Constantinescu A, Pick U, Handelman G J, Haramaki N, Han D, Podda M, Tritschler H J, Packer L
Department of Molecular and Cell Biology, University of California, Berkeley 94720-3200, USA.
Biochem Pharmacol. 1995 Jul 17;50(2):253-61. doi: 10.1016/0006-2952(95)00084-d.
Reduction of exogenous lipoic acid to dihydrolipoate is known to occur in several mammalian cells and tissues. Dihydrolipoate is a potent radical scavenger, and may provide significant antioxidant protection. Because lipoic acid appears in the bloodstream after oral administration, we have examined the reduction of exogenous lipoate by human erythrocytes. Normal human erythrocytes reduced lipoate to dihydrolipoate only in the presence of glucose; deoxyglucose did not substitute for glucose, indicating that the reduction of lipoate requires glucose metabolism. Furthermore, the reduction was shown to be NADPH dependent. Erythrocytes isolated from a human subject with a genetic deficiency of glucose-6-phosphate dehydrogenase (and, therefore, deficient in the formation of NADPH) did not reduce lipoate. Dehydroepiandrosterone, a specific inhibitor of glucose-6-phosphate dehydrogenase, inhibited lipoate reduction. Our findings imply that some of the reduction of exogenous lipoic acid is catalysed by glutathione reductase, a flavoprotein dehydrogenase; mitomycin C, an inhibitor of FAD-dependent reductases, inhibited lipoate reduction by erythrocytes, and glutathione reductase purified from human erythrocytes was observed to reduce lipoic acid in a cell-free system. We further explored these findings with erythrocyte ghosts and liposomes. Our results indicate that a transport system exists for alpha-lipoic acid and dihydrolipoate; resealed erythrocyte ghosts, containing trapped lipoamide dehydrogenase and pyridine nucleotides, reduced externally added lipoate. By contrast, liposomes prepared with enzyme and pyridine nucleotides did not catalyze reduction of lipoate. This work indicates that uptake of exogenous lipoate and reduction to dihydrolipoate by normal human erythrocytes may contribute to oxidant protection in the human bloodstream.
已知在几种哺乳动物细胞和组织中会发生外源性硫辛酸还原为二氢硫辛酸的过程。二氢硫辛酸是一种强效自由基清除剂，可能提供显著的抗氧化保护。由于口服后硫辛酸会出现在血液中，我们研究了人类红细胞对外源性硫辛酸的还原作用。正常人红细胞仅在有葡萄糖存在的情况下才将硫辛酸还原为二氢硫辛酸；脱氧葡萄糖不能替代葡萄糖，这表明硫辛酸的还原需要葡萄糖代谢。此外，该还原过程显示为NADPH依赖性。从一名葡萄糖-6-磷酸脱氢酶基因缺陷的人类受试者中分离出的红细胞（因此缺乏NADPH的形成）不能还原硫辛酸。脱氢表雄酮是葡萄糖-6-磷酸脱氢酶的特异性抑制剂，可抑制硫辛酸的还原。我们的研究结果表明，外源性硫辛酸的部分还原是由谷胱甘肽还原酶催化的，谷胱甘肽还原酶是一种黄素蛋白脱氢酶；丝裂霉素C是一种FAD依赖性还原酶的抑制剂，可抑制红细胞对硫辛酸的还原，并且观察到从人类红细胞中纯化的谷胱甘肽还原酶在无细胞系统中可还原硫辛酸。我们用红细胞血影和脂质体进一步探究了这些发现。我们的结果表明，存在α-硫辛酸和二氢硫辛酸的转运系统；含有被困的硫辛酰胺脱氢酶和吡啶核苷酸的重封红细胞血影可还原外部添加的硫辛酸。相比之下，用酶和吡啶核苷酸制备的脂质体不能催化硫辛酸的还原。这项工作表明，正常人红细胞对外源性硫辛酸的摄取以及将其还原为二氢硫辛酸可能有助于人类血液中的抗氧化保护。
