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Radionuclides and heavy metals
Published in Rym Salah-Tazdaït, Djaber Tazdaït, Phyto and Microbial Remediation of Heavy Metals and Radionuclides in the Environment, 2022
Rym Salah-Tazdaït, Djaber Tazdaït
Hg2+ and Hg0 can interact with carbon to form organic Hg compounds such as monomethylmercury, dimethylmercury, ethylmercury, and phenyl-mercury. Monomethylmercury (MeHg) is the most frequently encountered in the environment and can be easily accumulated in the tissues of organisms, including humans, causing severe toxic effects. MeHg can result from both abiotic and biotic processes. The biotic process is dominant in the environment and occurs by the action of soil and aquatic microorganisms, mainly in anaerobic conditions. This process consists mainly of transferring a methyl group from a donor molecule (methylcobalamin) to mercuric cation through the acetyl-coenzyme A pathway (Ma, Du, and Wang 2019, 1901). It is worth noting that Hg0 can be formed from MeHg (Lehnherr and St. Louis 2009, 5692) or Hg2+ (Amyot, Mierle, Lean, and McQueen 1994, 2366) through abiotic photoreduction, which occurs in aquatic media.
The Biomethylation and Cycling of Selected Metals and Metalloids in Aquatic Sediments
Published in Renato Baudo, John P. Giesy, Herbert Muntau, Sediments:, 2020
Methylcobalamin appears to be the only biological methylating agent capable of transferring a methyl group to the inorganic Hg+2 ion. It can transfer methyl groups as a carbanion (CH3−) and a methyl radical (CH3·) to produce CH3Hg+ and (CH3)2Hg under both aerobic and anaerobic conditions (Wood et al. 1968, Hill et al. 1970, Adin & Espenson 1971, Neujahr & Bertilsson 1971, Schrauzer et al. 1971, 1973, Wood 1971, 1974, 1987, Ridley et al. 1977). Therefore, methylcobalamin serves as the methyl donor in the bacterial methylation of Hg and other heavy metals (Shapiro & Schlenk 1965, Wood et al. 1968, Neujahr & Bertilsson 1971, Wood 1974, 1975). Significant quantities of methylcobalamin may be available in the sediments because it is a common coenzyme in both aerobic and anaerobic bacteria (Wood 1972).
Precision Medicine for Alzheimer’s Disease Prevention
Published in Shaker A. Mousa, Raj Bawa, Gerald F. Audette, The Road from Nanomedicine to Precision Medicine, 2020
Cara L. Berkowitz, Lisa Mosconi, Olivia Scheyer, Aneela Rahman, Hollie Hristov, Richard S. Isaacson
Similar to APOE, MTHFR genotype status may allow for targeted AD prevention interventions. B-vitamin supplementation (cyanocobalamin, folic acid, and B6) has been shown to slow cognitive decline in individuals with elevated homocysteine levels [55, 56]. Several trials have studied a combination of B vitamins to determine whether lowering homocysteine can impact cognitive function and/or brain pathology [55]. While there is limited evidence thus far, individuals with one or more MTHFR polymorphisms may potentially benefit from genotype-specific recommendations. For example, as individuals with certain MTHFR polymorphisms have decreased catalytic ability of the MTHFR protein, replacing the traditional B-vitamins with their methylated counterparts (methylcobalamin for cyanocobalamin and methyltetrahydrofolate [5-MTH] for folic acid) that do not require hepatic conversion to active forms may increase the outcomes. One study demonstrated that 5-MTH supplementation in individuals with C677T and A1298C polymorphisms significantly increased the serum folate concentration when compared to folic acid, but it did not result in differences in the serum homocysteine concentration [57]. Additional studies evaluating the impact of methylated B-vitamins for specific MTHFR polymorphisms and AD risk may therefore help to advance the field of precision medicine for AD prevention.
Mercury methylation by anaerobic microorganisms: A review
Published in Critical Reviews in Environmental Science and Technology, 2019
Ming Ma, Hongxia Du, Dingyong Wang
Early studies of Hg methylation by anaerobic microorganisms were mainly focused on methanogens. This is because methanogens produce a structurally vitamin B12-like derivative, which produces methylcobalamin. Methylcobalamin was previously considered as the only methyl donor for Hg(II), and hence methanogens were the major bacterial subjects in early Hg methylation studies. Interestingly, the methylation capability of methanogens has gone through a conflicted process as first confirmed, then almost negated, and finally reaffirmed to have ‘robust capacities’.