The peroxidase and peroxynitrite reductase activity of human erythrocyte peroxiredoxin 2

Peroxiredoxin 2 (Prx2) is a 2-Cys peroxiredoxin extremely abundant in the erythrocyte. The peroxidase activity was studied in a steady-state approach yielding an apparent KM of 2.4 μM for human thioredoxin and a very low KM for H2O2 (⩽0.7 μM). Rate constants for the reaction of peroxidatic cysteine with the peroxide substrate, H2O2 or peroxynitrite, were determined by competition kinetics, k2 = 1.0 × 108 and 1.4 × 107 M−1 s−1 at 25 °C and pH 7.4, respectively. Excess of both oxidants inactivated the enzyme by overoxidation and also tyrosine nitration and dityrosine were observed with peroxynitrite treatment. Prx2 associates into decamers (5 homodimers) and we estimated a dissociation constant Kd < 10−23 M4 which confirms the enzyme exists as a decamer in vivo. Our kinetic results indicate Prx2 is a key antioxidant enzyme for the erythrocyte and reveal red blood cells as active oxidant scrubbers in the bloodstream.