An Expanded Function for Superoxide Dismutase

Abstract

α-Hydroxyalkylperoxyl radicals were generated from the primary and secondary alcohols methanol, ethanol and 2-propanol in N₂O/O₂-saturated aqueous solutions by pulse radiolysis. These radicals reduced a ferric iron porphyrin complex, tetrakis-(4-N-methylpyridyl)porphine, with diffusiontontrolled rate constants. The extreme sensitivity of the shift of the Soret absorption band in this reaction was used to determine, by competition kinetics, the reactivity of the peroxyl radicals with different proteins. Only native Cu, Zn-superoxide dismutase and metallothionein showed competitive behavior, with SOD exhibiting rate constants close to the dismutation rate for O₂-. Metallothionein was slower by a factor of 30 with hydroxymethylperoxyl radicals. We propose, that SOD has unique properties of the protein surface in addition to the prosthetic copper site, having possibly evolved as a 'general-purpose radical-scavenging protein'.

Key Words:

• Alcohol peroxyl radicals
• competition kinetics
• iron porphyrin complex
• pulse radiolysis
• rate constants.