To assess the disulfide reduction capacity of intact cells, EA.hy926 endothelial cells were incubated with alpha-lipoic acid in the presence of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). alpha-Lipoic acid was reduced within cells to dihydrolipoic acid, which could be quantified upon efflux from the cells as reduction of DTNB. Uptake of both alpha-lipoic acid and alpha-lipoamide occurred at least in part via a medium chain fatty acid transporter, based on inhibition by octanoate. alpha-Lipoic acid was reduced within cells by pyridine nucleotide-disulfide oxidoreductases, since it is not reduced by GSH and since its reduction was inhibited by carmustine. Nonetheless, reduction was also dependent on the cellular redox environment, since it was inhibited by the redox cycling of menadione, by decreasing intracellular GSH, and by reduction of dehydroascorbate. Together, these results show that alpha-lipoic acid-dependent DTNB reduction provides a simple method to assess the disulfide-reducing capacity of intact cells, especially as determined by pyridine nucleotide-disulfide oxidoreductases. (c) 2006 Elsevier Inc. All rights reserved.