The chemistry of lipid alkoxyl radicals and their role in metal-amplified lipid peroxidation

Reaction of polyunsaturated fatty acid hydroperoxides with metal complexes generates lipid alkoxyl radicals and metal-oxo complexes. Lipid alkoxyl radicals are presumed to be the species responsible for metal-amplified lipid peroxidation because of the chemical analogy of simple organic alkoxyl radicals to the hydroxyl radical. However, polyunsaturated fatty acid alkoxyl radicals exhibit a rich and diverse chemistry that is dominated by intramolecular cyclization to epoxyallylic radicals. Studies described herein demonstrate that the equilibrium between cyclization and ring-opening of epoxyallylic radicals lies overwhelmingly toward cyclization. Thus lipid alkoxyl radicals have a steady-state concentration that is so low that their contribution to metal-amplified lipid peroxidation is insignificant. In fact, the species responsible for metal amplification of lipid peroxidation appears to be the epoxyperoxyl radical formed by coupling the epoxyallylic radical to molecular oxygen.