Free radicals of acetaminophen: their subsequent reactions and toxicological significance

The oxidation of acetaminophen to the corresponding phenoxyl free radical and N-acetyl-p-benzoquinone imine by mammalian peroxidases is discussed. The acetaminophen free radical is very reactive--forming dimers, and, ultimately, melanin-like polymeric products. A model compound, leading to more stable metabolites, can be obtained by introduction of methyl groups next to the oxygen, to produce 3,5-dimethylacetaminophen. The electron spin resonance spectrum of this free radical could be completely analyzed. The phenoxyl radical of the dimethyl analog does not form polymers or bind with nucleophiles. N-Acetyl-p-benzoquinone imine, a hepatic metabolite of acetaminophen, and its analog N-acetyl-3,5-dimethyl-p-benzoquinone imine are metabolized by rat liver microsomes and NADPH to their corresponding p-aminophenoxyl free radicals. The p-aminophenoxyl free radical formation could be suppressed by the deacetylase inhibitors sodium fluoride and paraoxon. Substitution of NADPH-cytochrome P-450 reductase for rat liver microsomes eliminates the deacetylase activity and results in the direct reduction of N-acetyl-3,5-dimethyl-p-benzoquinone imine to the 3,5-dimethylacetaminophen phenoxyl free radical. Neither the acetaminophen nor the 3,5-dimethylacetaminophen phenoxyl radical reduces oxygen to form superoxide or reacts with oxygen in any other detectable way.