Methyl transfer from methyl vitamin B 12

The inhibition of corrinoid-catalyzed oxidation of mercaptoethanol by methyl iodide: mechanistic implications

The cobalamin coenzymes (5'-deoxyadenosyl- and methylcobalamin) and their cobinamide counterparts (5'-deoxyadenosyl- and methylcobinamide) catalyze the oxidation of 2-mercaptoethanol to its disulfide with hydrogen peroxide formation under aerobic conditions. The reactions are blocked by methyl iodide. Inhibition by methyl iodide is apparently due to the formation of the trans dialkyl corrinoids: methyl(adenosyl)cobalamin, dimethylcobalamin, methyl(adenosyl)cobinamide, and dimethylcobinamide, respectively. When the reaction system is illuminated with visible light, inhibition is released and a dramatic enhancement in the rate of oxygen consumption occurs. For reactions catalyzed by adenosyl- and methylcobalamin and then inhibited by methyl iodide, the rates observed during photolysis approach those obtained with aquacobalamin. For reactions catalyzed by adenosyl- and methylcobinamide and then inhibited by methyl iodide, the rates observed during photlysis approach those obtained with diaquacobinamide. Thus, both trans axial carbon-cobalt bonds in the putative dialkyl corrinoid are homolyzed during photolysis. In contrast to these results, the catalysis of the aerobic oxidation of 2-mercaptoethanol by aquacobalamin is only weakly inhibited by methyl iodide. This observation suggests that aquacob(II)alamin is produced during the catalysis of this reaction. Superoxide, the anticipated product of the reaction between aquacob(II)alamin and dioxygen, is formed during aquacobalamin-catalyzed 2-mercaptoethanol oxidation since superoxide dismutase decreases the rate of oxygen consumption by 50%. However, the enzyme has no effect on oxygen uptake during reactions catalyzed by cobalamin coenzymes and their cobinamide counterparts. These corrinoid catalysts apparently transfer two electrons to dioxygen from cobalt(I) intermediates formed during the reactions. Nitrogenous bases inhibit corrinoid-catalyzed thiol oxidation by competing with 2-mercaptoethanol for axial-ligand coordination sites on the catalyst. In contrast to the inhibition observed with methyl iodide, visible light has no effect on the inhibition obtained with nitrogenous bases.

Gold-resistant bacteria: excretion of a cystine-rich protein by Pseudomonas cepacia induced by an antiarthritic drug

P. cepacia bacteria adapted to growth in a chemically defined medium containing millimolar concentrations of Au(I) thiolates including the antiarthritic drug Au(I) thiomalate. The bacteria became very large, accumulated polyhydroxybutyrate and gold, and excreted a yellow protein ("thiorin"), which caused foaming of the culture medium. Thiorin was shown by 1H-NMR, amino acid analysis, and gel filtration chromatography to be of low molecular weight (ca. 9500) and to contain predominantly Cys (oxidized), Glx, and Gly.

Methyl transfer from methylcobalamin to diaquocobinamide

The transfer of the methyl group from methylcobalamin to diaquocobinamide in aqueous solution has been demonstrated by proton, carbon-13, and phosphorus-31 nuclear magnetic resonance spectroscopy. The products of this reaction are aquocobalamin and the methylaquocobinamides. Dicyanocobinamide and the cyanoaquocobinamides do not serve as methyl acceptors, while ligands such as pyridine and histidine reduce the rate of the transfer reactions. The methyl transfer is not affected by oxidizing agents such as O2, N2O, and H2O2, suggesting that the reaction does not involve free Co(I) or Co(II) corrinoids. The pH dependence of the rate of the transfer reaction from methylcobalamin to diaquocobinamide demonstrates that methylcobalamin in the "base-on" form and diaquocobinamide are the most effective methyl donor and acceptor, respectively. The most plausible mechanism for the transfer reaction involves the one-electron oxidation of methylcobalamin by diaquocobinamide to a methylcobalamin radical cation and cob(II)inamide. The very unstable methylcobalamin radical cation releases a methyl radical, which reacts with cob(II)inamide to generate the methylaquocobinamides.

The biochemistry of toxic elements

In 1968 it was discovered that mercury could be biomethylated to give methylmercury as the major product (Wood, Kennedy & Rosen, 1968; Jensen & Jernelöv, 1969). This discovery has proved to be important in two respects: (a) it provided us with the recognition that biological systems are capable of synthesizing very toxic organometallic compounds from relatively innocuous inorganic precursors, and (b) it opened the door to the study of organometallic compounds produced by biological mechanisms in the aqueous environment.