Differential effects of toxic metal compounds on the activities of Fpg andXPA, two zinc finger proteins involved in DNA repair

Even though not mutagenic, compounds of the carcinogenic metals nickel, cad mium, cobalt and arsenic have been shown previously to inhibit nucleotide e xcision repair and base excision repair at low, non-cytotoxic concentration s. Since some toxic metals have high affinities for SH groups, we used the bacterial formamidopyrimidine-DNA glycosylase (Fpg protein) and the mammali an XPA protein as models to Investigate whether zinc finger structures in D NA repair enzymes are particularly sensitive to carcinogenic andlor toxic m etal compounds. Concentrations of less than or equal to1 mM Ni(II), Pb(II), As(III) or Co(II) did not affect the activity of the Fpg protein significa ntly. Tn contrast, the enzyme was inhibited in a dose-dependent manner by C d(II), Cu(Ir) or Hg(II), starting at concentrations of 50 muM, 5 muM acid 5 0 nM, respectively. Simultaneous treatment with Cd(Ir) or Cu(II) and Zn(II) partly prevented the inhibitions, while no reversal of inhibition was obse rved when Zn(II) was added after Cd(II) or Cu(II), In the case of Hg(II), Z n(II) had no protective effect independent of the time of its addition; how ever, the enzyme activity was completely restored by glutathione. Regarding XPA, Hg(II), Pb(II) or As(III) did not diminish its binding to an UV-irrad iated oligonucleotide, while Cd(II), Co(II), Cu(II) and Ni(II) reduced its DNA-binding ability. Simultaneous treatment with Zn(II) prevented largely t he inhibition induced by Cd(II), Co(II), and Ni(II), but only slightly in t he case of Cu(II). Collectively, the results indicate that both proteins we re Inhibited by Cd(II) and Cu(II), XPA was additionally inactivated by Ni(I I) and Co(II), and Fpg but not XPA was strongly affected by Hg(II). Even th ough other mechanisms of protein inactivation cannot be completely excluded , zinc finger structures may be sensitive targets for toxic metal compounds , but each zinc finger protein has unique sensitivities.