The glyoxalase system consists of glyoxalase I and glyoxalase II. Glyoxalase I catalyzes the conversion of methylglyoxal (CH(3)COCHO), a metabolite derived from glycolysis, with glutathione to S-D-lactoylglutathione, while glyoxalase II hydrolyses this glutathione thiolester to D-lactic acid and glutathione. Since methylglyoxal is toxic due to its high reactivity, the glyoxalase system is crucial to warrant the efficient metabolic flux of this reactive aldehyde. The budding yeast Saccharomyces cerevisiae has the sole gene (GLO1) encoding the structural gene for glyoxalase I. Meanwhile, this yeast has two isoforms of glyoxalase II encoded by GLO2 and GLO4. The expression of GLO1 is regulated by Hog1 mitogen-activated protein kinase and Msn2/Msn4 transcription factors under highly osmotic stress conditions. The physiological significance of GLO1 expression in response to osmotic stress is to combat the increase in the levels of methylglyoxal in cells during the production of glycerol as a compatible osmolyte. Deficiency in GLO1 in S. cerevisiae causes pleiotropic phenotypes in terms of stress response, because the steady state level of methylglyoxal increases in glo1Δ cells thereby constitutively activating Yap1 transcription factor. Yap1 is crucial for oxidative stress response, although methylglyoxal per se does not enhance the intracellular oxidation level in yeast, but it directly modifies cysteine residues of Yap1 that are critical for the nucleocytoplasmic localization of this b-ZIP transcription factor. Consequently, glyoxalase I can be defined as a negative regulator of Yap1 through modulating the intracellular methylglyoxal level.