Polarography of cobalamins and cobinamides

Hogenkamp, Harry P. C.; Holmes, Steven J.

doi:10.1021/bi00811a004

Cited by 72 publications

(22 citation statements)

References 21 publications

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“…SCE as measured under very different conditions*). Much related to this work is an early report on the reduction potentials of a series of 10 RCbl"' by Hogenkamp and Holmes [8]. The authors observed that primary unactivated RCb1""s undergo polarographic reduction at -1.37 to -1.39 V but that two RCb1""s with R = CH,COOH or CH,COOMe are reduced surprisingly at -0.84 and -1.14 V (us.…”

mentioning

confidence: 83%

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Zhou

Tinembart

Scheffold

et al. 1990

Helvetica Chimica Acta

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The irreversible-reduction potentials of 26 alkylcob(II1)alamins (RCbl"'; la-z) and 26 alkylcob(I1I)yrinates (RCby'"'; 2a-2) (E, la-z and Ep 2a-2, resp.) have been measured in situ by single-scan voltammetry of hydroxocob(II1)alamin hydrochloride (vitamin BIZb. HCl; 1) or heptamethyl cob(1I)yrinate perchlorate (ClO.,'Cby'"; 2) in presence of the corresponding alkyl halides (RX; 3-2) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E"(R-Co"' + eC%R' + Co'). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl"' and RCby'"' shift E,(la-z) and E (2a-2) towards positive potentials. Linear correlations have been found between Ep(la-z) (EP(2a-z)) of RCbl" (R'Cby'"') and the pK, of RH (or the Tuft v*-or the Hummeft o--values of R) within each class of R, i.e. MeCbl"' (Me'Cby'"'), primary RCbl"' (RCby'"') and secondary RCbll'P(R'Cby'"L). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co,C-bond of an alkylcobalamin (Scheme 2, la-lu), a key step in the vitamin-B,,-catalized C,C-bond formation.

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mentioning

confidence: 83%

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Zhou

Tinembart

Scheffold

et al. 1990

Helvetica Chimica Acta

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“…A difference spectrum was taken : the unchanged solution which was supposed to contain methylcobalamin served as sample, the illuminated one in which methylcobalamin should have been destroyed by light was placed into the reference light beam. [12], it shouldbe expectedthat theenzyme bound BIZ derivative is reduced to the bivalent state only. Fig.…”

Section: Methodsmentioning

confidence: 99%

The Vitamin B12-Dependent Methionine Synthetase. The Cycle of Transmethylation

Rüdiger

1971

Eur J Biochem

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The preactivation of the vitamin B,,-dependent methionine synthetase (5-methyl-5,6,7,8-tetrahydrofolate-homocysteine methyltransferase) from Escherichia coli B which is known to lead to a methylcobalamin enzyme has been shown by spectrophotometry to be initiated by a reduction of the cob(II1)alamin (Biza) to a cob(I1)alamin (B12r) enzyme. This one electron reduction is sufficient to add the methyl kryptocation of the potent methyl donor S-adenosylmethionine to the coenzyme, since Bizr, to a small extent, disproportionates to yield Biza and B12s (cob(I)alamin), the latter being trapped from the equilibrium by 8-adenosylmethionine. Within the transmethylation cycle, methylcobalamin enzyme donates its methyl group to homocysteine, the resulting Bizs enzyme is remethylated by the less potent methyl donor 5-methyltetrahydrofolate thus closing the cycle. The latter reaction can be simulated by model experiments with the free coenzyme. From the experimental conditions, conclusions may be drawn concerning the active site of the holoenzyme. According to this scheme, methionine can only be synthesized if methylated B,, enzyme is available to the cell. As has been shown earlier [6], this is the case only to a limitcd extent. Furthermore, it must be assumed that the cell is largely able to prevent oxidation of the enzyme-bound Bizs, a cobalamin species which is known to be extremely susceptible to oxidation, and that oxidized enzyme which has escaped the cycle can be methylated, thus entering it again. In the present work, the preactivation step was investigated by spectrophotometry. Of the reactions within the cycle, the methylation of BlZs by 5-CH3-H,folate was shown to occur in a model system.Unusual Abbreviations. 5-CH3-H,folate, 5-methyl-5,6,7,8-tetmhydrofolate; H,folate, 5,6,7,8-tetrahydrofolate; Biza, cob(I1l)alamin; Bizr, cob(I1)alamin; Bizs, cob(1)alamin. MATERIALS AND METHODSThe purification of methionine synthetase has been described [6]. This enzyme differs from other preparations [7,8] by its high molecular weight in gel filtration and by its stability towards inhibition by halogenated hydrocarbons. From its absorbance a t 542 nm, its molecular weight and an average ~& f = 9000 of the cobalamin a peak [9], the preparation used in this study can be calculated to consist of 40 methionine synthetase. Its specific activity was 60 pmoles methionine formed x h-l x mg protein-l. 5-CH3-H, folate and 8-adenosylmethionine were prepared as described earlier [3,10], aquocobalamin was purchased from Merck (Darmstadt). Spectra were recorded in a Cary 14 spectrophotometer.For methylating Bizs by 5-CH3-H,folate (Fig. 3), I ml 0.6 mM B-CH,-H,folate in 0.5 M CH,COOH/ CH,COONa buffer pH 4.0 a t 0 "C was added anaerobically to 2 ml 0.05 mM Bizs, freshly prepared from aquocobalamin and sodium borohydride. After 5 min at 0 "C in the dark, the mixture was aerated. One part was kept dark a t room temperature, another one was illuminated (100 W, 30 min, distance 30 em). A difference spectrum was taken : the unchanged solution which wa...

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“…No oxidation (anodic) peak was registered in the whole tested range of pH of supporting electrolyte (pH 3-12) with all three working electrodes. The cathodic response could be considered as a reduction of cobalt ion from Co 2 + to Co 1 + [13][14][15][16][17]. The highest cathodic current response of OH-Cbl could be recorded in slightly acidic solutions, specifically in B-R buffer of pH 5 (m-AgSAE) and pH 6 (p-AgSAE, HMDE), respectively, as it is obvious from Figure 3.…”

Section: Voltammetric Behavior Of Oh-cbl In Dependence On Ph Of the Smentioning

confidence: 99%

“…The first electrochemical studies of polarographic behavior of cobalamins originated from fifties of 20 th century [13,14]. These and later papers dealt especially with the adsorption of cobalamins on mercury surface or with changes in valence of the cobalt atom from three (B 12a ) to two (B 12r ) or/and from two to one (B 12s ) [13][14][15][16][17]. It was found that B 12a provided two well-developed cathodic waves (E p1 = À0.04 V, E p2 = À0.96 V vs. SCE, in acetic medium) on mercury electrodes which corresponded to two one-electron reduction steps to B 12r and subsequently to B 12s and similarly B 12r provided one one-electron step caused by reduction of cobalt (Co 2 + + 1e À !…”

Section: Introductionmentioning

confidence: 99%

Silver Solid Amalgam Electrode as a Tool for Monitoring the Electrochemical Reduction of Hydroxocobalamin

Bandžuchová¹,

Šelešovská²,

Navrátil³

et al. 2012

Electroanalysis

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The voltammetric behavior of hydroxocobalamin (OH‐Cbl), one form of vitamin B12, has been studied using two modifications of silver solid amalgam electrode: polished (p‐AgSAE) and mercury meniscus modified (m‐AgSAE). All results were compared with these achieved using hanging mercury drop electrode (HMDE). Various voltammetric methods, like cyclic voltammetry, DC voltammetry, differential pulse voltammetry and elimination voltammetry with linear scan, were applied to investigate the electrochemical behavior of OH‐Cbl. Working conditions of differential pulse voltammetry were found and the achieved limits of detection (LD=1.5 nmol L−1 (m‐AgSAE) and 3.3 nmol L−1 (p‐AgSAE)) proved high sensitivity. The applicability of the proposed method was verified by analysis of vitamin preparation.

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Polarography of cobalamins and cobinamides

Cited by 72 publications

References 21 publications

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

The Vitamin B12-Dependent Methionine Synthetase. The Cycle of Transmethylation

Silver Solid Amalgam Electrode as a Tool for Monitoring the Electrochemical Reduction of Hydroxocobalamin

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