Are the beneficial effects of 'antioxidant' lipoic acid mediated through metabolism of reactive sulfur species?

The health benefits of lipoic acid (LA) are generally attributed to mitigating the harmful effects of reactive oxygen species (ROS). ROS are chemically similar to reactive sulfur species (RSS) and signal through identical mechanisms. Here we examined the effects of LA on RSS in HEK293 cells using HS and polysulfide (PS) specific fluorophores, AzMC and SSP4. We show that LA concentration-dependently increased both HS and PS. Physioxia (5% O) augmented the effects of LA on HS production but decreased PS production. Thiosulfate, a known substrate for reduced LA, and an intermediate in the catabolism of HS enhanced the effects of LA on HS and PS production. Inhibiting peroxiredoxins with conoidin A and gluraredoxins with tiopronin augmented the effects of LA on PS and HS, respectively while decreasing glutathione with buthionine-sulfoximine (BSO) or diethyl maleate (DEM) decreased the stimulatory effect of LA on HS production but augmented LA's effect on PS. Aminooxyacetate (AOA) and propargylglycine (PPG), inhibitors of HS production from cysteine partially inhibited LA augmentation of HS production and further decreased the LA effect when applied concurrently with BSO and DEM. The selective and cell-permeable HS scavenger, SS20, inhibited the effects of LA on cellular HS. Estimates of single-cell HS production suggest that 0.1-0.2% of O consumption is used to metabolize HS and these requirements may increase to 1-2% with 1 mM LA. Collectively, these results suggest that LA rescues HS from irreversible oxidation and that the effects of LA on RSS directly confer antioxidant, anti-inflammatory and cytoprotective responses. They also suggest that TS may be an effective supplement to increase the efficacy of LA in clinical settings.