The effect of myoglobin on the stability of the hydroxyl-radical adducts of 5,5 dimethyl-1-pyrolline-N-oxide (DMPO), 3,3,5,5 tetramethyl-1-pyrolline-N-oxide (TMPO) and 1-alpha-phenyl-tert-butyl nitrone (PBN) in the presence of hydrogen peroxide.
The hydroxyl radical adducts of 5,5 dimethyl-1-pyrolline-N-oxide (DMPO) and 3,3,5,5 tetramethyl-1-pyrolline-N-oxide (TMPO) formed in the presence of hydrogen peroxide and FeII are normally quite stable, but in the presence of 5-20 micromolar myoglobin their ESR signals decay rapidly. This decay probably reflects further oxidation of the adduct to nonparamagnetic products. The ESR signal of the hydroxyl radical adduct of 1-alpha-phenyl-tert-butyl nitrone (PBN) formed under similar conditions is subject to non-heme dependent attenuation, possibly via hydroxyl radical scavenging, but not to heme dependent decay. Hydrogen peroxide readily converts myoglobin to its ferryl (FeIV) derivative, and this centre may be responsible for the oxidation of the DMPO and TMPO adducts. The different behaviour of PBN may be due to differences in susceptibility to ferrylmyoglobin mediated oxidation, or to steric differences controlling access to the heme pocket of myoglobin, and is relevant to the choice of spin trap for biological experiments aimed at detecting hydroxyl radicals in the presence of myoglobin or other heme proteins.