Wolak Maria, Stochel Grazyna, van Eldik Rudi
Faculty of Chemistry, Jagiellonian University, 30060 Krakow, Poland.
Inorg Chem. 2006 Feb 6;45(3):1367-79. doi: 10.1021/ic051300q.
The reactions of aquacobalamin (Cbl(III)H2O, vitamin B12a) and reduced cobalamin (Cbl(II), vitamin B12r) with the nitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueous solution at pH 7.4. UV-vis and NMR spectroscopic studies and semiquantitative kinetic investigations indicated complex reactivity patterns for the studied reactions. The detailed reaction routes depend on the oxidation state of the cobalt center in cobalamin, as well as on the structure of the nitrosothiol. Reactions of aquacobalamin with GSNO and SNAP involve initial formation of Cbl(III)-RSNO adducts followed by nitrosothiol decomposition via heterolytic S-NO bond cleavage. Formation of Cbl(III)(NO-) as the main cobalamin product indicates that the latter step leads to efficient transfer of the NO- group to the Co(III) center with concomitant oxidation of the nitrosothiol. Considerably faster reactions with Cbl(II) proceed through initial Cbl(II)-RSNO intermediates, which undergo subsequent electron-transfer processes leading to oxidation of the cobalt center and reduction of the nitrosothiol. In the case of GSNO, the overall reaction is fast (k approximately 1.2 x 10(6) M(-1) s(-1)) and leads to formation of glutathionylcobalamin (Cbl(III)SG) and nitrosylcobalamin (Cbl(III)(NO-)) as the final cobalamin products. A mechanism involving the reversible equilibrium Cbl(II) + RSNO <==> Cbl(III)SR + NO is suggested for the reaction on the basis of the obtained kinetic and mechanistic information. The corresponding reaction with SNAP is considerably slower and occurs in two distinct reaction steps, which result in the formation of Cbl(III)(NO-) as the ultimate cobalamin product. The significantly different kinetic and mechanistic features observed for the reaction of GSNO and SNAP illustrate 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.
在pH 7.4的水溶液中研究了水合钴胺素（Cbl(III)H2O，维生素B12a）和还原型钴胺素（Cbl(II)，维生素B12r）与亚硝基硫醇S-亚硝基谷胱甘肽（GSNO）和S-亚硝基-N-乙酰青霉胺（SNAP）的反应。紫外可见光谱和核磁共振光谱研究以及半定量动力学研究表明，所研究的反应具有复杂的反应模式。详细的反应途径取决于钴胺素中钴中心的氧化态以及亚硝基硫醇的结构。水合钴胺素与GSNO和SNAP的反应涉及最初形成Cbl(III)-RSNO加合物，随后通过异裂S-NO键断裂导致亚硝基硫醇分解。形成Cbl(III)(NO-)作为主要的钴胺素产物表明，后一步导致NO-基团有效地转移到Co(III)中心，同时亚硝基硫醇被氧化。与Cbl(II)的反应明显更快，通过最初的Cbl(II)-RSNO中间体进行，随后经历电子转移过程，导致钴中心氧化和亚硝基硫醇还原。在GSNO的情况下，总体反应很快（k约为1.2×10(6) M(-1) s(-1)），并导致形成谷胱甘肽钴胺素（Cbl(III)SG）和亚硝酰钴胺素（Cbl(III)(NO-)）作为最终的钴胺素产物。根据获得的动力学和机理信息，提出了一个涉及可逆平衡Cbl(II) + RSNO <==> Cbl(III)SR + NO的反应机理。与SNAP的相应反应明显较慢，分两个不同的反应步骤进行，最终形成Cbl(III)(NO-)作为钴胺素产物。观察到的GSNO和SNAP反应在动力学和机理特征上的显著差异说明了亚硝基硫醇结构对其与生物分子金属中心反应活性的重要影响。简要讨论了结果的潜在生物学意义。
