METAL-MEDIATED OXIDATIVE DAMAGE TO CELLULAR AND ISOLATED DNA BY CERTAIN TRYPTOPHAN-METABOLITES

The tryptophan metabolites 3-hydroxyanthranilic acid (3-HAA) and 3-hyd roxykynurenine (3-HKyn) are carcinogens. DNA damage by 3-HAA and 3-HKy n in the presence of metal ions was investigated as a potential mechan ism of their carcinogenicity. Pulsed held gel electrophoresis showed t hat in the presence of Mn(II), 3-HAA and 3-HKyn induced DNA double-str and breaks in cultured human cells. DNA single-strand breaks were obse rved with alkali treatment. The enhancing effect of catalase inhibitor and the inhibitory effect of o-phenanthroline on the strand breakage indicated the involvement of H2O2 and endogenous transition metal ion. Damage to DNA fragments obtained from c-Ha-ras-1 protooncogene was in vestigated by a DNA sequencing technique. 3-HAA and 3-HKyn induced pip eridine-labile sites frequently at thymine and guanine residues in the presence of Cu(II). The inhibitory effects of bathocuproine and catal ase on Cu(II)-mediated DNA damage suggest that Cu(I) and H2O2 have imp ortant roles in the production of active species causing DNA damage. T he Cu(II)-mediated DNA damage was enhanced by preincubation of 3-HAA w ith Mn(II). UV-visible spectroscopy showed that Mn(II) and Cu(II) enha nced the rate of autoxidation of 3-HAA in different ways. These result s suggest that in the presence of Mn(II) or Cu(II), these tryptophan m etabolites produce H2O2, which is activated by transition metal ion to cause damage to DNA both in the case of isolated DNA and cultured cel ls.