Testicular Injury to Rats Fed on Soybean Protein-Based Vitamin B12-Deficient Diet Can Be Reduced by Methionine Supplementation

We have previously reported that rats fed on a vitamin B12 (B12)-deficient diet containing 180 g soybean protein per kg diet showed marked histologic damage in their testes. In this paper, we report the effect of B12-deficiency on B12-dependent methionine synthase in the rats' testes and the effect of methionine supplementation of the diet on testicular damage. Rats were fed the soybean protein-based B12-deficient diet for 120 d. We confirmed that those rats were in serious B12-deficiency by measuring urinary methylmalonic acid excretion and B12 content in tissues. Methionine synthase activity in the testis of the B12-deficient rats was less than 2% of that in B12-supplemented (control) rats. To complement disrupted methionine biosynthesis, methionine was supplied in the diet. A supplement of 5 g D,L-methionine per kg diet to the B12-deficient diet did not affect urinary methylmalonic acid excretion of B12-deficient rats. The testicular histology of rats fed the methionine-supplemented B12-deficient diet was almost indistinguishable from that of control rats. Thus, we conclude that the lowered testicular methionine synthase activity is the primary cause of the histologic damage due to B12-deficiency and that methionine supplementation to the diet can reduce the damage. These findings would indicate the importance of the methionine synthase activity, especially for testicular function.

Activation of methionine synthase by insulin-like growth factor-1 and dopamine: a target for neurodevelopmental toxins and thimerosal

Methylation events play a critical role in the ability of growth factors to promote normal development. Neurodevelopmental toxins, such as ethanol and heavy metals, interrupt growth factor signaling, raising the possibility that they might exert adverse effects on methylation. We found that insulin-like growth factor-1 (IGF-1)- and dopamine-stimulated methionine synthase (MS) activity and folate-dependent methylation of phospholipids in SH-SY5Y human neuroblastoma cells, via a PI3-kinase- and MAP-kinase-dependent mechanism. The stimulation of this pathway increased DNA methylation, while its inhibition increased methylation-sensitive gene expression. Ethanol potently interfered with IGF-1 activation of MS and blocked its effect on DNA methylation, whereas it did not inhibit the effects of dopamine. Metal ions potently affected IGF-1 and dopamine-stimulated MS activity, as well as folate-dependent phospholipid methylation: Cu(2+) promoted enzyme activity and methylation, while Cu(+), Pb(2+), Hg(2+) and Al(3+) were inhibitory. The ethylmercury-containing preservative thimerosal inhibited both IGF-1- and dopamine-stimulated methylation with an IC(50) of 1 nM and eliminated MS activity. Our findings outline a novel growth factor signaling pathway that regulates MS activity and thereby modulates methylation reactions, including DNA methylation. The potent inhibition of this pathway by ethanol, lead, mercury, aluminum and thimerosal suggests that it may be an important target of neurodevelopmental toxins.

Acute valproate administration impairs methionine metabolism in rats

Valproate (VPA) is a drug widely used to treat epilepsy, but it has serious adverse effects including hepatotoxicity, teratogenicity and antifolate activity. The mechanism underlying VPA toxicity is unclear although an interaction with folate and other metabolites involved in methionine metabolism has been suggested. The present study was undertaken to evaluate potential changes in the metabolic function of the methionine cycle after acute exposure to a single dose of valproate. Female Wistar rats (n = 30) were treated with 400 mg/kg of VPA. Different groups of six rats were killed at 1 (t1), 3 (t3), 6 (t6), 9 (t9), and 24 (t24) hours after the injection. One group of rats was untreated (n = 6) and was considered the control group. The most pronounced effects of VPA administration were observed 1 h after drug injection. VPA induced a 56% reduction in methionine adenosyltransferase activity and a 54% reduction in plasma vitamin B-6. Increases in the hepatic concentration of S-adenosylhomocysteine and oxidized glutathione, and a reduction in the S-adenosylmethionine/S-adenosylhomocysteine transmethylation ratio also occurred at 1 h. All of these alterations, however, were normalized within 24 h, parallel with a decrease in serum VPA concentration. The acute effects of VPA suggest that the alterations in the methionine cycle could be the common mechanism underlying the hepatotoxic, teratogenic and antifolate effects of the drug.

Methionine synthase activity and sulphur amino acid levels in the rat liver tumour cells HTC and Phi-1

Methionine dependence has been reported in tumour cells and suggested as a possible target for chemotherapeutic drugs. The underlying defect has not been extensively researched, nor have levels of sulphur amino acids been examined in these cells. This study compared two rat liver tumour cell lines. One was found to be methionine dependent (HTC) and the other found to be methionine independent (Phi-1). The methionine-dependent cell line (HTC) was discovered to contain markedly less methionine synthase activity, the enzyme activity being less responsive to methionine concentration than in the methionine-independent cells (Phi-1). HTC cells had lower cysteine requirements and contained larger concentrations of reduced glutathione (GSH) and taurine than the Phi-1 cells. Also, in contrast to Phi-1 cells, no glutathione was found in the media of the HTC cells, although large quantities of cysteinylglycine were detected. These results suggested that differences in methionine synthase activity might be partly responsible for methionine dependence and that methionine-dependent cells may have different metabolic requirements for other sulphur amino acids.

Co-ordinate variations in methylmalonyl-CoA mutase and methionine synthase, and the cobalamin cofactors in human glioma cells during nitrous oxide exposure and the subsequent recovery phase

We investigated the co-ordinate variations of the two cobalamin (Cbl)-dependent enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase (MCM), and measured the levels of their respective cofactors, methylcobalamin (CH3Cbl) and adenosylcobalamin (AdoCbl) in cultured human glioma cells during nitrous oxide exposure and during a subsequent recovery period of culture in a nitrous oxide-free atmosphere (air). In agreement with published data, MS as the primary target of nitrous oxide was inactivated rapidly (initial rate of 0.06 h(-1)), followed by reduction of CH3Cbl (to <20%). Both enzyme activity and cofactor levels recovered rapidly when the cells were subsequently cultured in air, but the recovery was completely blocked by the protein-synthesis inhibitor, cycloheximide. During MS inactivation, there was a reduction of cellular AdoCbl and holo-MCM activity (measured in the absence of exogenous AdoCbl) to about 50% of pre-treatment levels. When the cells were transferred to air, both AdoCbl and holo-MCM activity recovered, albeit more slowly than the MS system. Notably, the regain of the holo-MCM and AdoCbl was enhanced rather than inhibited by cycloheximide. These findings confirm irreversible damage of MS by nitrous oxide; hence, synthesis of the enzyme is required to restore its activity. In contrast, restoration of holo-MCM activity is only dependent on repletion of the AdoCbl cofactor. We also observed a synchronous fluctuation in AdoCbl and the much larger hydroxycobalamin pool during the inactivation and recovery phase, suggesting that the loss and repletion of AdoCbl reflect changes in intracellular Cbl homoeostasis. Our data demonstrate that the nitrous oxide-induced changes in MS and CH3Cbl are associated with reversible changes in both MCM holoactivity and the AdoCbl level, suggesting co-ordinate distribution of Cbl cofactors during depletion and repletion.

Effects of streptozotocin-induced diabetes and of insulin treatment on homocysteine metabolism in the rat

An elevation in the concentration of total plasma homocysteine is known to be an independent risk factor for the development of vascular disease. Alterations in homocysteine metabolism have also been observed clinically in diabetic patients. Patients with either type 1 or type 2 diabetes who have signs of renal dysfunction tend to exhibit elevated total plasma homocysteine levels, whereas type 1 diabetic patients who have no clinical signs of renal dysfunction have lower than normal plasma homocysteine levels. The purpose of this study was to investigate homocysteine metabolism in a type 1 diabetic animal model and to examine whether insulin plays a role in its regulation. Diabetes was induced by intravenous administration of 100 mg/kg streptozotocin to Sprague-Dawley rats. We observed a 30% reduction in plasma homocysteine in the untreated diabetic rat. This decrease in homocysteine was prevented when diabetic rats received insulin. Transsulfuration and remethylation enzymes were measured in both the liver and the kidney. We observed an increase in the activities of the hepatic transsulfuration enzymes (cystathionine beta-synthase and cystathionine gamma-lyase) in the untreated diabetic rat. Insulin treatment normalized the activities of these enzymes. The renal activities of these enzymes were unchanged. These results suggest that insulin is involved in the regulation of plasma homocysteine concentrations by affecting the hepatic transsulfuration pathway, which is involved in the catabolism of homocysteine.

Compartmentation of folate metabolism in rat pancreas: nitrous oxide inactivation of methionine synthase leads to accumulation of 5-methyltetrahydrofolate in cytosol

Folate-dependent one-carbon metabolism and methylation reactions have been implicated in the secretory function of the pancreas. Because vitamin B-12 deficiency perturbs folate metabolism, we determined the effects of nitrous oxide inactivation of methionine synthase on the compartmentation of folate metabolism in rat pancreas. Rats were exposed to an atmosphere of nitrous oxide and oxygen (80 and 20%, respectively) for 18 h; control rats breathed air. Folate coenzyme concentrations were determined by HPLC and Lactobacillus casei microbiological assay of the cytosolic and mitochondrial fractions of pancreas, which contained 62 and 46%, respectively, of the total folate. In pancreas of control rats, cytosolic folates were 5-methyltetrahydrofolate (31% of total folates), tetrahydrofolate (54%) and 5- and 10-formyltetrahydrofolate (6 and 8%, respectively). In the rats exposed to nitrous oxide, cytosolic 5-methyltetrahydrofolate concentrations were significantly greater (59% of total folates) and tetrahydrofolate concentrations were significantly lower (32%) than in controls; however, total cytosolic folate levels were unaffected by nitrous oxide exposure. In controls, mitochondrial folates were composed of 5-methyltetrahydrofolate (9% of total folates), tetrahydrofolate (60%) and 5- and 10-formyltetrahydrofolate (22 and 10%, respectively). Exposure to nitrous oxide led to significantly lower total mitochondrial folates (1.49 +/- 0.18 vs. 0.75 +/- 0.29 nmol/g, control vs. nitrous oxide, P < 0.05). This was due to a significantly lower concentration of tetrahydrofolate and 5-formyltetrahydrofolate, but not of 5-methyl- or 10-formyltetrahydrofolate. The activity of methionine synthase was 85% lower (P < 0.001) in pancreatic extracts of rats exposed to nitrous oxide than in controls. These results show that cytosolic folates accumulate in pancreas as the 5-methyl derivative at the expense of other reduced folates, as happens in liver. However, in contrast to results in liver, the mitochondrial folate concentration was lower in the pancreas of rats exposed to nitrous oxide, and this decline was limited to the 5-formyl- and tetrahydrofolate derivatives.

Folate depletion induced by methotrexate affects methionine synthase activity and its susceptibility to inactivation by nitrous oxide

We compared the effects of methotrexate (MTX) and nitrous oxide on the methionine (Met) synthase system in two variants of a human glioma cell line. The cells were protected from cytotoxic effect of MTX by adding thymidine and hypoxanthine to the cell culture medium. MTX (0-1 microM) was associated with a dose- and time-dependent reduction in 5-methyltetrahydrofolate (5-methyl-THF) in both cell lines. Already after 3 hr of exposure, 5-methyl-THF was reduced by 50% and after additional 48 hr, the level was undetectable. In addition to reduction in folate level, homocysteine (Hcy) remethylation in intact cells was markedly inhibited as judged by an increased export of Hcy from the cells, and Met synthase activity in cell extracts and level of cellular methylcobalamin (CH3Cbl) declined. MTX reduced Hcy remethylation and CH3Cbl level more efficiently than nitrous oxide. In both cell variants, the inactivation of Met synthase by nitrous oxide was almost completely prevented in cells pre-exposed to MTX. This indicates that there is no catalytic turnover in cells exposed to MTX, and emphasizes the importance of the sequence of administration for synergistic effect of this drug combination. In conclusion, our data show that MTX through depletion of 5-methyl-THF reduces both the Met synthase activity and the cellular CH3Cbl level. Moreover, the effect of MTX on the Hcy remethylation is more pronounced than the inhibition caused by nitrous oxide. These observations should be taken into account in studies on MTX pharmacodynamics.

Effects of repeated intermittent exposures to nitrous oxide on central neurotransmitters and hepatic methionine synthetase activity in CD-1 mice

The central neurotransmitters and hepatic methionine synthetase (MS) appear to play an important role in mediating the side effects associated with N2O exposure. Male CD-1 mice were exposed to 0, 50, 500, and 5,000 ppm of N2O 6 hr per day, 5 days a week for 2 or 13 weeks. One day after the last day of exposure, the animals were decapitated and steady state concentrations of norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3-methoxy-4-hydroxy-mandelic acid (VMA), 3-methoxy-4-hydroxyphenyl glycol (MOPEG), dihydroxphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) were determined in six discreet brain regions using electrochemical high-performance liquid chromatography. Hepatic MS activity was measured using a newly developed non-isotopic method. After a 2-week exposure to 5,000 ppm N2O, levels of NE and DA in some brain regions were significantly increased and were accompanied by significant decreases in the levels of their major metabolites. Serotonin levels were significantly decreased in certain brain regions. After the 13-week exposure to 5,000 ppm N2O, levels of NE, DA, and 5-HT significantly increased in the hypothalamus. Hepatic MS activity was not affected at any dose level of N2O used. The alterations in neurotransmitter levels may be related to the reported clinical and behavioral effects associated with N2O misuse or occupational exposures.

Scavenging system developed for the Magill anesthetic circuit for use in the dental office

Numerous potential problems have been associated with long term or occupational exposure to both nitrous oxide and halothane. Despite the lack of firmly established cause-and-effect relationships, particularly in humans, it would seem prudent to use techniques that minimize operator exposure. With this in mind, a scavenging system for use in both conscious sedation and general anesthetic techniques was developed which fulfills the requirements of both general dentists as well as those administering general anesthesia. This paper describes this system and its adaptation to the commonly used Magill circuit. It also briefly reviews the factors involved in potential toxicity caused by long term exposure to nitrous oxide and halothane.