Probing the free radicals formed in the metmyoglobin–hydrogen peroxide reaction

The reaction between metmyoglobin and hydrogen peroxide results in the two-electron reduction of H2O2 by the protein, with concomitant formation of a ferryl-oxo heme and a protein-centered free radical. Sperm whale metmyoglobin, which contains three tyrosine residues (Tyr-103, Tyr-146, and Tyr-151) and two tryptophan residues (Trp-7 and Trp-14), forms a tryptophanyl radical at residue 14 that reacts with O2 to form a peroxyl radical and also forms distinct tyrosyl radicals at Tyr-103 and Tyr-151. Horse metmyoglobin, which lacks Tyr-151 of the sperm whale protein, forms an oxygen-reactive tryptophanyl radical and also a phenoxyl radical at Tyr-103. Human metmyoglobin, in addition to the tyrosine and tryptophan radicals formed on horse metmyoglobin, also forms a Cys-110-centered thiyl radical that can also form a peroxyl radical. The tryptophanyl radicals react both with molecular oxygen and with the spin trap 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS). The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) traps the Tyr-103 radicals and the Cys-110 thiyl radical of human myoglobin, and 2-methyl-2-nitrosopropane (MNP) traps all of the tyrosyl radicals. When excess H2O2 is used, DBNBS traps only a tyrosyl radical on horse myoglobin, but the detection of peroxyl radicals and the loss of tryptophan fluorescence support tryptophan oxidation under those conditions. Kinetic analysis of the formation of the various free radicals suggests that tryptophanyl radical and tyrosyl radical formation are independent events, and that formation of the Cys-110 thiyl radical on human myoglobin occurs via oxidation of the thiol group by the Tyr-103 phenoxyl radical. Peptide mapping studies of the radical adducts and direct EPR studies at low temperature and room temperature support the conclusions of the EPR spin trapping studies.