A novel aldo-keto reductase (AKR) from Escherichia coli has been cloned, expressed and purified. This protein, YghZ, is distantly related ( 6 40%) to mammalian aflatoxin dialdehyde reductases of the aldo-keto reductase AKR7 family and to potassium channel L-subunits in the AKR6 family. The enzyme has been placed in a new AKR family (AKR14), with the designation AKR14A1. Sequences encoding putative homologues of this enzyme exist in many other bacteria. The enzyme can reduce several aldehyde and diketone substrates, including the toxic metabolite methylglyoxal. The K m for the model substrate 4-nitrobenzaldehyde is 1.06 mM and for the endogenous dicarbonyl methylglyoxal it is 3.4 mM. Overexpression of the recombinant enzyme in E. coli leads to increased resistance to methylglyoxal. It is possible that this enzyme plays a role in the metabolism of methylglyoxal, and can influence its levels in vivo.