Methylglyoxal detoxification in Escherichia coli
Methylglyoxal is a naturally occurring toxic electrophile which, in E. coli, can be detoxified by several routes. This study has focused on the glutathione-dependent glyoxalase system. The genes encoding glyoxalase I (gloA) and glyoxalase II (gloB) were identified in the E. coli genome and cloned into the multi-copy plasmid pHG165. A null mutant in glyoxalase I (ΔgloA::KanR) was also created. Cells overexpressing glyoxalase I exhibit elevated rates of detoxification and enhanced tolerance of methylglyoxal. Analysis of the ΔgloA mutant has revealed that growth and viability are quite normal, unless the cell is challenged with methylglyoxal either added exogenously or synthesized by the cells. The mutant strain has a low rate of detoxification of methylglyoxal; only 20 -30 % of the level of the detoxification compared to the parent strain. Thus, the glutathione-dependent glyoxalase system was shown to be the dominant pathway for the methylglyoxal detoxification in E. coli. Glyoxalase I mutant cells rapidly lose viability when exposed to methylglyoxal as cell viability parallels the rate of detoxification. Glyoxalase I is the rate determining step in this pathway as overexpression of glyoxalase II does not affect the overall rate of metabolism of methylglyoxal. Methylglyoxal-elicited potassium efflux via the E. coli potassium channel KefB is enhanced in strains overexpressing glyoxalase I. Activation of KefB is diminished in the absence of functional glyoxalase I or in a strain overexpressing glyoxalase II. S-lactoylglutathione is the product of glyoxalase I and the substrate for glyoxalase II. This glutathione adduct is the major activator of the KefB and KefC systems. The glyoxalase I substrate, hemithiolacetal, could partially activate KefB and KefC when they were overexpressed on a multi-copy plasmid.