Stable free radical and benzoquinone imine metabolites of an acetaminophen analogue.

The enzymatic oxidation of the acetaminophen analogue 3',5'-dimethyl-4'-hydroxyacetanilide (3',5'-dimethylacetaminophen) with the horseradish peroxidase/hydrogen peroxide system forms a phenoxyl free radical metabolite. The structure of this free radical is established by a complete analysis of the ESR spectrum and confirmed by deuterium isotope substitution. Concomitant with phenoxyl radical formation, N-acetyl-3,5-dimethyl-p-benzoquinone imine was detected by optical spectroscopy. The free radical is also formed by comproportionation in solutions of the quinone imine containing added 3',5'-dimethylacetaminophen. In contrast to acetaminophen, the imine and radical metabolites are stable and can be detected without resort to rapid-mixing techniques. Factors leading to the increased stability of these metabolites relative to those formed from acetaminophen are discussed in terms of the toxicity of acetaminophen.