Glutaredoxins catalyze the reduction of glutathione by dihydrolipoamide with high efficiency

Glutaredoxins (Grx) are small (approximately 12kDa) proteins which catalyze thiol disulfide oxidoreductions involving glutathione (GSH) and disulfides in proteins or small molecules. Here, we present data which demonstrate the ability of glutaredoxins to catalyze the reduction of oxidized glutathione (GSSG) by dihydrolipoamide (DHL), an important biological redox catalyst and synthetic antioxidant. We have designed a new assay method to quantify the rate of reduction of GSSG and other disulfides by reduced lipoamide and have tested a set of eight recombinant Grx from human, rat, yeast, and E. coli. Lipoamide dependent activity is highest with the large atypical E. coli Grx2 (k(cat)=3.235 min(-1)) and lowest for human mitochondrial Grx2a (k(cat)=96 min(-1)) covering a wider range than k(cat) for the standard reduction of hydroxyethyldisulfide (HED) by GSH (290-2.851 min(-1)). The lipoamide/HED activity ratio was highest for yeast Grx2 (1.25) and E. coli Grx2 and lowest for E. coli Grx1 (0.13). These results suggest a new role for Grxs as ancillary proteins that could shunt reducing equivalents from main catabolic pathways to recycling of GSSG via a lipoyl group, thus serving biochemical functions which involve GSH but without NAD(P)H consumption.