Generation of superoxide during the enzymatic action of tyrosinase

Evidence for the generation of superoxide anion in an enzymatic action of tyrosinase is reported. In the dopatyrosinase reaction, 1 mol of O2 is required for the production of 2 mol of dopaquinone, 1 mol of dopachrome, and 1/4 mol of O2-. Superoxide dismutase and 2-methyl-6-phenyl-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (a chemiluminescence probe and O2 trap) do not inhibit the rate of dopachrome formation from dopa in the presence of tyrosinase, indicating that free O2- is not utilized for metabolizing dopa. ESR studies for the accumulation of semiquinone radicals generated from tyrosine and N-acetyltyrosine in the presence of tyrosinase imply that O2- is not generated by the semiquinone + O2 reaction. Since the addition of H2O2 and dopa to tyrosinase promotes the release of O2- and formation of dopachrome, the Cu(II)O2-Cu(I) complex could be formed as a intermediate (an active form of tyrosinase); [Cu(II)]2 + H2O2 in equilibrium Cu(I)O2-Cu(II) + 2H+.