We examined the ability of horseradish peroxidase (HRP), an analog of human myeloperoxidase, to protect DNA against oxidative damage caused by peroxynitrite in the presence of chlorogenic acid (CGA), a naturally occurring polyphenol. Chlorogenic acid inhibits the formation of single strand breaks in supercoiled pBR322 DNA by acting as a scavenger of peroxynitrite. Horseradish peroxidase markedly enhances the extent of DNA protection by catalyzing the decomposition of peroxynitrite in the presence of CGA. Horseradish peroxidase alone does not inhibit peroxynitrite-induced DNA strand breaks, indicating that CGA is required as an electron donor to regenerate the active enzyme. The apparent second order rate constant for the HRP-mediated oxidation of CGA in the presence of peroxynitrite at pH 6.9 is 3.4 x 10(7) M(-1) s(-1). This high rate suggests that CGA and other dietary polyphenols might efficiently scavenge peroxynitrite in peroxidase-containing systems in vivo.