Transmethylation mechanism

Standard Reduction Potentials and Proposed Transmethylation Mechanisms... 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

The mechanism of transmethylation was then examined. A series of deuterium-labeled methylated compounds were synthesized by du Vigneaud s group, including arsenocholine, trimethylamine, dimethyl-glycine, and dimethylthetin. Of these only betaine and dimethylthetin served as methyl donors. In 1949 Dubnoff found that choline could only act as a donor under aerobic conditions, when it was oxidized to betaine. 

Cantoni, G.L. (1955). Enzymatic Mechanisms and the Biological Significance of Transmethylation Reactions, pp. 233-237, ibid. 

Procarbazine (Matulane) may autooxidize spontaneously, and during this reaction hydrogen peroxide and hydroxyl free radicals are generated. These highly reactive products may degrade DNA and serve as one mechanism of procarbazine-induced cytotoxicity. Cell toxicity also may be the result of a transmethylation reaction that can occur between the A-methyl group of procarbazine and the N7 position of guanine. 

Although three early papers briefly discussed reactions between methylcobalamin and mercury compounds (30-32), the most systematic investigation has come from Wood and co-workers (33). They proposed the mechanism shown in Fig. 1, with values for the various rate constants presented in Table I. Species 2 and 3, in which the benzimidazole nitrogen no longer bonds to the cobalt atom, are termed base-off compounds, whereas 1 is base-on methylcobalamin and 4 is aquocob(III)alamin, the usual product of aqueous transmethylation by 1. Each one of these species has a unique ultraviolet-visible spectrum, which allows quantitative studies by spectrophotometric techniques to be made (28, 32, 33). The mercuric acetate-1 exchange is so rapid that it must be studied using stopped-flow kinetic techniques (33). 

The mechanisms of transmethylation involving methylcobalamin have been extensively discussed by Wood and co-workers (189-191). They proposed a classification based on the standard reduction potentials for the elements (Table IV) and having three types of reaction ... 

In every case where a mechanism has been proposed for aqueous transmethylations involving methylmetals, the mechanism has been SK2 (44, 51. 59-66, 196). Available evidence strongly indicates that, from a... 

A free radical mechanism has been proposed for the reactions of CH3B12 with Cr(II) (53), thiols (199), and Sn(II) (50, 190). Wood has proposed the following mechanism for the transmethylation of Sn(II) ... 

As part of a theoretical examination of the factors controlling the catalytic efficiency of a transmethylation enzyme (catechol (9-mcthyltransferase), the reaction mechanism of the non-enzymic transmethylation of catechol by, -adcnosylmethionine (AdoMet, as modelled by sulfonium ion) has been elucidated by using ab initio and semiempirical quantum mechanical methods.97 The gas-phase reaction between catecholate and sulfonium is extremely fast, involving no overall barrier, and the reaction profile to some extent resembles that of a typical gas-phase, S N 2 reaction. However, in aqueous solution, this reaction is very slow, with a predicted barrier of 37.3 kealmol-1. Good agreement between calculated KIEs for the model reaction and measured KIEs for the enzymic reaction suggests that the transition states are similar. 

The mechanism of carbamate-resistance, formerly considered to be enhanced hydrolysis (e.g. carbaryl to 1-naphthol), was found to derive almost exclusively from hydroxylation at various points on the molecule, not only the aromatic leaving group but also the N-methyl on the carbamate (Fig. 3), as well as some desmethylation for good measure (14). Pyrethrin-resistance, at first considered to be due to hydrolysis of the alcohol-acid linkage, was also found to be due to an oxidation, occurring at the transmethyl group of the isobutenyl side-chain of the chrysanthemic acid (15). 

Methylation derivatization also has been successfully used for the transmethylation of esters and carbonates. For an ester, the reaction can take place by the following mechanism ... 

Miscellaneous Synthetic compounds - Procarbazine, (N-isopropyl-Of-(2 methylhydrazino)-g -toluamide hydrochloride), one of several derivatives of methylhydrazine which have antineoplastic activity against transplanted tumors, has proven clinically useful in the management of Hodgkin s disease. A second derivative, N-allophanoyl-a-(2 methylhydrazine)- -toluam-ide, appeared to have activity comparable to that of procarbazine in a preliminary clinical trial. 47 The mechanism by which procarbazine and related compounds produce cytotoxicity is not established. Procarbazine produced a short-lived inhibition of synthesis of DNA, RNA and protein when studied in a mouse lymphoma 48 the methyl group enters the 1-carbon pool and participates in transmethylation reactions. 49 Pertinence of these observations, as well as those previously described of molecular autoxidation to the drug s antitumor effects, awaits demonstration. 

During the course of biomethylation the methyl group is most likely transferred as a bridging intermediate rather than a free entity. Such an intermediate is assumed to form during an associative mechanism [4]. The methyl group may be electrophilic (cationic), radical or nucleophilic (anionic), depending on the specific donor moiety. A broad variety of methyl transfer reactions are therefore possible. Besides the two biological donors, methylcobalamin (see below) and S-adenosyl-methonine (1) nonenzymatic transmethylation is also possible in the natural environment, probably also very important for the formation and decomposition of metal methyl compounds [3b],... 

Methyl transfer from CFeSP to Ni is elosely followed by, or eoupled to, oxidation of the D site, yielding Ni —CH3 with Dqx- CO then binds to Ni and inserts to generate a Ni C cetyl species, which is subsequently attacked by CoA to generate the product. In this transformation, the D site increases electron density at Ni in some marmer, thus permitting Np to be an effective nucleophile for the transmethylation. The mechanism requires an ordered binding of methyl, followed by CO, while accounting for the stereochemical outcome of the chiral methyl experiments. 

AdoHcy is a strong competitive inhibitor of AdoMet-dependent methyla-tion reactions (Cantoni et al., 1979). Modulation by adenosylhomocysteinase of the concentration of AdoHcy relative to that of AdoMet therefore provides a mechanism for regulation of such transmethylations (see Poulton, this series, Vol. 7, Chapter 22). 

Transmethylation by Cobalamins.— Two mechanisms of methyl transfer from cobalamins are found, a homolytic process with metals that have reduction potentials less than 0.50 V and an electrophilic (carbanion heterolytic) process for metals with potentials greater than 0.85 V. The PF /ii couple is 0.76 V and both oxidation states are required for methylation. A mechanism is proposed in which [Ptcy " binds through the propionamide and acetamide side-chains of the corrin thereby promoting formation of the base on form and activating the Co— C bond. The methylation step involves [PtCIe] " either in a redox switch where two electrons are abstracted from bound Pt followed by methyl transfer to the then bound Pt v or by direct electrophilic attack by [PtClel " ... 

An electrophilic attack mechanism is proposed " for the transmethylation reactions of organometallic complexes by methylcobalamin. Reaction rates show first-order dependences on methylcobalamin and the organometallic concentrations with second-order rate constants varying from 6.9 x 10 M s for MeHgOAc to 4.5 x 10 for MegSnOAc at 25 °C, pH 4.55, and ionic... 

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