Evidence for a one-electron mechanism of 2-aminofluorene oxidation by prostaglandin H synthase and horseradish peroxidase.

Abstract

Previous studies have shown that the primary arylamine carcinogen 2-aminofluorene (2-AF) is oxidized by the prostaglandin H synthase peroxidase to mutagenic and electrophilic products capable of covalent binding to macromolecules. The present study was designed to identify the potential reactive intermediate(s) responsible for binding, and to characterize further the metabolic intermediates in 2-AF peroxidation. Both prostaglandin H synthase and horseradish peroxidase, with H2O2, oxidize 2-AF to azofluorene, 2-aminodifluorenylamine (2-ADFA), 2-nitrofluorene, polymeric and nonorganic-extractable material. Both enzymes show greater activity at pH 5.0 than at pH 7.0. In the presence of either 2-t-butyl-4-methoxyphenol or 2,6-dimethylphenol, arylamine/phenol adducts were formed in high yield, with the nitrogen of either 2-AF or 2-ADFA coupled to the para position of the phenol (loss of -OCH3 with 2-t-butyl-4-methoxyphenol). These structures were confirmed by mass spectrometry and NMR spectroscopy. Acid hydrolysis of N-hydroxy-2-AF to yield the nitrenium ion, in the presence of a phenol, also results in adduct formation, but only at times greater than 2 h and in very limited yield. The peroxidase-catalyzed adduct formation, however is rapid (less than 2 min) and extensive. These and other data support a one-electron pathway for 2-AF peroxidation, with a free radical or a free radical-derived product responsible for binding to protein and DNA. An N-hydroxy intermediate may therefore not be obligatory in the enzymatic activation of 2-AF to a mutagenic product.