Biomethylation, of selenium

Chasteen TG, R Bentley (2002) Biomethylation of selenium and tellurium microorganisms and plants. Chem Rev 103 1-25. 

Reviews of the extensive biochemistry of selenium in bacteria are available 108,109 ug fenler for Disease Control summaries for Se toxicology are also available110 A more recent review of the processes of biomethylation of selenium and tellurium in microorganisms and plants is also available.111... 

Fig. 8. Proposed mechanism for the biomethylation of selenium. Adapted from Reamer and Zoller (106). Copyright 1980 by the American Association for the Advancement of Science. Dashed arrows represent pathways proposed by the present authors.

Carvalho KM, Gallardo-Williams MT, Benson RF, Martin DF (2003) Effects of selenium supplementation on four agricultural crops. J Agric Food Chem 51 704-709 Chasteen TG, Bentley R (2002) Biomethylation of selenium and tellurium microorganisms and plants. Chem Rev 103 1-25... 

At present, arsenic, mercury, selenium, and tin have been unequivocally established as undergoing biomethylation. There is substantial, though not completely unequivocal, evidence for the biomethylation of lead, tellurium, and thallium, at least under laboratory conditions. Various... 

The amount of selenium contributed to the air by other sources is not known. Microbial action within the soil may also contribute selenium to the air (Fishbein 1983). Selenium biomethylation volatilizes about 3,000 tons of selenium per year into the atmosphere, which eventually returns to earth in rainfall (NAS 1976a). Volcanic gas is suspected to be the major natural source of atmospheric selenium. Certain plants metabolize inorganic selenium compounds to volatile selenium in the forms of dimethyl selenide (Lewis et al. 1971) and dimethyl diselenide (Evans et al. 1968). Animals are also capable of volatilizing selenium and releasing dimethyl selenide in expired air (Schultz and Lewis 1940). 

Gao S, Tanji KK. 1995. Model for biomethylation and volatilization of selenium from agricultural evaporation ponds. J Environ Qual 24 191-197. 

The selenium species that are drawing most attention are Se(IV) and Se(VI) in water and sediments, and the biomethylated products (dimethylselenide and dimethyldi-selenide) that are spread into the environment (Camara et al. 1995). Se-species in food (including Se-cysteine and other species in yeast) are in the limelight (Crews 1998) because of their beneficial effect on human health and their increasing use as nutraceuticals. 

Biomethylation. Methylation and methyl transfer are important reactions in the organic metabolism of organisms (Mudd, 1973) and it has been known for some time that dimethyl selenide, dimethyl telluride and di- and trimethyl arsine are biosynthetic products of the metabolism of inorganic compounds selenium, tellurium and arsenic, respectively, by microorganisms (Challenger, 1951). 

Volatilisation is the microbial biochemical process that methylates heavy metals. Several methyl-metal complexes have significant vapour pressures at room temperature. In this way metals methylated by the microbes just boil ofF. The best known metals to be treated in this fashion are mercury, selenium, tellurium, arsenic and tin. Much of the biomethylation pathway is unknown it appears to be very complex [26]. As a result of the complexity of the volatilisation process it has not been used in effluent treatment and is not discussed further herein. 

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