Biomethylation

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

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

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. 

The biosynthesis of mutasterol (65) [45] has recently been determined [46] and, as found with strongylosterol (63), considerable stereoselectivity at C-24 was observed. The metabolism of mutasterol was shown to proceed via three biomethylations of desmosterol (34), epicodisterol (51) (rather than the epimeric codisterol (52) in strongylosterol biosynthesis) and 24,26-dimethyldesmosterol (66) as described in Scheme 9. 

Xestosterol (67), another unconventional sterol, was isolated as the major sterol of the Caribbean and Great Barrier Reef Xestospongia spp. [44], This sterol (Scheme 9) possesses a unique symmetrical double extension at C-26 and C-27 and a 24-methylene substituent. The biosynthesis again involves three successive biomethylations of desmosterol (34), epicodisterol (51) and 25(26)-dehydroaplysterol (68) (Scheme 9), a very similar sequence to that found for strongylosterol (63) and mutasterol (65). However, in this case, complete lack of stereospecificity was observed in the bioalkylation of codisterol (52) and epicodisterol (51). 

The origin of sterols with four and five biomethylations in the side-chain has not been investigated. Sutinasterol (72), the result of four biomethylations, was isolated as essentially the only sterol ( > 98% of the sterol mixture) of a Xestospongia species from Puerto Rico [47]. A trace sterol, 73, is the product of an... 

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... 

Pongratz and Hunmann [19], using differential pulse anodic scanning voltammetry, found low levels of methyl cadmium compounds in the Atlantic Ocean. Levels in the South Atlantic were approximately 700 pg/1, and those in the North Atlantic were below the detection limit of the method, i.e., below 470 pg/1. It is believed that these compounds were formed as a result of biomethylation of inorganic cadmium. 

B. Methyllead Compounds Found in the Natural Environment and Arising from Biomethylation... 

The vast majority of measurements of organolead compounds in the environment do not constitute evidence for biomethylation of lead. Most environmental organic lead comes from incomplete combustion or spillage of methyl- or ethyl-lead gasoline additives (viz tetraalkylleads or TALs). A literature search will produce several hundred TAL or ionic alkyllead results, but few of them are evidence for methylation in or by the environment. 

Discovery of methylleads in the absence of accompanying ethylleads is also not really evidence for biomethylation. Methylleads are sometimes used alone in petrol and ethyl-leads, being less stable, may decay faster than co-existing methylleads. Monomethyllead species are not observed as these are essentially unstable in aquatic media. 

Measurement in pristine prehistoric Antarctic ice has given lead concentrations which, in order to be accounted for, require a natural input of lead in prehistoric times of the order of 105 tonnes per year to the atmosphere. Biomethylation could be responsible for this input63. 

A number of negative reports have also arisen from incubation experiments. It has been pointed out that methylation of MesPb 1 species to Me4Pb may arise through a sulphide-mediated disproportionation68-70. MesPbOAc has been incubated with both sterilized and unsterilized lake sediments and in all cases similar amounts of Me4Pb were evolved, i.e. disproportionation without biomethylation can account for the results71. Use of labelled carbon and lead in a series of attempted biomethylation in cultures produced no evidence of biomethylation but did confirm that sulphide-promoted disproportionation is possible72. 

Huber and coworkers123 also reported biomethylation of Pb2+ and of Me3PbX. They followed the redistribution of Me3PbX in anaerobic cultures (bacteria from the surface of a natural lake gron under N2, or from the anaerobic sediment of a small pond), and observed a rate increase, but less Pb2+ and more Me4Pb were obtained than were expected from equation 4 ... 

A blank and also a sterile solution containing Pb2+ of methyllead compounds showed no Pb content in the methanolic solution after the same treatment. The author concluded that Me4Pb was produced in the biomethylation of Pb2+ by bacteria123. Thompson and Crerar122 also observed that about 0.03% of lead as Pb(NC>3)2 underwent methylation and trimethyllead acetate, (CH3)3PbOAc, was methylated nearly quantitatively in incubation experiments with marine sediments from the British Columbia coastline. 

Jarvie and coworkers conducted an unsuccessful search for lead biomethylation using sediments. They studied modified and abnormal sediments and culture systems, but could not find any definite evidence for lead biomethylation in any of the systems investigated128. 

Biomethylation is the preferred detoxification mechanism for inorganic arsenicals. 

Inorganic arsenicals are oxidized in vivo, biomethylated, and usually excreted rapidly in the urine, but organoarsenicals are usually not subject to similar transformations. 

Organometallic compounds are included as it is becoming increasingly apparent that these compounds occur extensively throughout the ecosystem either as direct pollutants eg alkylead compounds from automobile exhausts or by biomethylation of inorganic metals occurring in sediments. 

Reisinger et al. [21] used the gas chromatographic-atomic absorption spectrometric technique to demonstrate that biomethylation of inorganic lead does not account for the presence of organolead compounds in sediments. Sulphide induced chemical conversion of organic lead(IV) salts into alkyl lead compounds is, however, possible. 

Complexation of metals with organic compounds can also increase the toxicity of metals. This is the case with mercury, in which the organo-Hg species, methyl- and dimethylmercury, are fer more toxic than elemental or ionic mercury (Hg (aq)). The enhanced toxicity is caused by the increased tendency of the organo species to be retained, and therefore concentrated within, organisms. As discussed in Chapter 28.6.8, mercury is naturally biomethylated by bacteria under conditions of low pH and low... 

O2 concentrations, such as found in marine wetlands. High biomethylation rates have also been observed in coastal sediments. Because methylmercury is transferred up the food chain, the marine fish that occupy high trophic levels have very high mercury concentrations. In some cases, such as for tuna and swordfish, concentrations are high enough to pose human health risks. 

Biomethylation often occurs more readily at low redox potentials. 

Reamer DC, Zoller WH. 1980. Selenium biomethylation products from soil aud sewage sludge. Science 208 500-502. 

Ridley WP, Dizikes LJ, Woord JM. 1977. Biomethylation of toxic elements in the environment. Science 197 329-332. 

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