A HYDROGEN-DONATING MONOHYDROXAMATE SCAVENGES FERRYL MYOGLOBIN RADICALS

The addition of 25muM hydrogen peroxide to 20muM metmyoglobin produces ferryl (Fe(IV) = O) myoglobin. Optical spectroscopy shows that the fe rryl species reaches a maximum concentration (60-70%o of total haem) a fter 10 minutes and decays slowly (hours). Low temperature EPR spectro scopy of the high spin metmyoglobin (g = 6) signal is consistent with these findings. At this low peroxide concentration there is no evidenc e for iron release from the haem. At least two free radicals are detec table by EPR immediately after H2O2 addition, but decay completely aft er ten minutes. However, a longer-lived radical is observed at lower c oncentrations that is still present after 90 minutes. The monohydroxam ate N-methylbutyrohydroxamic acid (NMBH) increases the rate of decay o f the fenyl species. In the presence of NMBH, none of the protein-boun d free radicals are detectable; instead nitroxide radicals produced by oxidation of the hydroxamate group are observed. Similar results are observed with the trihydroxamate, desferrioxamine. ''Ferryl myoglobin' ' is still able to initiate lipid peroxidation even after the short-li ved protein free radicals are no longer detectable (E.S.R. Newman, C.A . Rice-Evans and M.J. Davies (1991) Biochemical and Biophysical Resear ch Communications 179, 1414-1419). It is suggested that the longer-liv ed protein radicals described here may be partly responsible for this effect. The mechanism of inhibition of initiation of lipid peroxidatio n by hydroxamate drugs, such as NMBH, may therefore be due to reductio n of the protein-derived radicals, rather than reduction of ferryl hae m.