Abstract

Acetaminophen (4'-hydroxyacetanilide) is metabolized by horseradish peroxidase to a reactive metabolite or metabolites that become covalently (irreversibly) bound to either mouse liver microsomal protein or bovine serum albumin. The time-dependent reaction requires the presence of both the enzyme and hydrogen peroxide. Although the binding is almost completely inhibited by either catalase (0.2 mg/ml) or ascorbic acid (1.5 mM), it is unaffected by superoxide dismutase (20 µg/ml). Glutathione also inhibits the binding (∼50% at a concentration of 0.1 mM) with formation of the same glutathione conjugate that is produced from acetaminophen and glutathione in the presence of mouse liver microsomal oxygenases. An acetaminophen radical is generated by horseradish peroxidase in the presence of hydrogen peroxide as determined by electron paramagnetic resonance spectroscopy. The radical rapidly disappears in the presence of microsomal protein, bovine serum albumin, or glutathione, and is quenched by ascorbic acid with the concomitant formation of the ascorbate radical. The acetaminophen radical can be photolytically generated and spin-trapped with 5,5-dimethyl-1-pyrroline-1-oxide. The hyperfine splitting constants A_H^β (18.7 G) and A_N (15.2 G) strongly suggest that the radical is primarily centered on a carbon atom. These results indicate that a reactive metabolite of acetaminophen is formed in a peroxidase-mediated reaction with properties similar to that which is produced in microsomal incubations, and that an acetaminophen radical is formed under the same peroxidative conditions.