Jimenez M, Garcia-Canovas F, Garcia-Carmona F, Lozano J A, Iborra J L
Biochem Pharmacol. 1984 Nov 15;33(22):3689-97. doi: 10.1016/0006-2952(84)90158-8.
Characterization of intermediates formed in the noradrenaline oxidation by mushroom tyrosinase and sodium periodate has been performed by rapid scanning spectrophotometry and graphical analysis of obtained spectra. In a pH range from 5.0 to 6.0, it has been possible to detect o-noradrenalinequinone-H+ as the first intermediate in these oxidations. The following steps for noradrenaline transformation into noradrenochrome would be: noradrenaline----o-noradrenalinequinone-H+----o- noradrenalinequinone----leukonoradrenochrome----noradreno chrome. It has been also verified that o-noradrenalinequinone-H+ is transformed into noradrenochrome at a constant ratio. The stoichiometry for this converstion followed the equation: 2-noradrenalinequinone-H+----noradrenaline + noradrenochrome. The pathway between noradrenaline and noradrenochrome has been studied as a system of various chemical reactions coupled to an enzymatic reaction. We have denominated this type of mechanism as an enzymatic-chemical-chemical mechanism, (E2CC). Whole rate constants for the implicated chemical steps at different pH and temperature values have been evaluated from measurement of the lag period arising from the accumulation of noradrenochrome that takes place when noradrenaline was oxidized at pH 5-6. The lag period was independent on enzyme concentration, but was increased when pH and/or temperature were increased. Rate constants pH independent for the deprotonation of noradrenalinequinone-H+ into noradrenalinequinone and for the internal cyclization of noradrenalinequinone into leukonoradrenochrome have been obtained. We conclude that this minor pathway of noradrenaline oxidation by tyrosinase follows a scheme similar to that established for L-dopa.
通过快速扫描分光光度法和对所得光谱的图形分析，对蘑菇酪氨酸酶和高碘酸钠氧化去甲肾上腺素过程中形成的中间体进行了表征。在pH值为5.0至6.0的范围内，已能够检测到邻去甲肾上腺素醌-H⁺作为这些氧化反应中的第一个中间体。去甲肾上腺素转化为去甲肾上腺素色素的后续步骤为：去甲肾上腺素→邻去甲肾上腺素醌-H⁺→邻去甲肾上腺素醌→白细胞去甲肾上腺素色素→去甲肾上腺素色素。还证实了邻去甲肾上腺素醌-H⁺以恒定比例转化为去甲肾上腺素色素。该转化的化学计量遵循方程：2-邻去甲肾上腺素醌-H⁺→去甲肾上腺素+去甲肾上腺素色素。去甲肾上腺素和去甲肾上腺素色素之间的途径已作为与酶促反应偶联的各种化学反应系统进行了研究。我们将这种类型的机制称为酶-化学-化学机制（E2CC）。通过测量在pH 5-6下氧化去甲肾上腺素时由于去甲肾上腺素色素积累而产生的延迟期，评估了不同pH和温度值下相关化学步骤的整体速率常数。延迟期与酶浓度无关，但在pH值和/或温度升高时会增加。已获得了与pH无关的去甲肾上腺素醌-H⁺去质子化为去甲肾上腺素醌以及去甲肾上腺素醌内环化为白细胞去甲肾上腺素色素的速率常数。我们得出结论，酪氨酸酶氧化去甲肾上腺素的这条次要途径遵循与L-多巴所确立的方案类似的方案。
