文摘
Promutagenic 8-oxo-7,8-dihydro-2'-deoxyguanosine(8-oxo-dG) levels are increased in DNAof animals exposed to carcinogenic metals, such as Ni(II).Besides being generated directly ingenomic DNA, 8-oxo-dG may be incorporated there from8-oxo-7,8-dihydro-2'-deoxyguanosine5'-triphosphate (8-oxo-dGTP), a product of oxidative damage to thenucleotide pool. TheEscherichia coli dGTPase MutT, and analogous dGTPases inrats and humans, have beensuggested as a defense against such incorporation because theyhydrolyze 8-oxo-dGTP to 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-monophosphate (8-oxo-dGMP). MutTand its mammaliancounterparts are Mg(II)-dependent enzymes. Ni(II), inturn, is known to interact antagonistically with Mg(II) in biological systems. Thus, we hypothesizedthat Ni(II) might inhibit theactivity of MutT. As an initial examination of this hypothesis, weconducted enzyme kineticstudies of MutT to determine the effect of Ni(II) on MutT activityand the mechanisms involved.As found, Ni(II) inhibited MutT in a concentration-dependentmanner when either dGTP or8-oxo-dGTP was the nucleotide substrate. Ni(II) wasdetermined to be an uncompetitiveinhibitor of MutT with respect to Mg(II) when dGTP was thesubstrate, with apparent Ki of1.2 mM Ni(II), and a noncompetitive inhibitor with respect toMg(II) when 8-oxo-dGTP wasthe substrate, with apparent Ki of 0.9 mMNi(II). Hence, the two metal cations did notcompetewith each other for binding at the MutT active site. This makes itdifficult to predict Ni(II)effects on 8-oxo-dGTPases of other species. However, based uponthe amino acid sequences ofhuman and rat MutT-like dGTPases, their capacity for Ni(II)binding should be greater thanthat of MutT. Whether this could lead to stronger inhibition ofthose enzymes by Ni(II), ornot, remains to be investigated.
