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Cytotoxic methylglyoxal is detoxified by the two-enzyme glyoxalase system. Glyoxalase I (GlxI) catalyzes conversion of non-enzymatically produced methylglyoxal-glutathione hemithioacetal into its corresponding thioester. Glyoxalase II (Glx II) hydrolyzes the thioester into d-lactate and free glutathione. Glyoxalase I and II are metalloenzymes, which possess mononuclear and binuclear active sites, respectively. There are two distinct classes of GlxI; the first class is Zn²⁺-dependent and is composed of GlxI from mainly eukaryotic organisms and the second class is composed of non-Zn²⁺-dependent (but Ni²⁺ or Co²⁺-dependent) GlxI enzymes (mainly prokaryotic and leishmanial species). GlxII is typically Zn²⁺-activated, containing Zn²⁺ and either Fe³⁺/Fe²⁺ or Mn²⁺ at the active site depending upon the biological source. To address whether two classes of GlxII might exist, glyoxalase II from Escherichia coli was …