Brown Stephanie E, Ross Meredith F, Sanjuan-Pla Alejandra, Manas Abdul-Rahman B, Smith Robin A J, Murphy Michael P
Medical Research Council Dunn Human Nutrition Unit, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, UK.
Free Radic Biol Med. 2007 Jun 15;42(12):1766-80. doi: 10.1016/j.freeradbiomed.2007.02.033. Epub 2007 Mar 12.
Lipoic acid (LA) is a widely used antioxidant that protects mitochondria from oxidative damage in vivo. Much of this protection is thought to be due to the reduction of LA to dihydrolipoic acid (LAH(2)). This reduction is catalyzed in vivo by thioredoxin, thioredoxin reductase (TrxR), and lipoamide dehydrogenase. We hypothesized that specifically targeting LA to mitochondria, the site of most cellular reactive oxygen species production, would make it a more effective antioxidant. To do this, we made a novel molecule, MitoLipoic acid, by attaching lipoic acid to the lipophilic triphenylphosphonium cation. MitoL was accumulated rapidly within mitochondria several-hundred fold driven by the membrane potential. MitoL was reduced to the active antioxidant dihydroMitoLipoic acid by thioredoxin and by lipoamide dehydrogenase but not by TrxR. In isolated mitochondria or cells MitoL was only slightly reduced (5-10%), while, in contrast, LA was extensively reduced. This difference was largely due to the reaction of LA with TrxR, which did not occur for MitoL. Furthermore, in cells MitoL was quantitatively converted to an S-methylated product. As a consequence of its lack of reduction, MitoL was not protective for mitochondria or cells against a range of oxidative stresses. These results suggest that the protective action of LA in vivo may require its reduction to LAH(2) and that this reduction is largely mediated by TrxR.
硫辛酸（LA）是一种广泛使用的抗氧化剂，可在体内保护线粒体免受氧化损伤。人们认为这种保护作用很大程度上归因于LA还原为二氢硫辛酸（LAH₂）。这种还原在体内由硫氧还蛋白、硫氧还蛋白还原酶（TrxR）和硫辛酰胺脱氢酶催化。我们推测，将LA特异性靶向到大多数细胞活性氧产生的部位——线粒体，会使其成为一种更有效的抗氧化剂。为此，我们通过将硫辛酸连接到亲脂性三苯基膦阳离子上，制备了一种新型分子——线粒体硫辛酸（MitoLipoic acid）。在膜电位的驱动下，MitoL在线粒体内迅速积累，达到数百倍。MitoL可被硫氧还蛋白和硫辛酰胺脱氢酶还原为活性抗氧化剂二氢线粒体硫辛酸，但不能被TrxR还原。在分离的线粒体或细胞中，MitoL仅略有还原（5 - 10%），而相比之下，LA被大量还原。这种差异很大程度上是由于LA与TrxR发生了反应，而MitoL不会发生这种反应。此外，在细胞中，MitoL定量转化为一种S - 甲基化产物。由于其缺乏还原作用，MitoL对线粒体或细胞抵御一系列氧化应激没有保护作用。这些结果表明，LA在体内的保护作用可能需要将其还原为LAH₂，并且这种还原很大程度上由TrxR介导。
