John 5. Thayer University of Cincinnati Cincinnati, Ohio 45221
Biological Methylation Its nature and scope
The most intimate involvement of organometallic compounds with living systems occurs in the process termed "biological methylation." This process bonds a methyl group to some substrate, either by direct transfer or by use of some precursor moiety. The term1 was first coined by Challenger (1-3). He found that oxy compounds of arsenic, selenium and tellurium would have the oxygens replaced by methyl groups through the action of molds. More recently, dimethylarsine has been reported to form from action of Methanobacterium strain M.0.H. on disodium arsenate (4). All available evidence indicates that biological methylation is a general process for living organisms, though not necessarily always through the same mechanism. People working with selenium or tellurium compounds have to he careful in their handling techniques, lest these compounds he accidentally ingested and later excreted as the malodorous, gaseous dimethyl compounds! Most metabolic and mechanism studies. however. have been done either in uitro or with microorganisms. Methylcobalamin, a derivative of Vitamin BIZ containing a cobalt-methyl bond, appears to be the crucial intermediate. Numerous studies on the action of methylcohalamin and related compounds have been reported (414). The majority of these have concentrated on the cobalt atom, which is the active site. Stereochemistry and suhstitution on the organic portion do play a part, however; epimerization at one of the rina positions causes a loss of 90% of the BIZ activity (15). ~ethylcohalaminserves in the formation of methane by anaerobic bacteria (14. 16). These hacteria use decaying vegetation as their source of material, and methane is the final product from the reduction of cellulose. Since this occurs most frequently in marshy areas, methane was originally called "swamp gas." Adenosine-5'4riphosphate (ATP) and hydrogen are also required for methane formation. Whether methylcobalamin forms methane directly, or whether it is first converted to another compound that then forms methane, has not yet been decided (16). The mechanism of biological methylation by bacteria has been studied by McBride and Wolfe (4). They found that 1. The process occurs stepwise 2. Ethylcobalamin did not transfer an ethyl group, indicating
methylmercuric ion, CHsHg+). Full details are given elsewhere (18-20). Inorganic mercury (usually mercuric chloride) enters natural waters. The methanogenic bacteria from mud on lake bottoms convert this compound, via hiological methylation, to gaseous dimethylmercury. Dimethylmercury may he converted to methylmercury in two ways, exchange (8) (CHJ2Hg + HgCI, 2CH8HgCI (1) or by acid cleavage. +
+
(CHJ~H~
that methylation is enzymatic rather than purely chemical
Arsenate is reduced to arsenite, which is methylated to form methylarsonic acid, CHsAs(:O)(OH)z. Further methylation gives cacodylic acid, (CH&As(:O)OH which is reduced to the final product, dimethylarsine, (CHdzAsH. This contrasts with Challenger's findings that sodium methylarsonate and sodium cacodylate formed trimethylarsine exclusively (2, 3). The reasons for this difference remain to be determined. Impetus for the study of biological methylation has come from concern about methylmercury (actually, 'Biological methylation now generally refers to the formation of methyl groups from one-carbon precursors. Methyl transfer is usually termed "transmethylation." This article uses "biological methylation" in its older sense. 390 /Journal of Chemical Education
-+
CH,H~+
+
CH,
(2)
Both methylmercury and dimethylmercury enter the water, where they are absorbed by aquatic organisms comprising some part of a food chain. These organisms then are devoured by others. The concentration of methylmercury increases as one moves along the chain (a process called "biological magnification"). Since fishes are usually near the top of such chains, they build up high concentrations. Methylmercury is metabolized more slowly than any other form of mercury; the half-life for methylmercury excretion is 70 days in humans (18) and several hundred days in fishes (19). Fish-eating predators (birds, seals, humans) therefore ingest a substantial quantity of methylmercury, with deleterious results (18). How general is biological methylation? Can other metals be methylated and therefore possibly rendered more dangerous? Enough work has been reported to give some preliminary answers, though i t should be recognized that they are rather speculative. Methylcobalamin may be considered a biological analog of the commonly used Grignard reagent. Available evidence indicates that it methylates substrates stepwise (4, 9). In order for a second methyl group to be introduced onto an atom, the monomethyl compound must have a certain degree of stability. Three metals present themselves as possible candidates because of their relation to mercury in the Periodic Table: cadmium, thallium, and lead. All three are toxic, and lead and cadmium have become pollution problems in their own right. Methylcadmium compounds are not stable towards water, so are unlikely to form from biological methylation in any quantity. Monometbylthallium and -lead compounds are unknown, because of the easy decomposition CH,TIX,
3. Demethylated methylcobalamin (Blzr) does not cause al-
kylation, indicating that the eobalamin derivatives do not function solely as reducing agents
H+
CH,X
+
TIX
(3)
CH,PhX, CH,X + PhX, (4) This difference may he further illustrated by the reaction of the acetates of mercury, thallium, and lead with certain olefins R' p -+
R'R2C=CWR'
+
Hg(OAc),
I
+
I
Rz--C-C-R' i I
(5)
All attemptqto methylate these metals have been unsuccessful (7, 8, 21). This is fortunate, because the dimethylderivatives of both metals are hoth stable towards water and highly toxic. In the presence of some complexing agent, though, inorganic lead or thallium might he converted to the stable dimethyl compounds. Alternatively, aryl compounds of these metals might be methylated to the more toxic methylmetals, though phenylmercuric acetate metabolism by microorganisms does not yield any methylmercury compounds (22). Further research is needed to settle these questions. Metalloids do form stahle monomethyl derivatives and thus are promising candidates for methylation. Arsenic and tellurium have alreadv heen discussed. Silicon.. eermanium, phosphorus, tin ahd antimony are other possihilities. Challenaer reported that vhosvhorus com~oundsdid not form uolitile methyl compounds, and thatpotassium antimony1 tartarate was converted to antimony oxide by molds (17). However, he also reported that mercury compounds were not methylated under these conditions. Therefore the possibility of biological methylation of these elements is still considerable and deserves attention. Tin especially warrants investigation, since trimethyltin compounds are known to show toxic effects. As may he seen, biological methylation and its numerous ramifications have drawn considerable attention.
-
Much remains yet to h i discovered about this process. As long as methylmercury remains as a topic of concern (and indications point to its staying that way for many years), biological methylation and other interactions of organometals with living organisms will be the focus and source of considerable research. Literature Cited Ill (21 131 14) 15) 16)
Thayer.J. S . . J . CHEM.EDUC.,48,80611971). Challenger. F., C h m Roo., 36.315119451. Challenger. F., Buon. Rou., 3,25511955). McRride,B.C.,andWolfe,R.S.,Bioch
