Metal-mediated oxidative damage to cellular and isolated DNA by certain tryptophan metabolites

The tryptophan metabolites 3-hydroxyanthranilic acid (3-HAA) and 3-hydroxykynurenine (3-HKyn) are carcinogens. DNA damage by 3-HAA and 3-HKyn in the presence of metal ions was investigated as a potential mechanism of their carcinogenicity. Pulsed field gel electrophoresis showed that in the presence of Mn(II), 3-HAA and 3-HKyn induced DNA double-strand breaks in cultured human cells. DNA single-strand breaks were observed with alkali treatment. The enhancing effect of catalase inhibitor and the inhibitory effect of o-phenanthroline on the strand breakage indicated the involvement of H 2 O 2 and endogenous transition metal ion. Damage to DNA fragments obtained from c-Ha-ras-1 protooncogene was investigated by a DNA sequencing technique. 3-HAA and 3-HKyn induced piperidine-labile sites frequently at thymine and guanine residues in the presence of Cu(II). The inhibitory effects of bathocuproine and catalase on Cu(II)-mediated DNA damage suggest that Cu(I) and H 2 O 2 have important roles in the production of active species causing DNA damage. The Cu(II)-mediated DNA damage was enhanced by preincubation of 3-HAA with Mn(II). UV-visible spectroscopy showed that Mn(II) and Cu(II) enhanced the rate of autoxidation of 3-HAA in different ways. These results suggest that in the presence of Mn(II) or Cu(II), these tryptophan metabolites produce H 2 O 2 , which is activated by transition metal ion to cause damage to DNA both in the case of isolated DNA and cultured cells