alpha-Lipoic acid dependent regeneration of ascorbic acid from dehydroascorbic acid in rat liver mitochondria

Rat liver mitochondria were examined for their ability to reduce dehydroascorbic acid to ascorbic acid in an alpha-lipoic acid dependent or independent manner. The alpha-lipoic acid dependent reduction was stimulated by factors that increased the NADH dependent reduction of alpha-lipoic acid to dihydrolipoic acid in coupled reactions. Optimal conditions for dehydroascorbic acid reduction to ascorbic acid were achieved in the presence of pyruvate, alpha-lipoic acid, and ATP. Electron transport inhibitors, rotenone and antimycin A, further enhanced the dehydroascorbic acid reduction. The reactions were strongly inhibited by 1 mM iodoacetamide or sodium arsenite. Mitoplasts were qualitatively similar to intact mitochondria in dehydroascorbate reduction activity. Pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase reduced dehydroascorbic acid to ascorbic acid in an alpha-lipoic acid, coenzyme A, and pyruvate or alpha-ketoglutarate dependent fashion. Dehydroascorbic acid was also catalytically reduced to ascorbic acid by purified lipoamide dehydrogenase in an alpha-lipoic acid (K0.5 = 1.4 +/- 0.8 mM) and lipoamide (K0.5 = 0.9 +/- 0.3 mM) dependent manner.