The inhibitory effects of nitrous oxide and methylmercury in vivo on methionine synthase (EC 2.1.1.13) activity in the brain, liver, ovary and spinal cord of the rat

Hair Mercury Level is Associated with Anemia and Micronutrient Status in Children Living Near Artisanal and Small-Scale Gold Mining in the Peruvian Amazon

Anemia has been widely studied in global health contexts because of severe nutritional deficiency, and more recently, inflammatory status, but chemical exposures are rarely considered. Until recently, "anemia" was used synonymously with "iron deficiency anemia (IDA)" in global health settings. However, only 50% of anemia cases worldwide are IDA. Environmental toxicology studies of anemia risk have generally focused on populations in developed countries, albeit with high exposure to environmental toxicants, such as lead or cadmium. In the developing world, toxicant exposures commonly coexist with other risk factors for anemia. In particular, artisanal and small-scale gold mining (ASGM) communities are at risk for dietary methylmercury exposure through contaminated fish consumption, and for anemia due to food insecurity and infectious and chronic diseases. Here, we report analysis of total hair mercury content, hemoglobin, and serum micronutrient levels in children < 12 years of age (N = 83) near ASGM in the Peruvian Amazon. Forty-nine percent (N = 29/59) of those aged < 5 years were anemic (< 11 g/dL) and 52% (N = 12/23) of those aged 5-11 years (< 11.5 g/dL). Few children were stunted, wasted, or micronutrient deficient. Median total hair mercury was 1.18 μg/g (range: 0.06-9.70 μg/g). We found an inverse association between total mercury and hemoglobin (β = -0.12 g/dL, P = 0.06) that persisted (β = -0.14 g/dL, P = 0.04) after adjusting for age, sex, anthropometrics, and vitamin B₁₂ in multivariate regression. This study provides preliminary evidence that methylmercury exposure is associated with anemia, which is especially relevant to children living near ASGM.

Design, synthesis, and enzyme kinetics of novel benzimidazole and quinoxaline derivatives as methionine synthase inhibitors

Methionine synthase catalyzes the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine, producing methionine and tetrahydrofolate. Benzimidazole and deazatetrahydrofolates derivatives have been shown to inhibit methionine synthase by competing with the substrate 5-methyltetrahydrofolate. In this study, a novel series of substituted benzimidazoles and quinoxalines were designed and assessed for inhibitory activity against purified rat liver methionine synthase using a radiometric enzyme assay. Compounds 3g, 3j, and 5c showed the highest activity against methionine synthase (IC₅₀: 20 μM, 18 μM, 9 μM, respectively). Kinetic analysis of these compounds using Lineweaver-Burk plots revealed characteristics of mixed inhibition for 3g and 5c; and uncompetitive inhibition for 3j. Docking study into a homology model of the rat methionine synthase gave insights into the molecular determinants of the activity of this class of compounds. The identification of these drug-like inhibitors could lead the design of the next generation modulators of methionine synthase.

The inactivation of methionine synthase in isolated rat hepatocytes by sodium nitroprusside

Methionine synthase, the enzyme that catalyses the transfer of a methyl group from 5-methyl tetrahydrofolate to homocysteine via the cofactor methylcobalamin, is one of the two established mammalian enzymes that utilise a biologically active vitamin B-12 derivative. Through its substrates, products and downstream metabolites, methionine synthase is directly involved in the sulphur amino acid pathways, polyamine biosynthesis, biological methylations and one-carbon-unit transfers. Rat liver methionine synthase was shown to be inactivated by the nitric oxide donor sodium nitroprusside. The inactivation occurred during the treatment of isolated rat hepatocytes in a time-dependent and dose-dependent manner with an apparent IC50 value of 170 microM. Highly purified rat liver methionine synthase was inactivated in a partially irreversible manner with an apparent IC50 value of 10 microM. The inactivation has been attributed to nitric oxide released by sodium nitroprusside. Since biomolecules possessing transition state metals are targets for nitric oxide, the possibility of a nitric oxide-cobalamin interaction could explain the observed inactivation. Nitric oxide is directly involved in different aspects of liver metabolic functions both under physiological and pathological conditions like sepsis and inflammation. The nitric-oxide-induced inactivation of methionine synthase could offer a rational explanation for the cellular and cytotoxic effects of this highly reactive molecule.

The effect of nitrous oxide on S-adenosylmethionine levels in mouse brain

Mice were exposed to nitrous oxide (50%) for up to 24 h and S-adenosylmethionine (SAMe) levels measured in corpus striatum and cerebellum, areas with high and low catecholamine turnover rates, respectively. After 4 h, levels were 21 and 8% and after 6 h, 33 and 14% lower than controls in striatum and cerebellum, respectively. Thus, the effect was more pronounced in corpus striatum, the area with the presumed higher rate of catecholamine O-methylation. With continued exposure to nitrous oxide SAMe concentrations in the two areas returned to nearly normal at 24 h. The observation that levels did not continue to decline, and even returned towards control levels, while animals were still in the presence of the gas suggests that a mechanism other than that of methionine synthase inhibition may have been responsible for the initial effect. Alternatively, some other source of SAMe may have become available to compensate for the inhibition of the enzyme.