REMOVAL OF HYDROGEN-PEROXIDE BY THIOL-SPECIFIC ANTIOXIDANT ENZYME (TSA) IS INVOLVED WITH ITS ANTIOXIDANT PROPERTIES - TSA POSSESSES THIOL PEROXIDASE-ACTIVITY

The thiol-specific antioxidant protein (TSA) protects glutamine synthe tase from inactivation by a metal-catalyzed oxidation (MCO) system com prised of dithiothreitol (DTT)/Fe3+/O-2 but not by the ascorbate/Fe3+/ O-2 MCO system. The removal of sulfur-centered radicals or H2O2 has be en proposed as the protective mechanism of TSA. Like catalase, TSA pre vents the initiation of the rapid O-2 uptake phase during MCO of DTT b ut causes only partial inhibition when added after the reaction is wel l into the propagation phase. Stoichiometric studies showed that the a ntioxidant property of TSA is, at least in part, due to its ability to catalyze the destruction of H2O2 by the overall reaction 2 RSH + H2O2 --> RSSR + H2O. Results of kinetic studies demonstrate that the remov al of H2O2 by TSA correlates with its ability to protect glutamine syn thetase from inactivation. In the presence of thioredoxin, TSA is more active, whereas C170S (an active mutant of TSA in which cysteine 170 was replaced by a serine) and open reading frame 6 (a human antioxidan t protein homologous to TSA with only one conserved cysteine residue) are only slightly affected. The thiol specificity of the protective ac tivity of TSA derives from the fact that the oxidized form of TSA can be converted back to its sulfhydryl form by treatment with thiols but not by ascorbate.