A new look at a time-worn system: oxidation of CuZn-SOD by H2O2

This work summarizes observations from numerous investigators on the reaction of the copper-zinc superoxide dismutase with hydrogen peroxide at physiological pH in order to propose a likely sequence of events that leads to 2-oxo-histidine formation, copper loss, inactivation, and random and site-specific peptide fragmentation. New data is presented for the bovine liver enzyme that indicate copper is lost as the copper(I) form which immediately reacts with bathocuproine disulfonate to form the characteristic complex that absorbs at 485 nm. Studies in TRIS buffer ruled out the loss of copper(II) followed by reduction of the high potential copper(II)-bathocuproine disulfonate complex by buffer because TRIS is known not to reduce this complex. The rate of loss of copper(I) is not affected by the spin trap, 5,5'-dimethylpyrolline-N-oxide (DMPO), nor by replacing oxygen with argon in the reaction. In addition, changes in the native electrophoretic pattern that are correlated with copper loss and not peptide fragmentation are also unaffected by DMPO, argon, EDTA, or DTPA. These data are taken as indirect evidence that the formation of 2-oxo-histidine is the first oxidative event, unaffected by DMPO, that occurs at the bound oxidant and leads to loss of copper(I). Peptide fragmentation and the peroxidative activity of the dismutase are discussed in light of these observations.