| Abstract
| - The reactions of aquacobalamin (Cbl(III)H2O, vitamin B12a) and reduced cobalamin (Cbl(II), vitamin B12r) with thenitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueoussolution at pH 7.4. UV−vis and NMR spectroscopic studies and semiquantitative kinetic investigations indicatedcomplex reactivity patterns for the studied reactions. The detailed reaction routes depend on the oxidation state ofthe cobalt center in cobalamin, as well as on the structure of the nitrosothiol. Reactions of aquacobalamin withGSNO and SNAP involve initial formation of Cbl(III)−RSNO adducts followed by nitrosothiol decomposition viaheterolytic S−NO bond cleavage. Formation of Cbl(III)(NO-) as the main cobalamin product indicates that thelatter step leads to efficient transfer of the NO- group to the Co(III) center with concomitant oxidation of thenitrosothiol. Considerably faster reactions with Cbl(II) proceed through initial Cbl(II)−RSNO intermediates, whichundergo subsequent electron-transfer processes leading to oxidation of the cobalt center and reduction of thenitrosothiol. In the case of GSNO, the overall reaction is fast (k ≈ 1.2 × 106 M-1 s-1) and leads to formation ofglutathionylcobalamin (Cbl(III)SG) and nitrosylcobalamin (Cbl(III)(NO-)) as the final cobalamin products. A mechanisminvolving the reversible equilibrium Cbl(II) + RSNO ⇄ Cbl(III)SR + NO is suggested for the reaction on the basisof the obtained kinetic and mechanistic information. The corresponding reaction with SNAP is considerably slowerand occurs in two distinct reaction steps, which result in the formation of Cbl(III)(NO-) as the ultimate cobalaminproduct. The significantly different kinetic and mechanistic features observed for the reaction of GSNO and SNAPillustrate the important influence of the nitrosothiol structure on its reactivity toward metal centers of biomolecules.The potential biological implications of the results are briefly discussed.
- Reactions of aquacobalamin (Cbl(III)H2O) and reduced cobalamin (Cbl(II)) with nitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueous solution at pH 7.4. Reactions with aquacobalamin involve initial formation of Cbl(III)−RSNO adducts followed by transfer of the NO- group to the Co(III) center. Reactions with Cbl(II) proceed through Cbl(II)−RSNO intermediates which undergo subsequent electron-transfer processes. Different kinetic and mechanistic features observed for the reactions with GSNO and SNAP, respectively, illustrate the important influence of the nitrosothiol structure on its reactivity toward metal centers of biomolecules.
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