liver aryl sulfotransferase in vitro. All of the secondary, but none of the primary nitroalkanes examined, induced significant DNA repair in rat hepatocytes. Also, the nitronates of all of the secondary nitroalkanes, but none of the primary nitroalkanes, served as substrates for the aryl sulfotransferase-catalysed production of 8-aminoguanosine and 8-oxoguanosine from guanosine in vitro. In a carcinogenicity assay using male F344 rats, the secondary nitroalkanes, 2-nitrobutane and 3-nitropentane, produced a highly significant incidence of hepatocarcinoma with metastases to the lungs, whereas the primary nitroalkane, 1-nitrobutane, was not carcinogenic. While a low incidence of hepatocarcinoma was also produced by cyclopentanone oxime, the results were not statistically significant. Since the secondary nitroalkane, 2-nitropropane, in contrast to the primary nitroalkane, 1-nitropropane, was also previously shown to be hepatocarcinogenic in rats, it is probable that secondary nitroalkanes constitute a hitherto unrecognized class of chemical carcinogens.

Animals↗
PubMed1995

Effects of dietary restriction and fasting on selected rat liver enzymes of xenobiotic metabolism and on AOM-induced DNA guanine methylation in rat liver and colon.

Using five- to eight-week-old male F344 rats and a high-fat (23.5% corn oil) modified AIN-76A diet, we examined the effects of dietary restriction (a 3-wk 30% reduction of food intake with respect to ad libitum-fed controls) or complete fasting (2 days without food) on the activities of hepatic xenobiotic metabolizing enzymes in vitro and on azoxymethane- (AOM) induced formation of O6-methylguanine and 7-methylguanine in liver and colon DNA in vivo. Compared with ad libitum-fed rats, fasting increased total liver cytochrome P450 by 32%, microsomal aniline hydroxylase by 270%, N-nitrosodimethylamine demethylase by 270%, and azoxymethane hydroxylase by 320%. Liver benzo[a]pyrene (BP) hydroxylase and glutathione-S-transferase were decreased by 39% and 21%, respectively, whereas NADPH cytochrome c reductase and UDP glucuronyltransferase were unchanged. DNA methylation in the livers of fasted animals was 20-31% greater six hours after a 15 mg/kg sc injection of AOM than in ad libitum-fed controls, whereas DNA methylation in the colon was slightly lower. In three-week diet-restricted animals. there were small but not statistically significant changes in the various enzyme activities and in AOM-induced DNA methylation compared with the ad libitum-fed controls, with the exception of BP hydroxylase, which showed a 26% decrease. However, the trends in the increase or decrease of each parameter, although small in magnitude, were similar to those observed in the case of fasting, suggesting that the effects might become significant if the duration of diet restriction were prolonged. The enhancement of AOM metabolism in rat liver by fasting, leading to increased liver DNA methylation, is different from that produced by chemical inducers, such as ethanol, where no increase in liver DNA methylation is observed.

Aniline Hydroxylase↗
PubMed1994

Effects of green and black tea on hepatic xenobiotic metabolizing systems in the male F344 rat.

1. The induction of phase I and II enzymes in the liver of the male F344 rat drinking 2% (w/v) solutions of green or black tea for 6 weeks was investigated. Also studied were glutathione (GSH) and cyst(e)ine in blood, liver and kidney, as well as serum cholesterol, HDL cholesterol, triglycerides, and total and free testosterone. 2. The total carbon monoxide-discernible liver P450, b5 and NADPH-cytochrome c(P450) reductase activities were similar in all groups. 3. There were significant increases in liver of rat drinking green or black tea of P4501A1, 1A2 and 2B1 activities, but no change in P4502E1 and 3A4 activities. Of the phase II enzymes, UDP-glucuronyltransferase was increased, but glutathione S-transferase was not. 4. Serum GSH was higher in the group administered black tea, but GSH and cyst(e)ine in other groups was at control levels. Serum cholesterol was lower in rat given black compared with green tea. Triglycerides had a declining trend after green and black tea exposure compared with water controls. Free and total testosterone were not affected. 5. Thus, beverages widely used by man altered host biochemistry as regards specific phase I and II enzymes in liver of rat and specific serum parameters.

Animals↗
PubMed1993

2-Nitropropane-induced liver DNA and RNA base modifications: differences between Sprague-Dawley rats and New Zealand white rabbits.

2-Nitropropane (2-NP), a hepatocarcinogen in male Sprague-Dawley rats but not, under the same conditions, in male New Zealand White rabbits, induces characteristic base modifications in rat liver DNA and RNA including increases in 8-oxoguanine and the formation of 8-aminoguanine. We compared the levels of these modifications in the two animal species at 6, 18 and 42 h after a single i.p. treatment with 1.12 mmol/kg 2-NP. Significantly less nucleic acid base modifications were found to be produced in rabbit liver than in rat liver. Thus, the relative resistance of the rabbit to the hepatocarcinogenicity of 2-NP correlates with decreased levels of 2-NP-induced liver DNA and RNA base damage.

Animals↗
PubMed1991

Comparison of oxidative damage to rat liver DNA and RNA by primary nitroalkanes, secondary nitroalkanes, cyclopentanone oxime, and related compounds.

The hepatocarcinogen 2-nitropropane causes oxidative damage to liver DNA and RNA after administration to rats; increases in 8-hydroxydeoxyguanosine and formation of an unknown moiety (DX1) in DNA, plus increases in 8-hydroxyguanosine and the appearance of two unidentified peaks (RX1 and RX2) in RNA were observed by high-performance liquid chromatography of nucleosides from 2-nitropropane-treated rats using electrochemical detection (E. S. Fiala et al, Cancer Res., 49:5518-5522, 1989). In the present study, damage to Sprague-Dawley rat liver RNA and DNA was assessed to determine whether the characteristic pattern of oxidative nucleic acid damage caused by 2-nitropropane also occurred after i.p. administration of primary nitroalkanes, other secondary nitroalkanes, 2-methyl-2-nitropropane (a tertiary nitroalkane), and cyclopentanone oxime. All of the secondary nitroalkanes and cyclopentanone oxime significantly increased levels of 8-hydroxyguanine in both DNA and RNA and caused the appearance of DX1, RX1 and RX2. The primary nitroalkanes and the tertiary nitroalkane 2-methyl-2-nitropropane did not cause a similar pattern of nucleic acid damage. The rates of reprotonation of nitronates of the secondary nitroalkanes to the respective un-ionized neutral forms at pH 7.7 were more than 20-fold less than the rates of reprotonation of primary nitroalkane nitronates, suggesting that the anionic nitronates, rather than neutral compounds, are more immediately responsible for the DNA and RNA damage observed in vivo. Since 8-hydroxyguanine is a miscoding lesion in DNA, these results suggest the possibility, still to be rigorously tested, that hepatocarcinogenicity may be associated not only with 2-nitropropane but also with other secondary nitroalkanes as well as with those ketoximes that are capable of being converted to secondary nitroalkanes in vivo.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1991

Mechanism of benzylselenocyanate inhibition of azoxymethane-induced colon carcinogenesis in F344 rats.

Benzylselenocyanate (BSC), a novel organoselenium compound, has been found to inhibit azoxymethane (AOM)-induced colon carcinogenesis in rats during initiation. To investigate its mechanism of action, we examined the effects of BSC feeding on the following parameters: (a) metabolism of [14C]AOM to 14CO2 in vivo; (b) metabolic activation of AOM to MAM and of MAM to formic acid and methanol by rat liver microsomes in vitro; and (c) AOM-induced DNA methylation in rat livers and colons. Five-week-old male F344 rats were fed modified (23% corn oil) AIN-76A diets containing 0 (control), 25, or 50 ppm of BSC or benzylthiocyanate (BTC), a sulfur analogue of BSC which does not inhibit the colon carcinogenicity of AOM. After 3 weeks, rats were either sacrificed for the isolation of liver microsomes or were given 15 mg/kg of [14C]AOM s.c. to determine the rate of carcinogen metabolism in vivo. No difference in [14C]AOM metabolism was found between rats fed the BTC diets and those fed the control diet. In contrast, the rate of [14C]AOM metabolism, as determined by exhaled radioactivity, was 2-3 times higher in rats fed the BSC diets. While liver microsomes from rats fed the BTC diets metabolized AOM and MAM at rates not significantly different from those obtained with control liver microsomes, the metabolic activation of AOM as well as of MAM was stimulated severalfold when assayed with liver microsomes from rats fed the BSC diets. An increase in total liver cytochrome P-450 was also observed in the BSC-fed rats. Following the administration of 15 mg/kg AOM, significantly less O6-methylguanine and 7-methylguanine was present in the colon DNA from rats consuming the BSC diets than in rats fed the BTC or control diets. The body weight gains of rats fed the 25- and 50-ppm BSC-containing diets for 3 weeks were less (27 and 43%, respectively) than those of rats fed either the control or BTC-containing diets. These results indicate that dietary BSC significantly induces the hydroxylation of AOM and the oxidation of MAM in rat liver. An increase in the rates of AOM and MAM metabolism in the liver due to enzyme induction by BSC will result in decreased delivery of MAM to the colon via the bloodstream. This will be reflected in decreased DNA alkylation, as observed, and is likely to be a major factor in the inhibition of AOM-induced colon carcinogenesis by BSC.

Animals↗
PubMed1991

Evaluation of secondary nitroalkanes, their nitronates, primary nitroalkanes, nitrocarbinols, and other aliphatic nitro compounds in the Ames Salmonella assay.

The secondary nitroalkanes 2-nitropropane, 2-nitrobutane, 3-nitropentane and nitrocyclopentane, as well as their anionic forms (nitronates); the primary nitroalkanes 1-nitropropane, 1-nitrobutane, and 1-nitropentane and their respective nitronates; the nitrocarbinols 2-nitro-1-propanol, 2-nitro-1-butanol, 3-nitro-2-butanol, and 3-nitro-2-pentanol and their respective nitronates; 2-methyl-2-nitropropane, and 2-nitroso-2-nitropropane were tested in the Ames Salmonella assay using strains TA98, TA100 and TA102. Nitronates of the secondary nitroalkanes 2-nitropropane, 2-nitrobutane, 3-nitropentane, and nitrocyclopentane were significantly mutagenic in Salmonella strains TA100 and TA102 at 10-80 mumoles/plate, but the parent compounds were mutagenic at only a single dose level or were not mutagenic at all in the same dose range. The primary nitroalkanes and the nitrocarbinols were not mutagenic, or only marginally so, at the concentrations tested. The nitronates of the primary nitroalkanes and the nitrocarbinols reprotonated too rapidly under the conditions of the assay for adequate evaluation of mutagenicity. 2-Methyl-2-nitropropane was not mutagenic in strains TA100 and TA102; 2-nitroso-2-nitropropane was also not mutagenic in strains TA100 and TA102, but induced an equivocal mutagenic response in TA98. The positive Salmonella mutation data for the nitronates of the secondary nitroalkanes studied correlate very well with the very slow rate of reprotonation of secondary nitroalkane nitronates at pH 7.7 (Conaway et al. (1991) Cancer Res., 51, 3143), and provide further evidence that nitronates of secondary nitroalkanes, rather than the neutral parent forms with which they may be in equilibrium, are the more proximate mutagenic species.

Carcinogenicity Tests↗
PubMed1991

Metabolism of azoxymethane, methylazoxymethanol and N-nitrosodimethylamine by cytochrome P450IIE1.

The metabolism of azoxymethane (AOM), methylazoxymethanol (MAM) and N-nitrosodimethylamine (NDMA) by liver microsomes from acetone-induced rats as well as by a reconstituted system containing purified cytochrome P450IIE1 was examined. The products consisted of MAM from AOM; methanol and formic acid from MAM; and methylamine, formaldehyde, methanol, methylphosphate and formic acid from NDMA. Compared to liver microsomes from untreated rats, the metabolic activity of acetone-induced microsomes was approximately 4 times higher for all three carcinogens. Using the reconstituted system, the enzyme activities (nmol substrate metabolized/nmol P450/min) for AOM, MAM and NDMA were 2.88 +/- 1.14, 2.87 +/- 0.59 and 9.47 +/- 2.24 respectively. Incubations carried out in the presence of a monoclonal antibody to cytochrome P450IIE1 resulted in a 85-90% inhibition of all three reactions in this system. These results provide conclusive evidence that AOM, MAM and NDMA are metabolized by the same form of rat liver cytochrome P450. In addition, the stoichiometry of NDMA products formed in these reactions indicates that denitrosation, a presumed detoxication process, and alpha-hydroxylation, an activation reaction, are also catalyzed by the same cytochrome P450 isozyme.

Acetone↗
PubMed1990

Oxidative DNA and RNA damage in rat liver due to acetoxime: similarity to effects of 2-nitropropane.

Acetoxime (ACO) and 2-nitropropane (2-NP), both industrially important chemicals and known hepatocarcinogens in rats, induced increased levels of 8-hydroxy-guanine in liver DNA and RNA of male Sprague-Dawley and F344 rats after either oral or i.p. administration. Both compounds also produced qualitatively the same patterns of other apparent modifications of liver DNA and RNA nucleosides, discernible by HPLC with electrochemical detection. Six hours after administration, the effects of 2-NP on liver nucleic acids were more pronounced in F344 rats than in Sprague-Dawley rats, suggesting that 2-NP may prove to be a stronger carcinogen in the F344 strain. The effects of ACO, a weaker carcinogen than 2-NP, were less than those of the nitroalkane in both rat strains. These results suggest that the hepatocarcinogenicity of ACO, like that of 2-NP, may depend on increased generation of reactive oxygen species capable of producing DNA and RNA base damage in rat liver. In addition, the data support the hypothesis that the hepatocarcinogenicity of ACO depends on its partial in vivo N-oxidation to 2-NP.

Administration, Oral↗
PubMed1990

Sex and organ differences in oxidative DNA and RNA damage due to treatment of Sprague-Dawley rats with acetoxime or 2-nitropropane.

The hepatocarcinogens 2-nitropropane and acetoxime have previously been found to induce a specific and qualitatively identical pattern of base damage in rat liver DNA and RNA, including the induction of increased levels of 8-hydroxyguanine. Because both 2-nitropropane and acetoxime are weaker carcinogens in female rats than male rats, we examined the ability of these chemicals to induce this pattern of damage in liver and kidney nucleic acids of male and female Sprague-Dawley rats 6 and 18 h after administration. Significantly lower levels of 8-hydroxydeoxyguanosine, 8-hydroxyguanosine and other presumed modified nucleosides discernible by high-performance liquid chromatography with electrochemical detection were found in liver nucleic acids of female rats at both time points. In addition, minimal alteration of nucleic acids was observed in the kidney, which is not a target organ for the carcinogenicity of either 2-nitropropane (2-NP) or acetoxime (ACO). These results support the hypothesis that the specific DNA alterations observed are relevant to the hepatocarcinogenicity of 2-NP and ACO.

Alkanes↗
PubMed1989

Oxidative DNA and RNA damage in the livers of Sprague-Dawley rats treated with the hepatocarcinogen 2-nitropropane.

2-Nitropropane (2-NP), a widely used industrial chemical, is a mutagen in bacteria and a powerful hepatocarcinogen in Sprague-Dawley rats. In contrast, 1-nitropropane (1-NP) is not mutagenic and does not appear to be carcinogenic. Thus far, the mechanism of carcinogenicity of 2-NP has not been examined. We report in the present work that i.p. treatment of male Sprague-Dawley rats with 100 mg/kg 2-NP results in a 3.6-fold increase (P less than 0.01) in the levels of 8-hydroxydeoxyguanosine as well as in the appearance of an additional electrochemically active species, presumably a modified deoxynucleoside, in liver DNA hydrolysates 6 h after dosing. Treatment with 2-NP also induces an 11-fold increase (P less than 0.0001) in the levels of 8-hydroxyguanosine in rat liver RNA, and results in the appearance of two new electrochemically active species (RX1 and RX2), presumably modified nucleosides. Small, statistically not significant increases of 8-hydroxyguanosine in RNA and of 8-hydroxydeoxyguanosine in DNA, as well as the induction of much smaller amounts of RX2 (but apparently not RX1) in rat liver RNA, are also observed following analogous treatment with 1-NP. Since the presence of 8-hydroxyguanine, a product of the attack of hydroxyl radicals (or other reactive oxygen species) on guanine, can cause DNA misreplication [Kuchino et al., Nature (Lond.), 327: 77-79, 1987], our findings are consistent with a mechanism of hepatocarcinogenicity of 2-NP based on damage to nucleic acids from the intracellular generation of reactive forms of oxygen and/or the 2-NP free radical.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1988

Inhibition by dietary ethanol of experimental colonic carcinogenesis induced by high-dose azoxymethane in F344 rats.

Epidemiological studies have shown an association between consumption of alcoholic beverages and increased occurrence of large bowel carcinoma, but studies in experimental models of colonic carcinogenesis have produced conflicting results. We assessed the effects of chronic dietary ethanol consumption during the preinduction and induction phase (period of acclimatization and carcinogen administration) in a high-dose azoxymethane-treated rat model (14 mg/kg/wk for 10 wk). Ten-wk-old male Fischer 344 rats were given 33% of calories as ethanol or no ethanol (controls). Pair-feeding with Lieber-DeCarli-type liquid diets provided comparable total carbohydrates, proteins, fats, and calories. After 3 wk of dietary acclimatization, injections of azoxymethane (AOM) were given s.c. to all rats in Wk 1 to 10. At necropsy in Wk 25, dramatic suppression of gastrointestinal tumorigenesis was evident in the ethanol-fed group: the prevalence of colonic tumors was 5% as compared with 91% in controls; and the prevalence of small bowel tumors was 0% versus 74% (P less than 0.0001). In an analogous study of [14C]AOM metabolism, exhaled 14CO2 was decreased in the ethanol-fed rats, indicating suppression of AOM metabolism. Similarly, in the ethanol-fed rats the levels of the DNA adducts O6-methylguanine and 7-methylguanine 24 h after AOM injection were reduced in the colonic mucosa to 14 +/- 7% and 61 +/- 11% of controls and in the liver to 80 +/- 9% and 86 +/- 6 of controls. By contrast, rats changed from the ethanol diet to no-ethanol diet for 12 h prior to the dose of [14C]AOM metabolized the carcinogen at a faster rate than controls, indicating loss of suppression with cessation of ethanol intake along with induction of metabolizing enzymes; DNA adduct levels were reduced in the colonic mucosa to 90 +/- 13% and 76 +/- 9% of controls and in the liver to 81 +/- 6% and 85 +/- 3% of controls. Our findings indicate that dietary ethanol during the preinduction and induction phase of the AOM model dramatically inhibits tumorigenesis, even with high dosage of carcinogen, and suggest that: (a) inhibition of tumorigenesis may result from suppression of metabolic activation of AOM and the consequent reduced formation of DNA adducts during the induction (initiation) phase of the model; (b) these anti-initiation effects of ethanol are unrelated to the epidemiological association between consumption of alcoholic beverages and large bowel cancer; and (c) mechanisms of action of agents found to modulate carcinogenesis in experimental models should be determined before the results can be generalized to human beings.

Alkylation↗
PubMed1987

Effects of chronic dietary ethanol on in vivo and in vitro metabolism of methylazoxymethanol and on methylazoxymethanol-induced DNA methylation in rat colon and liver.

We examined the metabolism of 14C-labeled methylazoxymethanol (MAM) in male F344 rats pair-fed for 21.0 days either a liquid control diet, an isocaloric liquid diet containing 6.6% ethanol by volume (continuous ethanol diet), or the ethanol diet for 20.5 days followed by the control diet for 0.5 day (interrupted ethanol diet). Compared to rats fed the control liquid diet, metabolism of [1,2-14C]MAM acetate to exhaled 14CO2 was inhibited by 25 to 42% in rats fed the continuous ethanol diet, but was initially stimulated by 90% in rats given the interrupted ethanol diet. MAM-induced DNA methylation, as reflected in 7-methylguanine and O6-methylguanine content 24 h after carcinogen administration, was inhibited in the colon mucosae of rats fed the interrupted ethanol diet by 52 to 54%, and an even greater inhibition (71 to 86%) of DNA methylation occurred in the colon mucosae of rats fed the continuous ethanol diet. Liver DNA methylation was significantly inhibited (by 32 to 42%) only in those rats fed the continuous ethanol diet. Liver microsomes isolated from rats fed the 3 diets metabolized MAM to formic acid and methanol in vitro, but liver microsomes from rats fed the continuous ethanol diet were 12 to 15 times more active than liver microsomes from rats fed the control diet. Liver microsomes isolated from rats fed the interrupted ethanol diet were only 3 to 5 times more active in MAM metabolism than liver microsomes from rats fed the control diet, indicating very rapid turnover of the ethanol-induced enzyme(s) catalyzing the oxidation of MAM. Although chronic ethanol feeding enhanced the activity of liver microsomes for MAM metabolism, ethanol was found to inhibit the reaction competitively. Hepatocytes isolated from rats fed the continuous ethanol diet were considerably more sensitive to MAM-induced unscheduled DNA synthesis than hepatocytes isolated from rats given the control liquid diet, indicating that the stimulation of MAM metabolism by dietary ethanol results in increased DNA damage, observable in an in vitro system. Thus, the increased metabolic activation of MAM, due to enzyme induction by ethanol which is observed in vitro, is not reflected in increased liver DNA methylation in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Alcoholism↗
PubMed1987

Effects of timing and quantity of chronic dietary ethanol consumption on azoxymethane-induced colonic carcinogenesis and azoxymethane metabolism in Fischer 344 rats.

Epidemiological studies have shown an association between consumption of alcoholic beverages and carcinoma of the large bowel, but studies in experimental models of colonic carcinogenesis have yielded conflicting results. We assessed the effects on azoxymethane-induced colonic carcinogenesis of both timing of chronic dietary ethanol consumption relative to carcinogen administration and quantity of ethanol consumption. Ten-week-old male Fischer 344 rats were given 11%, 22%, or 33% of calories as reagent ethanol or no ethanol by pair feeding with Lieber-DeCarli-type liquid diets providing comparable total carbohydrates, proteins, fats, and calories. Ten weekly s.c. injections of the bowel carcinogen azoxymethane (AOM), 7 mg/kg, were given to all rats in weeks 1-10. Three experimental groups were given their respective ethanol diet during acclimatization and AOM administration (preinduction and induction phases) and then were given the no-ethanol diet from week 11 until sacrifice in week 26 (postinduction phase). Three other groups received the no-ethanol diet during acclimatization and AOM administration and then were changed to their respective ethanol diet until sacrifice. The control AOM group received the no-ethanol diet throughout the study. Suppression of colonic tumorigenesis occurred in the groups with high levels of chronic dietary ethanol consumption during acclimatization and AOM administration: in the 33% and 22% diet groups, the prevalence of colonic tumors was 3% and 20% as compared with 50% in control (P less than 0.001 and P less than 0.02, respectively). Tumorigenesis in the left colon was more affected than in the right colon, as tumor prevalence in the left colon was decreased in both the 33% and 22% diet groups (0% in both versus 24% in control, P less than 0.005), whereas prevalence in the right colon was decreased only in the 33% diet group (3% versus 38%, P less than 0.001). By contrast, prevalence of colonic tumors in the 11% diet group was not significantly different from control. Chronic dietary ethanol consumption after AOM administration had no effect on tumor outcome, regardless of quantity of consumption. In an analogous study of [14C]AOM metabolism in rats fed the 33% diet during acclimatization and AOM administration, 14CO2 was exhaled at a slower rate than in rats fed no-ethanol diet (P = 0.05), indicating suppression of AOM metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Enhancement of rat liver microsomal metabolism of azoxymethane to methylazoxymethanol by chronic ethanol administration: similarity to the microsomal metabolism of N-nitrosodimethylamine.

We compared the metabolism of azoxymethane (AOM) and of N-nitrosodimethylamine (NDMA) by liver microsomes obtained from male F344 rats pair-fed for 3 weeks either a control liquid diet or an isocaloric liquid diet containing ethanol at a concentration of 6.6% by volume. High-performance liquid chromatographic analysis of the products of the microsomal metabolism of AOM showed that methylazoxymethanol was the only primary metabolite. While the formation of small (less than 4% of methylazoxymethanol) quantities of methanol and formaldehyde could also be detected in this reaction, these products could be accounted for almost entirely by the spontaneous decomposition of methylazoxymethanol. With NDMA as the substrate in the incubation system, the formation of methylamine, formaldehyde, methanol, and an additional, as yet unidentified metabolite was detected. Liver microsomes obtained from rats fed the ethanol-containing diet up to the time of sacrifice were 12-18 times more active in the metabolism of both AOM and NDMA than liver microsomes obtained from rats fed the control, ethanol-free diet for the same period. When rats fed the ethanol diet for 20.5 days were fed the control diet for 0.5 days and then sacrificed, only a 2- to 3-fold increase in the metabolism of both AOM and NDMA by liver microsomes was observed, indicating that cessation of ethanol intake results in a rapid decrease of the ethanol-induced metabolic enzymes. Hepatocytes isolated from ethanol-fed rats showed a significantly enhanced sensitivity to AOM- as well as to NDMA-induced unscheduled DNA synthesis, indicating that the increased rate of microsomal metabolism induced by ethanol is associated with enhanced carcinogen activation in vitro. The metabolism of AOM and NDMA by liver microsomes was inhibited to similar extents by carbon monoxide, pyrazole, sodium azide, aminoacetonitrile, imidazole, and ethanol. In addition, both ethanol and NDMA were found to inhibit competitively the microsomal metabolism of AOM. These results suggest that AOM and NDMA are metabolized by very similar, indeed perhaps the same rat liver microsomal enzyme(s).

Animals↗
PubMed1987

Differential effects of 4-iodopyrazole and 3-methylpyrazole on the metabolic activation of methylazoxymethanol to a DNA methylating species by rat liver and rat colon mucosa in vivo.

Using a hybrid ion-exchange reverse phase HPLC system, we found that F344 rat liver microsomes, in the presence of an NADPH-generating system, can metabolize methylazoxymethanol (MAM), a colon and liver carcinogen, to methanol and formic acid. This is in contrast to the spontaneous decomposition of MAM which yields methanol and formaldehyde. The metabolism of MAM by rat liver microsomes is sensitive to inhibition by carbon monoxide as well as to inhibition by 3-methylpyrazole (3-MeP) and 4-iodopyrazole (4-IP), with 4-IP being more potent in this respect than 3-MeP. Pretreatment of rats with 4-IP decreased the level of MAM acetate-induced DNA methylation in both the liver and the colon mucosa. In contrast, pretreatment with 3-MeP decreased MAM acetate-induced liver DNA methylation, but increased DNA methylation in the colon mucosa. This differential effect of the two compounds on DNA methylation in the liver and the colon suggests that different enzymes are responsible for activation of the carcinogen in the two organs.

Alcohol Dehydrogenase↗
PubMed1987

Enhanced mutagenicity of 2-nitropropane nitronate with respect to 2-nitropropane--possible involvement of free radical species.

The mutagenicities of the neutral and the anionic (nitronate) forms of 2-nitropropane (2-NP) were compared in S. typhimurium strains TA98, TA100 and TA102. The latter is a special strain sensitive to compounds producing oxidative damage to DNA at thymine-adenine base pair loci. Neutral 2-NP was not mutagenic in TA98, and produced significant mutagenic response in TA100 and TA102 only at levels of 55 mumoles/plate. In contrast, 2-NP nitronate was significantly mutagenic in TA98 at 14 mumoles/plate and, in strains TA100 and TA102, at approximately 4 mumoles/plate. Inclusion of S9 slightly increased the mutagenicity of 2-NP nitronate in TA102, but decreased it in TA100. The mutagenic response in TA102, but not in TA100, was inhibited by DMSO, a scavenger of hydroxyl radicals, in a dose-dependent manner. When 2-NP nitronate was incubated with thymidine and horseradish peroxidase-H2O2, the formation of 2-NP dimer (2,3-dimethyl-2,3-dinitrobutane), a condensation product of 2-NP free radicals, was observed. This was accompanied by 2-NP nitronate-dependent oxidation of thymidine to thymidine glycol, thymine glycol, 5-hydroxymethyldeoxyuridine and thymine, indicating that hydroxyl radicals and/or other reactive oxygen species capable of causing thymidine damage are also formed in this reaction. As a working hypothesis, we suggest that the genotoxicity of 2-NP nitronate may be due to the generation of DNA-damaging reactive forms of oxygen or 2-NP free radicals.

Alkanes↗