The mutagen and carcinogen cadmium is a high-affinity inhibitor of the zinc-dependent MutLα endonuclease

MutL alpha (MLH1-PMS2 heterodimer), which acts as a strand-directed endonuclease during the initiation of eukaryotic mismatch repair, has been postulated to function as a zinc-dependent enzyme [Kosinski J, Plotz G, Guarne A, Bujnicki JM, Friedhoff P (2008) J Mol Biol 382: 610-627]. We show that human MutLa copurifies with two bound zinc ions, at least one of which resides within the endonuclease active site, and that bound zinc is required for endonuclease function. Mutagenic action of the carcinogen cadmium, a known inhibitor of zinc-dependent enzymes, is largely due to selective inhibition of mismatch repair [Jin YH, et al. (2003) Nat Genet 34: 326-329]. We show that cadmium is a potent inhibitor (apparent K-i similar to 200 nM) of MutLa endonuclease and that cadmium inhibition is reversed by zinc. We also show that inhibition of mismatch repair in cadmium-treated nuclear extract is significantly reversed by exogenous MutL alpha but not by MutS alpha (MSH2MSH6 heterodimer) and that MutL alpha reversal depends on integrity of the endonuclease active site. Exogenous MutL alpha also partially rescues the mismatch repair defect in nuclear extract prepared from cells exposed to cadmium. These findings indicate that targeted inhibition of MutLa endonuclease contributes to cadmium inhibition of mismatch repair. This effect may play a role in the mechanism of cadmium carcinogenesis.