Bicarbonate enhances the hydroxylation, nitration, and peroxidation reactions catalyzed by copper, zinc superoxide dismutase - Intermediacy of carbonate anion radical

The effect of bicarbonate anion (HCO3-) on the peroxidase activity of copper, zinc superoxide dismutase (SOD1) was investigated using three structurally different probes: 5,5'-dimethyl-1-pyrroline N-oxide (DMPO), tyrosine, and 2,2'-azino-bis- [3-ethylbenzothiazolinel-6-sulfonic acid (ABTS), Results indicate that HCO3- enhanced SOD/H2O2-dependent (i) hydroxylation of DMPO to DMPO-OH as measured by electron spin resonance, (ii) oxidation and nitration of tyrosine to dityrosine, nitrotyrosine, and nitrodityrosine as measured by high pressure liquid chromatography, and (iii) oxidation of ARTS to the ARTS cation radical as measured by UV-visible spectroscopy. Using oxygen-17-labeled water, it was determined that the oxygen atom present in the DMPO-OH adduct originated from H2O and not from H2O2. This result proves that neither free hydroxyl radical nor enzyme-bound hydroxyl radical was involved in the hydroxylation of DMPO. We postulate that HCO3- enhances SOD1 peroxidase activity via formation of a putative carbonate radical anion. This new and different perspective on HCO3--mediated oxidative reactions of SOD1 may help us understand the free radical mechanism of SOD1 and related mutants linked to amyotrophic lateral sclerosis.