Segura-Aguilar J, Lind C
Department of Biochemistry, University of Stockholm, Sweden.
Chem Biol Interact. 1989;72(3):309-24. doi: 10.1016/0009-2797(89)90006-9.
Dopamine (DA) is rapidly oxidized by Mn3(+)-pyrophosphate to its cyclized o-quinone (cDAoQ), a reaction which can be prevented by NADH, reduced glutathione (GSH) or ascorbic acid. The oxidation of DA by Mn3+, which appears to be irreversible, results in a decrease in the level of DA, but not in a formation of reactive oxygen species, since oxygen is neither consumed nor required in this reaction. The formation of cDAoQ can initiate the generation of superoxide radicals (O2-.) by reduction-oxidation cycling, i.e. one-electron reduction of the quinone by various NADH- or NADPH-dependent flavoproteins to the semiquinone (QH.), which is readily reoxidized by O2 with the concomitant formation of O2-.. This mechanism is believed to underly the cytotoxicity of many quinones. Two-electron reduction of cDAoQ to the hydroquinone can be catalyzed by the flavoprotein DT diaphorase (NAD(P)H:quinone oxidoreductase). This enzyme efficiently maintains DA quinone in its fully reduced state, although some reoxidation of the hydroquinone (QH2) is observed (QH2 + O2----QH. + O2-. + H+; QH. + O2----Q + O2-.). In the presence of Mn3+, generated from Mn2+ by O2-. (Mn2+ + 2H+ + O2-.----Mn3+ + H2O2) formed during the autoxidation of DA hydroquinone, the rate of autoxidation is increased dramatically as is the formation of H2O2. Furthermore, cDAoQ is no longer fully reduced and the steady-state ratio between the hydroquinone and the quinone is dependent on the amount of DT diaphorase present. The generation of Mn3+ is inhibited by superoxide dismutase (SOD), which catalyzes the disproportionation of O2-. to H2O2 and O2. It is noteworthy that addition of SOD does not only result in a decrease in the amount of H2O2 formed during the regeneration of Mn3+, but, in fact, prevents H2O2 formation. Furthermore, in the presence of this enzyme the consumption of O2 is low, as is the oxidation of NADH, due to autoxidation of the hydroquinone, and the cyclized DA o-quinone is found to be fully reduced. These observations can be explained by the newly-discovered role of SOD as a superoxide:semiquinone (QH.) oxidoreductase catalyzing the following reaction: O2-. + QH. + 2H+----QH2 + O2. Thus, the combination of DT diaphorase and SOD is an efficient system for maintaining cDAoQ in its fully reduced state, a prerequisite for detoxication of the quinone by conjugation with sulfate or glucuronic acid. In addition, only minute amounts of reactive oxygen species will be formed, i.e. by the generation of O2-., which through disproportionation to H2O2 and further reduction by ferrous ions can be converted to the hydroxyl radical (OH.). Absence or low levels of these enzymes may create an oxidative stress on the cell and thereby initiate events leading to cell death.
多巴胺（DA）可被焦磷酸锰（Ⅲ）迅速氧化为其环化邻醌（cDAoQ），该反应可被NADH、还原型谷胱甘肽（GSH）或抗坏血酸抑制。锰（Ⅲ）对DA的氧化似乎是不可逆的，会导致DA水平降低，但不会产生活性氧，因为该反应既不消耗氧气也不需要氧气。cDAoQ的形成可通过氧化还原循环引发超氧阴离子自由基（O₂⁻·）的生成，即各种依赖NADH或NADPH的黄素蛋白将醌单电子还原为半醌（QH·），半醌很容易被O₂再氧化并伴随O₂⁻·的形成。这种机制被认为是许多醌类细胞毒性的基础。黄素蛋白DT黄递酶（NAD(P)H：醌氧化还原酶）可催化cDAoQ双电子还原为对苯二酚。该酶能有效地将DA醌维持在完全还原状态，尽管会观察到对苯二酚（QH₂）有一些再氧化现象（QH₂ + O₂→QH· + O₂⁻· + H⁺；QH· + O₂→Q + O₂⁻·）。在DA对苯二酚自氧化过程中由O₂⁻·（Mn²⁺ + 2H⁺ + O₂⁻·→Mn³⁺ + H₂O₂）生成的锰（Ⅲ）存在的情况下，自氧化速率会急剧增加，H₂O₂的形成也会增加。此外，cDAoQ不再完全被还原，对苯二酚和醌之间的稳态比率取决于存在的DT黄递酶的量。超氧化物歧化酶（SOD）可抑制锰（Ⅲ）的生成，它催化O₂⁻·歧化为H₂O₂和O₂。值得注意的是，添加SOD不仅会导致锰（Ⅲ）再生过程中形成的H₂O₂量减少，实际上还能防止H₂O₂的形成。此外，在这种酶存在的情况下，O₂的消耗量很低，由于对苯二酚的自氧化，NADH的氧化也很低，并且发现环化的DA邻醌被完全还原。这些观察结果可以用SOD作为超氧化物：半醌（QH·）氧化还原酶这一新发现的作用来解释，它催化以下反应：O₂⁻· + QH· + 2H⁺→QH₂ + O₂。因此，DT黄递酶和SOD的组合是将cDAoQ维持在完全还原状态的有效系统，这是醌通过与硫酸盐或葡萄糖醛酸结合进行解毒的前提条件。此外，只会形成极少量的活性氧，即通过生成O₂⁻·，O₂⁻·通过歧化为H₂O₂并进一步被亚铁离子还原可转化为羟基自由基（OH·）。这些酶的缺乏或低水平可能会对细胞造成氧化应激，从而引发导致细胞死亡的事件。
