Peroxidation of linoleate at physiological pH: hemichrome formation by substrate binding protects against metmyoglobin activation by hydrogen peroxide

Peroxidation by metmyoglobin, MbFe(III), by metmyoglobin/hydrogen peroxide, MbFe(III)/H2O2 , to yield the myoglobin ferryl radical (√MbFe(IV)=O), or by ferrylmyoglobin, MbFe(IV)=O, was investigated at physiological pH (7.4) in oil-in-water linoleate emulsions. Linoleate peroxidation was followed using second derivative ultraviolet (UV)-spectroscopy for monitoring formation of conjugated dienes and quantitative determination of specific linoleate hydroperoxides by liquid chromatography with photodiode absorption detection. Modifications of myoglobins during lipid peroxidation were followed simultaneously by changes in the Soret absorption band (410 or 424 nm), and in the visible absorption region (from 450 to 700 nm), combined with electron spin resonance (ESR) spectroscopy for direct detection of changes in the spin state of the iron center. In contrast to MbFe(IV)=O, MbFe(III) and MbFe(III)/H2O2 were not able to initiate linoleate peroxidation in oil-in-water emulsions, and MbFe(III) was converted, by binding of linoleate (but not methyl linoleate), to a low-spin hemichrome derivate, HMbFe(III), with the distal histidine reversibly bound to the iron center. HMbFe(III) is ineffective in initiating lipid peroxidation and cannot be activated to √MbFe(IV)=O or MbFe(IV)=O by addition of moderate amounts of H2O2. Addition of MbFe(III) to linoleate emulsions containing H2O2 results in the competitive formation of √MbFe(IV)=O and HMbFe(III) in favor of HMbFe(III), and little linoleate peroxidation is detected, demonstrating the inherent protection, at physiologic pH, against peroxidation by reversible binding of the substrate to the potential myoglobin catalyst.