The eosinophil peroxidase-hydrogen peroxide-bromide system of human eosinophils generates 5-bromouracil, a mutagenic thymine analogue

Eosinophils use eosinophil peroxidase, hydrogen peroxide (H2O2), and bromid e ion (Br-) to generate hypobromous acid (HOBr), a brominating intermediate . This potent oxidant may play a role in host defenses against invading par asites and eosinophil-mediated tissue damage. In this study, we explore the possibility that HOBr generated by eosinophil peroxidase might oxidize nuc leic acids. When we exposed uracil, uridine, or deoxyuridine to reagent HOB r, each reaction mixture yielded a single major oxidation product that comi grated on reversed-phase HPLC with the corresponding authentic brominated p yrimidine. The eosinophil peroxidase-H2O2-Br- system also converted uracil into a single major oxidation product, and the yield was near-quantitative. Mass spectrometry, HPLC, W-visible spectroscopy, and NMR spectroscopy iden tified the product as 5-bromouracil. Eosinophil peroxidase required H2O2 an d Br- to produce 5-bromouracil, implicating HOBr as an intermediate in the reaction. Primary and secondary bromamines also brominated uracil, suggesti ng that long-lived bromamines also might be physiologically relevant bromin ating intermediates. Human eosinophils used the eosinophil peroxidase-H2O2- Br- system to oxidize uracil. The product was identified as 5-bromouracil b y mass spectrometry, HPLC, and UVvisible spectroscopy. Collectively, these results indicate that HOBr generated by eosinophil peroxidase oxidizes urac il to 5-bromouracil. Thymidine phosphorylase, a pyrimidine salvage enzyme, transforms 5-bromouracil to 5-bromodeoxyridine, a mutagenic analogue of thy midine. These findings raise the possibility that halogenated nucleobases g enerated by eosinophil peroxidase exert cytotoxic and mutagenic effects at eosinophil-rich sites of inflammation.