Peroxiredoxin 2 (Prx2), a thiol-dependent peroxidase, is the third most abundant protein in the erythrocyte, and its absence in knock-out mice gives rise to hemolytic anemia. We have found that in human erythrocytes, Prx2 was extremely sensitive to oxidation by H(2)O(2), as dimerization was observed after exposure of 5 x 10(6) cells/mL to 0.5 muM H(2)O(2). In contrast to Prx2 in Jurkat T lymphocytes, Prx2 was resistant to overoxidation (oxidation of the cysteine thiol to a sulfinic/sulfonic acid) in erythrocytes. Reduction of dimerized Prx2 in the erythrocyte occurred very slowly, with reversal occurring gradually over a 20-minute period. Very low thioredoxin reductase activity was detected in hemolysates. We postulate that this limits the rate of Prx2 regeneration, and this inefficiency in recycling prevents the overoxidation of Prx2. We also found that Prx2 was oxidized by endogenously generated H(2)O(2), which was mainly derived from hemoglobin autoxidation. Our results demonstrate that in the erythrocyte Prx2 is extremely efficient at scavenging H(2)O(2) noncatalytically. Although it does not act as a classical antioxidant enzyme, its high concentration and substrate sensitivity enable it to handle low H(2)O(2) concentrations efficiently. These unique redox properties may account for its nonredundant role in erythrocyte defense against oxidative stress.