Free-radical metabolites of acetaminophen and a dimethylated derivative.

The oxidation of acetaminophen (4'-hydroxyacetanilide) to the corresponding N-acetyl-p-benzoquinone imines by plant and mammalian peroxidases is discussed. The acetaminophen free radical (N-acetyl-4-aminophenoxyl) has been reported as an intermediate. It is very reactive and forms melanin-like polymeric products. Application of a fast-flow system makes it possible to detect the transient species and clearly distinguish it from persistent paramagnetic melanin polymers. A model system, leading to more stable metabolites, can be obtained by introduction of methyl groups next to the oxygen, 3',5'-dimethylacetaminophen (3',5'-dimethyl-4'-hydroxyacetanilide). The ESR spectrum of the free radical formed could be completely analyzed and confirmed by deuterium substitution. The data are consistent with the assignment to a phenoxyl free radical (N-acetyl-2,6-dimethyl-4-amino-phenoxyl). Its formation is discussed in terms of substrate, hydrogen peroxide and enzyme concentration dependence. It is believed to be formed via a direct one-electron oxidation of 3',5'-dimethyl-4'-hydroxy-acetanilide. The radical does not form polymers or react with nucleophiles. Its redox behavior is discussed. The possible reaction of these phenoxyl free radicals with oxygen is thought to be negligible.