Dithiothreitol Induces the Sacrificial Antioxidant Property of Human Serum Albumin in a Metal-Catalyzed Oxidation and γ-Irradiation System

The species •OH or H2O2 are produced by both metal-catalyzed oxidation (MCO) of reducing equivalents and γ-irradiation. Intact or Cys-34-modified human serum albumin (HSA) was significantly degraded in the MCO system containing dithiothreitol (DTT) as electron donor, but as long as it lasted, HSA prohibited •OH or H2O2 from initiating molecular damage of DNA. However, in the GSH and ascorbate (nonthiol) MCO system, HSA was not sacrificially degraded, and indeed accelerated the formation of DNA strand breaks. In the γ-irradiation system producing •OH from H2O, only DTT attenuated the generation of DNA strand breaks by HSA. It did not degrade more H2O2 in the presence of reduced GSH (thiol-linked peroxidase) than in its absence. Therefore it would seem that in an MCO system, the antioxidant activity of HSA depends on the effectiveness of reducing equivalents to induce exposure of a functional group scavenging the •OH or H2O2 species, by reduction of its disulfide-bonds. In the presence of DTT, disulfide bonds in HSA were quantitatively reduced to cysteinyl residues but not significantly reduced by ascorbate or GSH. In conclusion, the antioxidant activity of HSA in the DTT MCO system is thought not to involve a thiol-specific peroxidase function, but to be due to sacrificial antioxidant activity by functional groups, such as hydrophobic residues exposed on the protein surface by DTT.