Jacobsen D W, Pezacka E H, Brown K L
Department of Cell Biology, Cleveland Clinic Foundation, Ohio 44195.
J Inorg Biochem. 1993 Apr 1;50(1):47-63. doi: 10.1016/0162-0134(93)80013-y.
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.
钴胺素辅酶（5'-脱氧腺苷钴胺素和甲基钴胺素）及其钴胺酰胺类似物（5'-脱氧腺苷钴胺酰胺和甲基钴胺酰胺）在有氧条件下催化2-巯基乙醇氧化为其二硫化物并生成过氧化氢。这些反应被甲基碘阻断。甲基碘的抑制作用显然是由于分别形成了反式二烷基类咕啉：甲基（腺苷基）钴胺素、二甲基钴胺素、甲基（腺苷基）钴胺酰胺和二甲基钴胺酰胺。当反应体系用可见光照射时，抑制作用解除，耗氧速率显著提高。对于由腺苷钴胺素和甲基钴胺素催化然后被甲基碘抑制的反应，光解过程中观察到的速率接近用水钴胺素获得的速率。对于由腺苷钴胺酰胺和甲基钴胺酰胺催化然后被甲基碘抑制的反应，光解过程中观察到的速率接近用二水钴胺酰胺获得的速率。因此，在光解过程中，假定的二烷基类咕啉中的两个反式轴向碳 - 钴键均发生均裂。与这些结果相反，水钴胺素对2-巯基乙醇的有氧氧化催化作用仅受到甲基碘的微弱抑制。这一观察结果表明，在该反应的催化过程中会生成水合钴胺（II）。超氧化物是水合钴胺（II）与双氧反应的预期产物，在水钴胺素催化2-巯基乙醇氧化过程中会形成超氧化物，因为超氧化物歧化酶可使耗氧速率降低50%。然而，该酶对钴胺素辅酶及其钴胺酰胺类似物催化的反应中的氧气摄取没有影响。这些类咕啉催化剂显然从反应过程中形成的钴（I）中间体向双氧转移两个电子。含氮碱通过与2-巯基乙醇竞争催化剂上的轴向配体配位位点来抑制类咕啉催化的硫醇氧化。与甲基碘引起的抑制作用不同，可见光对含氮碱引起的抑制作用没有影响。
