Reduction of molecular oxygen by redox active thiols: Comparison of glutathione, N-acetylcysteine, cysteine, and homocysteine

Abstract

The reaction properties of the thiol compounds, cysteine (Cys), N- acetyl-L-cysteine (NAC), the reduced form glutathione (GSH), and homocysteine (HCS) were compared. The main purpose of this study was to find a thiol-based anti-oxidant suitable for biological experiments and to provide clear reasoning for its selection. The availability of thiol compounds to generate superoxide by reducing molecular oxygen (O2) at a hyperthermal temperature was discussed. An oxidative atmosphere, i.e., superoxide generation by the hypoxanthine-xanthine oxidase reaction, hydroxyl radical generation by X-ray irradiation, or direct one-electron oxidation by ferricyanide, was prepared in a reaction mixture containing 0.1 mM TEMPOL and 1 mM test compound, and the EPR signal decay of TEMPOL was observed. A reaction mixture containing 0.1 mM TEMPOL and 1 mM thiol compound was incubated at 44°C, and the EPR signal decay of TEMPOL was observed. Thiols could function as H-donors to the oxoammonium cation and produce the hydroxylamine form of TEMPOL in an oxidative atmosphere. Thiols could also irreversibly react with the oxoam- monium cation. GSH and Cys could reduce O2 to form superoxide/ hydroperoxyl radical at hyperthermal temperatures, but HCS and NAC could not reduce O2. GSH and Cys may cause reductive stress, whereas NAC is a simple tractable antioxidant.