Methylglyoxal detoxification by E. coli aldo-keto reductase

Abstract

Methyglyoxal is a toxic metabolite known to accumulate in various cell types. We detected in vivo conversion of methylglyoxal (MG) to acetol in MG-accumulating Escherichia coli cells by $^1H-NMR$ spectroscopy. Search for homologs of the mammalian aldo-keto reductases, reported to exhibit activity to MG, revealed nine open-reading frames from the E. coli genome. Based on both sequence similarities and preliminary screening with $^1H-NMR$ for crude proteins of the corresponding mutant strains, we selected five genes, yafB, yqhE, yeaE, yghZ, and yajO, for further study. Quantitative assessment of acetol production including its metabolite 1,2-propanediol from the crude extracts of these mutants indicated that the yafB, yqhE, and yghZ genes are significantly involved in the metabolic conversion of MG in the presence of NADPH. Enzymatic properties of these proteins with the Km values of 2.1 - 6.9 mM are consistent with the in vivo and cell-free activities of MG conversion. Substrate specificities of these enzymes toward various aldehydes and ketones demonstrate their similarities to the mammalian aldo-keto reductases. The results imply that the glutathione-independent detoxification of MG can occur through multiple pathways of aldo-keto reductases, leading to a generation of acetol.