Sensitivity of Escherichia coli (MutT) and human (MTH1) 8-oxo-dGTPases to in vitro inhibition by the carcinogenic metals, nickel(II), copper(II), cobalt(II) and cadmium(II)

The toxicity of Ni(II), Co(II) and Cu(II) in animals, and that of Cd(II) incultured cells, has been associated with generation of the promutageniclesion 8-oxo-7,8-dihydroguanine (8-oxoguanine) in DNA, among other effects.One possible source of this base may be 8-oxo-7,8-dihydro-2'-deoxyguanosine-5'-triphosphate (8-oxo-dGTP), a product ofoxidative damage to the nucleotide pool, from which it is incorporated intoDNA. To promote such incorporation, the metals would have to inhibitspecific cellular 8-oxo-dGTPases that eliminate 8-oxo-dGTP from thenucleotide pool. The present study was designed to test such inhibition invitro on 8-oxo-dGTPases from two different species, the human MTH1 proteinand Escherichia coli MutT protein. In the presence of Mg(II), the naturalactivator of 8-oxo-dGTPases, all four metals were found to inhibit bothenzymes. For MTH1, the IC50 values (+/- SE; n = 3-4) were 17 +/- 2 microMfor Cu(II), 30 +/- 8 microM for Cd(II), 376 +/- 71 microM for Co(II) and801 +/- 97 microM for Ni(II). For MutT, they were 60 +/- 6 microM forCd(II), 102 +/- 8 microM for Cu(II), 1461 +/- 96 microM for Ni(II) and 8788+/- 1003 microM for Co(II). Thus, Cu(II) and Cd(II) emerged as muchstronger inhibitors than Ni(II) and Co(II), and MTH1 appeared to begenerally more sensitive to metal inhibition than MutT. Interestingly, inthe absence of Mg(II), the activity of the enzymes could be restored byCo(II) to 73% of that with Mg(II) alone for MutT, and 34% for MTH1, theother metals being much less or non-effective. The difference insensitivity to metal inhibition between the two enzymes may reflect thedifferences in the amino acid ligands, especially the cysteine ligand,outside their evolutionarily conserved Mg(II)-binding active sites, whichmight indicate predominantly non-competitive or uncompetitive mechanism ofthe inhibition. The overall results suggest that inhibition of 8-oxo-dGTPases may be involved in the mechanisms of induction of the 8-oxoguanine lesion in DNA by the metal ions studied, especially the non-redox-active Cd(II) cation.