Effect of polyvalent metal ions on the reactivity of human o-methylguanine-dna methyltransferase

Polyvalent metal ions are highly effective in inhibiting human O⁶-methylguanine-DNA methyltransferase, the repair protein responsible for the removal of the promutagenic and presumably procarcinogenic adduct, O⁶-alkylguanine, in DNA. The sulfhydryl group-reacting metal ions (Cd²⁺, Zn²⁺, Hg²⁺, Pb²⁺) completely inhibited the reaction at concentrations of 100-500 μM while other metal ions Al³⁺, Fe³⁺) required concentrations of 1 mM or greater for significant reduction of the reaction rate. Inhibition by the former group of metals could be reversed by dithiothreitol but not by reduction of the reaction rate. Inhibition by the former group of metals could be reversed by dithiothreitol but not by EDTA, while the opposite was true for the second group. Under conditions of partial inhibition of the initial reaction rate by either Hg²⁺ or Al³⁺, the extent of reaction was not significantly affected, indicating reversible binding of these ions.