Enhancement of hydroxylation and deglycosylation of 2'-deoxyguanosine by carcinogenic nickel compounds.

The effects of nickel subsulfide (Ni3S2) and nickel chloride [Ni(II)] on hydroxylation and deglycosylation of pure 2'-deoxyguanosine (dG) and on hydroxylation of guanine (Gu) residues in calf thymus DNA in the absence or presence of hydrogen peroxide (H2O2) and/or ascorbate (Ascb) were studied with the use of high-performance liquid chromatography. Incubation of 0.75 mM dG with 5 mg Ni3S2/ml (particle size less than 5 microns) at 37 degrees C in aerated 50 mM Tris/HCl buffer, pH 7.4, resulted in slow hydroxylation of dG to 8-hydroxy-2'-deoxyguanosine (8-OH-dG). This effect was greatly enhanced by 20 mM H2O2. Ni(II) alone at concentrations up to 10 mM was inactive but produced 8-OH-dG in the presence of 20 mM H2O2; the latter caused no dG hydroxylation by itself. Both Ni3S2 and Ni(II) increased the formation of 8-OH-dG from dG exposed to H2O2 + Ascb. At pH 7.4 and constant concentration of H2O2 and Ascb (20 and 8 mM, respectively), Ni(II) over the concentration range 1-10 mM raised the hydroxylation yield by up to five times that without Ni(II). Also, addition of 7.5 mM Ni(II) more than doubled the hydroxylation yield of Gu residues by the 20 mM H2O2 + 8 mM Ascb mixture (pH 7.4) in denatured DNA and doubled it in native DNA. Ni3S2 and Ni(II) alone had no effect on deglycosylation of dG and did not significantly influence the slow rate of Gu production from dG reacting with H2O2 or Ascb at pH 7.4. However, Ni(II), unlike Ni3S2, increased the extent of dG deglycosylation when added to the dG + H2O2 + Ascb system; 10 mM Ni(II) increased deglycosylation by a factor of 2.5 in 24 h. Thus, nickel carcinogens were shown for the first time to cause and/or enhance both hydroxylation and deglycosylation reactions of dG which may contribute to the observed genotoxic and carcinogenic effects of this metal.