Evidence that MutY is a monofunctional glycosylase capable of forming a covalent Schiff base intermediate with substrate DNA

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Abstract
The Escherichia coli adenine glycosylase MutY is involved in the repair of 7,8-dihydro-8-oxo-2′-deoxyguanosine (OG):A and G:A mispairs in DNA. DNA strand cleavage via β-elimination (β-lyase) activity coupled with MutY's removal of misincorporated adenine bases was sought using both qualitative and quantitative methods. The qualitative assays demonstrate formation of a Schiff base intermediate which is characteristic of DNA glycosylases catalyzing a concomitant β-lyase reaction. Borohydride reduction of the Schiff base results in the formation of a covalent DNA-MutY adduct which is easily detected in SDS-PAGE experiments. However, quantitative activity assays which monitor DNA strand scission accompanying base release suggest MutY behaves as a simple monofunctional glycosylase. Treatment with base effects DNA strand cleavage at apurinic/apyrimidinic (AP) sites arising via simple glycosylase activity. The amount of cleaved DNA in MutY reactions treated with base is much greater than that in non-base treated reactions, indicating that AP site generation by MutY is not associated with a concomitant β-lyase step. As standards, identical assays were performed with a known monofunctional enzyme (uracil DNA glycosylase) and a known bifunctional glycosylase/lyase (FPG), the results of which were used in comparison with those of the MutY experiments. The apparent inconsistency between the data obtained for MutY by the qualitative and quantitative methods underscores the current debate surrounding the catalytic activity of this enzyme, and a detailed explanation of this controversy is proposed. The work presented here lays ground for the identification of specific active site residues responsible for the chemical mechanism of MutY enzyme catalysis.