CYTOSOLIC AND MITOCHONDRIAL SYSTEMS FOR NADH-DEPENDENT AND NADPH-DEPENDENT REDUCTION OF ALPHA-LIPOIC ACID

In cellular, tissue, and organismal systems, exogenously supplied alph a-lipoic acid (thioctic acid) has a variety of significant effects, in cluding direct radical scavenging, redox modulation of cell metabolism , and potential to inhibit oxidatively-induced injury. Because reducti on of lipoate to dihydrolipoate is a crucial step in many of these pro cesses, we investigated mechanisms of its reduction. The mitochondrial NADH-dependent dihydrolipoamide dehydrogenase exhibits a marked prefe rence for R(+)-lipoate, whereas NADPH-dependent glutathione reductase shows slightly greater activity toward the S(-)-lipoate stereoisomer. Rat liver mitochondria also reduced exogenous lipoic acid. The rate of reduction was stimulated by substrates which increased the NADH conte nt of the mitochondria, and was inhibited by methoxyindole-2-carboxyli c acid, a dihydrolipoamide dehydrogenase inhibitor. In rat liver cytos ol, NADPH-dependent reduction was greater than NADH, and lipoate reduc tion was inhibited by glutathione disulfide. In rat heart, kidney, and brain whole cell-soluble fractions, NADH contributed more to reductio n (70-90%) than NADPH, whereas with liver, NADH and NADPH were about e qually active. An intact organ, the isolated perfused rat heart, reduc ed R-lipoate six to eight times more rapidly than S-lipoate, consisten t with high mitochondrial dihydrolipoamide dehydrogenase activity and results with isolated cardiac mitochondria. On the other hand, erythro cytes, which lack mitochondria, somewhat more actively reduced S- than R-lipoate. These results demonstrate differing stereospecific reducti on by intact cells and tissues. Thus, mechanisms of reduction of alpha -lipoate are highly tissue-specific and effects of exogenously supplie d alpha-lipoate are determined by tissue glutathione reductase and dih ydrolipoamide dehydrogenase activity. Copyright (C) 1996 Elsevier Scie nce Inc.