Studies on methanol toxicity and formate metabolism in isolated hepatocytes. The role of methionine in folate-dependent reactions

Folate as an Adjuvant Therapy in Methanol Poisoning

Folate and its derivatives have long been used as an adjunctive treatment in methanol poisoning. Methanol is ultimately metabolized to formate, the toxic compound. The accumulation of formate can lead to acidemia, retinal damage, visual impairment, and death. Formate is converted to carbon dioxide and water in a folate-dependent manner, and folate is often given in cases of methanol poisoning. In this paper, the evidence for folate as an adjunctive therapy in methanol poisoning is reviewed.

The toxicity of methanol

Methanol toxicity in humans and monkeys is characterized by a latent period of many hours followed by a metabolic acidosis and ocular toxicity. This is not observed in most lower animals. The metabolic acidosis and blindness is apparently due to formic acid accumulation in humans and monkeys, a feature not seen in lower animals. The accumulation of formate is due to a deficiency in formate metabolism which is, in turn, related, in part, to low hepatic tetrahydrofolate (H4 folate). An excellent correlation between hepatic H4 folate and formate oxidation rates has been shown within and across species. Thus, humans and monkeys possess low hepatic H4 folate levels, low rates of formate oxidation and accumulation of formate after methanol. Formate, itself, produces blindness in monkeys in the absence of metabolic acidosis. In addition to low hepatic H4 folate concentrations, monkeys and humans also have low hepatic 10-formyl H4 folate dehydrogenase levels, the enzyme which is the ultimate catalyst for conversion of formate to carbon dioxide. This review presents the basis for the role of folic acid-dependent reactions in the regulation of methanol toxicity.

Daunorubicin-induced mammary tumors in the rat

Eleven of 24 female Sprague-Dawley rats given a single i.v. injection of daunorubicin (10 mg/kg) developed mammary tumors within 8 months after the injection. Four of 12 rats given an intramammary injection of daunorubicin (4 or 8 micrograms) developed five mammary tumors in the injected area within 6.5 months of injection. Tissue distribution studies using tritiated daunorubicin revealed that the liver, kidney, lung, heart, and intestine had higher daunorubicin concentrations than mammary tissue during the first 24 h after i.v. injection. However, depletion of the drug from the internal organs was more rapid than from mammary tissue. Differences in ability to metabolize daunorubicin were compared in homogenates of isolated mammary epithelial cells and hepatocytes by high-performance liquid chromatography: after 90 min, hepatocytes metabolized about 70% of daunorubicin, whereas mammary epithelial cells did not metabolize the drug. Tritiated daunorubicin injected directly into rat mammary gland showed no metabolism in 24 h, and the drug did not get into the circulation. These results suggest that retention of daunorubicin because of the inability of mammary tissue to metabolize the drug is a cause of drug-induced mammary tumors in female Sprague-Dawley rats.

[The effect of nutrition on the metabolism of glycine]

It can be concluded on the base of data from literature and of our own results, that the metabolism of glycine is localized mainly in liver tissue. The main catabolic pathway of nutritional glycine proceeds via the glycine-cleavage enzyme, serinehydroxymethyltransferase and serinedehydratase or serine-pyruvate aminotransferase and via serine and pyruvate. The physiological significance of this metabolic pathway is estimated. The catabolism via the C1-pool is limited by the regeneration rate of tetrahydrofolic acid. Other pathways (via glyoxylate, SHEMIN-cycle, aminoacetone cycle) are only of minor significance for the catabolism of glycine originating from food.

Daunorubicin-induced DNA lesions in isolated rat hepatocytes and mammary epithelial cells

Daunorubicin, an anticancer drug, induces primarily mammary adenocarcinoma in Sprague-Dawley rats. We investigated daunorubicin-induced DNA lesions in enzymatically isolated mammary epithelial cells and hepatocytes from 7-8-week-old female Sprague-Dawley rats. Differences were observed in the type and quantity of DNA lesions in mammary epithelial cells and hepatocytes as determined by alkaline elution analysis. DNA single-strand breaks and proteinase-K-sensitive cross-linking lesions were observed in mammary epithelial cells. Hepatocytes appeared to have significantly lower relative frequencies of single-strand breaks than mammary epithelial cells when treated with daunorubicin (1.5-10.0 micrograms/10(6) cells). Hepatocytes displayed two types of cross-link. One form was sensitive to proteinase-K digestion, whereas the other form was insensitive. The metabolism of daunorubicin to the aglycone metabolites was substantially lower in mammary cells than in hepatocytes. However, the total uptake of the drug was similar in these two cell types. A metabolite, 7-deoxydaunorubicinol aglycone, was unable to induce single-strand breaks or cross-linking lesions in mammary epithelial cells. Both cell types exhibited a similar ability to repair radiation-induced single-strand breaks of DNA. However, the mammary cells may be less able to repair daunorubicin-mediated DNA damage. These results revealed that mammary epithelial cells are less able to metabolize the active mutagen/carcinogen, daunorubicin, than are hepatocytes. This, coupled with the observations of greater apparent DNA damage in mammary cells, may be of primary importance in the drug-induced carcinogenicity in the rat mammary tissue.

Inhibition of the low-Km mitochondrial aldehyde dehydrogenase by diethyl maleate and phorone in vivo and in vitro. Implications for formaldehyde metabolism

Formaldehyde can be oxidized primarily by two different enzymes, the low-Km mitochondrial aldehyde dehydrogenase and the cytosolic GSH-dependent formaldehyde dehydrogenase. Experiments were carried out to evaluate the effects of diethyl maleate or phorone, agents that deplete GSH from the liver, on the oxidation of formaldehyde. The addition of diethyl maleate or phorone to intact mitochondria or to disrupted mitochondrial fractions produced inhibition of formaldehyde oxidation. The kinetics of inhibition of the low-Km mitochondrial aldehyde dehydrogenase were mixed. Mitochondria isolated from rats treated in vivo with diethyl maleate or phorone had a decreased capacity to oxidize either formaldehyde or acetaldehyde. The activity of the low-Km, but not the high-Km, mitochondrial aldehyde dehydrogenase was also inhibited. The production of CO2 plus formate from 0.2 mM-[14C]formaldehyde by isolated hepatocytes was only slightly inhibited (15-30%) by incubation with diethyl maleate or addition of cyanamide, suggesting oxidation primarily via formaldehyde dehydrogenase. However, the production of CO2 plus formate was increased 2.5-fold when the concentration of [14C]formaldehyde was raised to 1 mM. This increase in product formation at higher formaldehyde concentrations was much more sensitive to inhibition by diethyl maleate or cyanamide, suggesting an important contribution by mitochondrial aldehyde dehydrogenase. Thus diethyl maleate and phorone, besides depleting GSH, can also serve as effective inhibitors in vivo or in vitro of the low-Km mitochondrial aldehyde dehydrogenase. Inhibition of formaldehyde oxidation by these agents could be due to impairment of both enzyme systems known to be capable of oxidizing formaldehyde. It would appear that a critical amount of GSH, e.g. 90%, must be depleted before the activity of formaldehyde dehydrogenase becomes impaired.

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.

The role of mitochondrial glutathione and cellular protein sulfhydryls in formaldehyde toxicity in glutathione-depleted rat hepatocytes

Depletion of cellular GSH by diethyl maleate (DEM) potentiates CH2O toxicity in isolated rat hepatocytes and it was postulated that this increase in toxicity is due to the further decrease in GSH caused by CH2O in DEM-pretreated hepatocytes (1). The present investigation was conducted to investigate further the effects of CH2O, DEM, and acrolein (a compound which is structurally related to CH2O and DEM) on subcellular GSH pools and on protein sulfhydryl groups (PSH). CH2O caused a decrease in cytosolic GSH but had no effect on mitochondrial GSH either in previously untreated hepatocytes or in DEM-pretreated hepatocytes in which GSH was approximately 25% of control. DEM decreased both cytosolic and mitochondrial GSH but it did not produce toxicity. Neither CH2O (up to 7.5 mM) nor DEM (20 mM) decreased PSH. However, in cells pretreated with 1 mM DEM, CH2O (7.5 mM) decreased PSH and this effect preceded cell death. Acrolein decreased both cytosolic and mitochondrial GSH and it also decreased PSH significantly prior to causing cell death. CH2O and acrolein stimulated phosphorylase alpha activity, indicative of an increase in cytosolic free Ca2+, by a PSH-independent and PSH-dependent mechanism, respectively. These results suggest that the further depletion of cellular GSH by CH2O in DEM-pretreated cells is not due to the depletion of mitochondrial GSH. CH2O toxicity in DEM-pretreated cells is, however, correlated with depletion of PSH. The critical sulfhydryl protein(s) responsible for cell death remain to be more clearly defined.

The role of 4-bromophenol and 4-bromocatechol in bromobenzene covalent binding and toxicity in isolated rat hepatocytes

4-Bromophenol and 4-bromocatechol are formed as metabolites of bromobenzene in vivo and in isolated rat hepatocytes. Both of these metabolites may potentially contribute to the hepatotoxicity of bromobenzene. Bromobenzene metabolism in hepatocytes isolated from phenobarbital-treated rats forms 0.12 to 0.17 mM 4-bromophenol and 4-bromocatechol in 2 hr, with 1 to 3 mM bromobenzene. The role of activated metabolites derived from 4-bromophenol and 4-bromocatechol in bromobenzene covalent binding and toxicity was investigated with isolated hepatocytes in suspension. The covalent binding of the phenol and the catechol was increased four- to eightfold by the addition of unlabeled bromobenzene. Two-hour incubations of 0.25 mM 14C-labeled 4-bromophenol or 4-bromocatechol with hepatocytes isolated from phenobarbital-treated rats resulted, under these conditions, in no significant toxicity, and approximately 4 and 25%, respectively, of the covalent binding associated with bromobenzene itself. Two- and six-hour incubations with higher 4-bromophenol and 4-bromocatechol concentrations demonstrated that 1 to 3 mM substrate concentrations were required for cytotoxicity. These results show that metabolically produced 4-bromophenol and 4-bromocatechol do not play significant roles in the production of bromobenzene cytotoxicity in isolated hepatocytes, and that they contribute only modestly to bromobenzene covalent binding.

Relationships between formaldehyde metabolism and toxicity and glutathione concentrations in isolated rat hepatocytes

The metabolism and toxicity of formaldehyde (CH2O) in isolated rat hepatocytes was found to be dependent upon the intracellular concentration of glutathione (GSH). Using hepatocytes depleted of GSH by treatment with diethyl maleate (DEM), the rate of CH2O (5.0 mM) disappearance was significantly decreased. Formaldehyde decreased the concentration of GSH in hepatocytes, probably by the extrusion of the CH2O-GSH adduct, S-hydroxymethylglutathione. Formaldehyde toxicity was potentiated in cells pretreated with 1.0 mM DEM as measured by the loss of membrane integrity (NADH stimulation of lactate dehydrogenase (LDH) activity) and an increase in lipid peroxidation (formation of thiobarbituric acid-reactive compounds). This potentiation of toxicity was both CH2O concentration-dependent and time-dependent. There was an excellent correlation between the increase in lipid peroxidation and the decrease in cell viability. L-Methionine (1.0 mM) both protected the cells from toxicity caused by the combination of 8.0 mM CH2O and 1.0 mM DEM and increased the cellular GSH concentration. The antioxidants, ascorbate, butylated hydroxytoluene (BHT) and alpha-tocopherol (10, 25 and 125 microM), all exhibited dose-dependent protection against toxicity produced by 8.0 mM CH2O and 1.0 mM DEM. At toxic concentrations of CH2O (10.0-13.0 mM), administered by itself, lipid peroxidation did not increase concomitantly with the decrease in cell viability and the addition of antioxidants (125 microM) did not influence CH2O toxicity. These results suggest that CH2O toxicity in GSH-depleted hepatocytes may be mediated by free radicals as a result of the effect of CH2O on a critical cellular pool of GSH. However, cells with normal concentrations of GSH are damaged by CH2O by a different mechanism.

Effect of acetaldehyde and cyanamide on the metabolism of formaldehyde by hepatocytes, mitochondria, and soluble supernatant from rat liver

Formaldehyde can be metabolized primarily by two different pathways, one involving oxidation by the low-Km mitochondrial aldehyde dehydrogenase, the other involving a specific, glutathione-dependent, formaldehyde dehydrogenase. To estimate the roles played by each enzyme in formaldehyde metabolism by rat hepatocytes, experiments with acetaldehyde and cyanamide, a potent inhibitor of the low-Km aldehyde dehydrogenase were carried out. The glutathione-dependent oxidation of formaldehyde by 100,000g rat liver supernatant fractions was not affected by either acetaldehyde or by cyanamide. By contrast, the uptake of formaldehyde by intact mitochondria was inhibited 75 to 90% by cyanamide. Acetaldehyde inhibited the uptake of formaldehyde by mitochondria in a competitive fashion. Formaldehyde was a weak inhibitor of the oxidation of acetaldehyde by mitochondria, suggesting that, relative to formaldehyde, acetaldehyde was a preferred substrate. In isolated hepatocytes, cyanamide, which inhibited the oxidation of acetaldehyde by 75 to 90%, produced only 30 to 50% inhibition of formaldehyde uptake by cells as well as of the production of 14CO2 and of formate from [14C]formaldehyde. The extent of inhibition by cyanamide was the same as that produced by acetaldehyde (30-40%). In the presence of cyanamide, acetaldehyde was no longer inhibitory, suggesting that acetaldehyde and cyanamide may act at the same site(s) and inhibit the same formaldehyde-oxidizing enzyme system. These results suggest that, in rat hepatocytes, formaldehyde is oxidized by cyanamide- and acetaldehyde-sensitive (low-Km aldehyde dehydrogenase) and insensitive (formaldehyde dehydrogenase) reactions, and that both enzymes appear to contribute about equally toward the overall metabolism of formaldehyde.

The effects of nitrous oxide on cobalamins, folates, and on related events

The anaesthetic gas nitrous oxide (N2O), when inhaled for longer than 6 hr, produces megaloblastic anemia in man. Longer term inhalation, as in addicts, produces a syndrome similar to that due to B12 neuropathy, and long term exposure to low concentrations results in an increased abortion rate and neuropathy, particularly in dental personnel. N2O acts by oxidizing vitamin B12 from the active reduced cob[I]alamin form to the inactive cob[III]alamin form. In turn, this inactivates the enzyme methionine synthetase which requires both B12 and folate as cofactors. In the rat, hepatic methionine synthetase is completely inactivated after 3 hr exposure to a mixture of equal parts of N2O/O2. There is an impared uptake of folate analogues by the liver so that the plasma folate level rises and thereafter there is a considerable loss of folate into the urine. Hepatic folate concentration falls to 25% within 10 days of N2O exposure. There is a failure to synthesize folate polyglutamate (the active folate coenzyme) from all other than formyltetrahydrofolate. As oxidization of the methyl of methionine is an important source of formyl, the failure of methionine synthesis in turn appears to lead to the failure in supply of formate and, hence, a lack of the formylfolate substrate.

Effect of 4-hydroxypyrazole on tryptophan and formate metabolism in isolated rat liver cells

1. 4-Hydroxypyrazole inhibits flux through tryptophan 2.3-dioxygenase in cells. The inhibition is apparently non-competitive with Ki = 0.15 mM. 2. Hydroxypyrazole inhibits the oxidation of formate to CO2 in liver cells. 3. Glycollate, which generates H2O2, stimulates formate oxidation. This process is inhibited by 4-hydroxypyrazole. 4. Methionine stimulates formate oxidation in cells and this stimulation is insensitive to 4-hydroxypyrazole. 5. It is concluded that, in freshly isolated liver cells, formate oxidation proceeds by a pathway involving catalase. In vivo, or when methionine is added to cell incubations, the pathway of oxidation involves tetrahydrofolate, and is insensitive to catalase inhibitors. 6. Methionine at physiological concentrations inhibits the activity of tryptophan 2,3-dioxygenase in isolated liver cells.

Acute toxicity of methanol in the folate-deficient acatalasemic mouse

Formate acidosis is the chief measurable biochemical characteristic of acute methanol toxicity in man. Its marked elevation in the blood stream of primates has been proposed to account for their much greater susceptibility versus rodents to methanol poisoning. Therefore, a study was undertaken to assess whether folic acid deficient (FAD) mice which accumulate formate are much more sensitive to the lethal effects of this alcohol than folic acid sufficient (FAS) mice. Moreover, because some formate is oxidized by catalase-H2O2 in rodents, but not in primates, we also compared the urinary excretion and blood plasma accumulation of formate and the methanol sensitivity of acatalasemic mice. Methanol-dosed C57BL/6Csb (acatalasemic) mice exhibit slightly lower LD50S than CSa (normal catalase) mice, irrespective of their folate state. CSb-FAD mice excreted much more formate and developed higher plasma formate concentrations (11-17 mM) than identically dosed CSa-FAD animals (6 mM). However, in no instance did a folate deficiency produce a large reciprocal decrease in the oral or i.p. LD50 that would be expected from a huge increase (greater than 10-fold) in the 24-h blood plasma formate level. A low methionine (0.2%) intake did not decrease the oral methanol LD50 of CSb-FAD mice, although excess dietary methionine (1.8%) did lower it from 7.1 to 6.4 g/kg. Methanol treated (4 g/kg) Csb-FAD mice excreted 30.8-48.2% of the oral dose as urinary formate, depending on the level of dietary methionine. Csb-FAS and -FAD mice which were given 2 g/kg sodium formate orally (LD50 = 4.7 and 3.7 g/kg) cleared this dose from the blood within 24 h and excreted 58% and 76% of it, respectively, in the urine. Our results indicate that the plasma formate concentration does not correlate well with methanol lethality in Csb-FAS vs. -FAD mice. In addition, urinary excretion, not oxidation, is the primary means by which mice, and probably rats, eliminate high levels of blood formate. Since the Csb-FAD mouse attains high plasma formate levels and low blood pH-values similar to those which have been reported for methanol poisoned monkeys, it appears to be of value as an inexpensive small animal model for further studies of lethal methanol toxicity and the contribution of formate to this process.

Dose-dependent toxicity of ethylene glycol monomethyl ether vapour in the rat

Wistar male rats were exposed by inhalation to 50, 100 or 400 ppm of ethylene glycol monomethyl ether (EGME) for 1 to 2 weeks. The overall hepatic drug oxidation reactions, O-deethylation of 7-ethoxycoumarin and 7-ethoxyresorufin and cytochrome P-450 content were only slightly affected by the EGME exposures. NADPH cytochrome c reductase activity showed a tendency toward a dose-dependent decrease in liver, the activity being 73% and 64% of that in the controls after one and two weeks of exposure, at 400 ppm respectively. UDP glucuronosyl transferase activity exhibited a dose-dependent enhancement in liver microsomes after exposure for two weeks to EGME. The enhancement was 1.3- 1.7- and 3.0 fold with exposure to 50, 100 and 400 ppm of EGME respectively. After exposure for one week the UDPglucuronosyltransferase activity in kidney microsomes was similarly enhanced. A dose-related increase in measurable UDPglucuronosyltransferase activity was also obtained in Triton X-100 treated hepatic microsomes. GSH levels of the liver and kidneys in EGME treated animals showed a tendency towards a dose-dependent increase. The activities of low-Km and high-Km aldehyde dehydrogenases in liver were decreased 6 - 14% of that in the controls with exposure to 400 ppm of EGME when glycolaldehyde was used as a substrate. Serum alanine aminotransferase activity was not influenced by inhalation exposures to EGME.