EFFICIENT REDUCTION OF LIPOAMIDE AND LIPOIC ACID BY MAMMALIAN THIOREDOXIN REDUCTASE

Reduction of the antioxidant lipoic acid has been proposed to be catal yzed in vivo by lipoamide dehydrogenase (LipDH) or glutathione reducta se (GR). We have found that thioredoxin reductase (TR) from calf thymu s, calf liver, human placenta and rat liver efficiently reduced both l ipoic acid and lipoamide with Michaelis-Menten type kinetics in NADPH- dependent reactions. In contrast to LipDH, lipoic acid was reduced alm ost as efficiently as lipoamide. Under equivalent conditions at 20 deg rees C, pH 8.0, mammalian TR reduced lipoic acid by NADPH 15 times mor e efficiently than the corresponding NADH dependent reduction catalyze d by LipDH (297 min(-1) for TR vs. 20.3 min(-1) for LipDH). Moreover, TR was 2.5 times faster in reducing lipoic acid with NADPH than in cat alyzing the reverse reaction (oxidation of dihydrolipoic acid with NAD P(+)). In contrast, LipDH was only 0.048 times as efficient in the for ward reaction as compared to the reverse reaction (using NADH and NAD( +)). We conclude that all or part of the previously described NADPH-de pendent lipoamide dehydrogenase (diaphorase) activities in mammalian s ystems should be attributed to TR. Our results suggest that in mammali an cells a significant part of the therapeutically important reduction of lipoic acid is catalyzed by thioredoxin reductase. (C) 1996 Academ ic Press, Inc.