Lead, biomethylation

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. 

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. 

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. 

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. 

Stereochemical retention of configuration around the saturated a-carbon can only result from the closed transition state, whereas the open transition state can lead to inversion (10, 193). Configurational inversions occur in many biological substrates, and much future work on biomethylation will likely involve the use of substituted methyl groups and the study of any optical changes that occur. Determination of the relative rates of cleavage for a series of normal and branched alkyl derivatives enables a distinction between retention or inversion in the SK2 (open) pathway (194). 

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 rate of methylation of mercury in anaerobically incubated estuarine sediments proved to be inversely related to salinity (267) this is consistent with results reported in Section II,A. Methylmercuric ion forms in sediments upon addition of HgCl2, with a lag phase of 1 month (268). Biomethylation by lake water columns and by sediments coincided, apparently being related to overall microbial activity, and showed periodic fluctuations (269). Topping and Davies have demonstrated that mercury can be methylated in the water column of a sea loch (270). As has previously been noted, tin compounds can be methylated by sediments (121-124), and this is also true for lead (134-136, 271). The relative proportions of biotic and abiotic methylation processes for such systems still remain to be determined. 

Although most research has been carried out with mercury, the possibility of a wider role for biomethylation of metals is suggested by reports of microbial formation of tetramethyl lead [(CH3)4Pb] in a number of lake sediments (Wong et al., 1975) and of methyl tin (CHjSn) by Pseudomonas sp. (Jemelov and Martin, 1975). 

Organic lead compounds have been introduced into the environment as pollutants, e.g. the fuel additive (anti knocking agent) tetraethyl lead, making it difficult to establish a biogenic origin of related compounds. However, methyl lead compounds, e.g. (CH3)4Pb have been discovered under environmental conditions, which are in accord with the biomethylation of lead [30]. In a comparative study, bioactive sediments methylated mercury, tin and lead substrates. Mercury underwent methylation ca. 10000 times faster than tin and lead, but lead was methylated more readily than tin [31]. 

Methyl thallium species have been discovered in natural water quite recently but thallium was reported to undergo biomethylation in vitro already more than two decades ago [32]. While the total thallium levels in oceans and lakes vary between 1.6 and 20.1 ng/L, levels exceeding 1000 ng/L have been found in industrial waste waters [33]. The proportion of [(CH3)2T1]+ to total thallium ranged up to 48%. The stability of the monomethyl derivatives varies in the order mercury(II) > thallium(III) 2> lead(IV) bismuth(V) [3b]. 

Monomethyl cadmium complexes were found in seawater and in arctic ice melts, where they reach levels of 1.2 ng/L (48% of total cadmium) [34]. Polar bacteria generate monoethyl cadmium species along with trimethyl lead compounds [30 b]. It seems that cadmium undergoes biomethylation under conditions similar... 

Biomethylation (enzymatic transmethylation) occurs in all living cells, and is an essential part of cellular metabolism, leading to methylation of proteins, nucleic acid bases, polysaccharides, and fatty acids. However, not all... 

The other concern in the 1970s and 1980s was whether or not inorganic lead could be biomethylated naturally, as in the case of mercury. In view of the bulk amounts of organoleads being used anyway at the time, this was not likely to be a key consideration in urban areas, but in pristine locations, natural lead methylation and transport might in some circumstances have led to environmental problems. In fact, there has not been any clear demonstration of such methylation, and it is unlikely to be a big environmental problem. Where organic lead compounds are detected in the remote environment, they are at very low levels, probably accountable by transport of these species. Evidence is considered in the next section. 

It is well established that microbiota plays a very significant role in the various transformations of arsenic, including mineralization/immobilization, oxidation/ reduction, and methylation/demethylation. Some of these biotransformations lead to less toxic forms of arsenic that can be used in the detoxification of the arsenic-contaminated environments. Biomethylation of arsenic results in formation of mono-, di-, and trimethylarsines which are volatile however, these gaseous arsenic forms are also toxic. In developing a bioremediation strategy to clean the... 

Other experiments indicated that conversion of lead(ll) compounds to Pb(CH3)4 was inconsistent and time-independent [130]. Lead contained in mine tailings is not mobilized in detectable amounts by biomethylation [125, 130]. A volatile organolead compound, presumably the methyllead derivative, was observed upon aerobic, not upon anaerobic, incubation of marine sediment with lead(ll) acetate [131]. Marine sediments inoculated with (CH3)3PbOOCCH3 produced Pb(CH3)4 in experiments employing flow-through conditions both under anaerobic and aerobic conditions. Production under anaerobic conditions was approximately threefold greater [130]. 

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