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 H2S and polysulfide (PS) specific fluorophores, AzMC and SSP4. We show that LA concentration-dependently increased both H2S and PS. Physioxia (5% O-2) augmented the effects of LA on H2S production but decreased PS production. Thiosulfate, a known substrate for reduced LA, and an intermediate in the catabolism of H2S enhanced the effects of LA on H2S and PS production. Inhibiting peroxiredoxins with conoidin A and gluraredoxins with tiopronin augmented the effects of LA on PS and H2S, respectively while decreasing glutathione with buthionine-sulfoximine (BSO) or diethyl maleate (DEM) decreased the stimulatory effect of LA on H2S production but augmented LA's effect on PS. Aminooxyacetate (AOA) and propargylglycine (PPG), inhibitors of H2S production from cysteine partially inhibited LA augmentation of H2S production and further decreased the LA effect when applied concurrently with BSO and DEM. The selective and cell-permeable H2S scavenger, SS20, inhibited the effects of LA on cellular H2S. Estimates of single-cell H2S production suggest that 0.1-0.2% of O-2 consumption is used to metabolize H2S and these requirements may increase to 1-2% with 1 mM LA. Collectively, these results suggest that LA rescues H2S 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.