The selectivity and inhibition of AlkB

AlkB is one of four proteins involved in the adaptive response to DNA alkylation damage in Escherichia coli and is highly conserved from bacteria to humans. Recent analyses have verified the prediction that AlkB is a member of the Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase family of enzymes. AlkB mediates repair of methylated DNA by direct demethylation of 1-methyladenine and 3-methylcytosine lesions. Other members of the Fe(II) and 2OG-dependent oxygenase family, including those involved in the hypoxic response, are targets for therapeutic intervention. Assays measuring 2OG turnover were used to investigate the selectivity of AlkB. 1-Methyladenosine, 1-methyl-2'-deoxyadenosine, 3-methylcytidine, and 3-methyl-2'-deoxycytidine all stimulated 2OG turnover by AlkB but were not demethylated indicating an uncoupling of 2OG and prime substrate oxidation and that oligomeric DNA is required for hydroxylation and subsequent demethylation. In contrast the equivalent unmethylated nucleosides did not stimulate 2OG turnover indicating that the presence of a methyl group in the substrate is important in initiating oxidation of 2OG. Stimulation of 2OG turnover by 1-methyladenosine was highly dependent on the presence of a reducing agent, ascorbate or dithiothreitol. Following the observation that AlkB is inhibited by high concentrations of 2OG, analogues of 2OG, including 2-mercaptoglutarate, were found to specifically inhibit AlkB. The flavonoid quercetin inhibits both AlkB and the 2OG oxygenase factor-inhibiting hypoxia-inducible factor (FIH) in vitro. FIH inhibition by quercetin occurs in the presence of excess iron indicating a specific interaction, while the inhibition of AlkB by quercetin is, predominantly, due to nonspecific iron chelation.