Buffinton G D, Ollinger K, Brunmark A, Cadenas E
Department of Pathology II, University of Linköping, Sweden.
Biochem J. 1989 Jan 15;257(2):561-71. doi: 10.1042/bj2570561.
DT-diaphorase catalysed the reduction of 1,4-naphthoquinones with hydroxy, methyl, methoxy and glutathionyl substituents at the expense of reducing equivalents from NADPH. The initial rates of quinone reduction did not correlate with either the half-wave reduction potential (E1/2) value (determined by h.p.l.c. with electrochemical detection against an Ag/AgCl reference electrode) or the partition coefficient of the quinones. After their reduction by DT-diaphorase the 1,4-naphthoquinone derivatives autoxidized at distinct rates, the extent of which was influenced by the nature of the substituents. Thus for the 1,4-naphthoquinone series the following order of rate of autoxidation was found: 5-hydroxy-1,4-naphthoquinone greater than 3-glutathionyl-1,4-naphthoquinone greater than 5-hydroxy-3-glutathionyl-1,4-naphthoquinone greater than 1,4-naphthoquinone greater than 2-hydroxy-1,4-naphthoquinone. For the 2-methyl-1,4-naphthoquinone (menadione) series the following order was observed: 5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-2-methyl-1,4-naphthoquinone greater than 2-methyl-1,4-naphthoquinone greater than 3-hydroxy-2-methyl-1,4-naphthoquinone. The autoxidized naphthohydroquinone derivatives were re-reduced by DT-diaphorase, thus closing a cycle of enzymic reduction in equilibrium autoxidation. This was expressed as an excess of NADPH oxidized over the initial concentration of quinone present as well as H2O2 formation. These findings demonstrate that glutathionyl conjugates of 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone and those of their respective 5-hydroxy derivatives are able to act as substrates for DT-diaphorase and that they also autoxidize at rates higher than those for the unsubstituted parent compounds. These results are discussed in terms of the cellular role of DT-diaphorase in the reduction of hydroxy- or glutathionyl-substituted naphthoquinones as well as the further conjugation of these hydroquinones with glucuronide or sulphate within the cellular milieu, thereby facilitating their disposal from the cells.
DT-黄递酶催化还原带有羟基、甲基、甲氧基和谷胱甘肽取代基的1,4-萘醌，反应以消耗NADPH的还原当量为代价。醌还原的初始速率与半波还原电位（E1/2）值（通过高效液相色谱结合电化学检测，以Ag/AgCl参比电极测定）或醌的分配系数均无相关性。经DT-黄递酶还原后，1,4-萘醌衍生物以不同速率自动氧化，其氧化程度受取代基性质影响。因此，对于1,4-萘醌系列，发现如下自动氧化速率顺序：5-羟基-1,4-萘醌＞3-谷胱甘肽基-1,4-萘醌＞5-羟基-3-谷胱甘肽基-1,4-萘醌＞1,4-萘醌＞2-羟基-1,4-萘醌。对于2-甲基-1,4-萘醌（甲萘醌）系列，观察到如下顺序：5-羟基-2-甲基-1,4-萘醌＞3-谷胱甘肽基-5-羟基-2-甲基-1,4-萘醌＞3-谷胱甘肽基-2-甲基-1,4-萘醌＞2-甲基-1,4-萘醌＞3-羟基-2-甲基-1,4-萘醌。自动氧化的萘氢醌衍生物可被DT-黄递酶再次还原，从而形成一个酶促还原与平衡自动氧化的循环。这表现为氧化的NADPH超过初始存在的醌浓度以及过氧化氢的生成。这些发现表明，1,4-萘醌和2-甲基-1,4-萘醌的谷胱甘肽共轭物及其各自的5-羟基衍生物能够作为DT-黄递酶的底物，并且它们的自动氧化速率也高于未取代的母体化合物。本文根据DT-黄递酶在还原羟基或谷胱甘肽取代的萘醌中的细胞作用，以及这些对苯二酚在细胞环境中与葡糖醛酸或硫酸盐的进一步共轭作用，从而促进它们从细胞中清除，对这些结果进行了讨论。
