Possesses glyoxalase I activity. Catalyzes the conversion of hemimercaptal, formed from methylglyoxal and glutathione, to S-lactoylglutathione. May be involved in oxidative stress response. (R)-S-lactoylglutathione = glutathione + methylglyoxal kcat is 1700 min(-1) with methylglyoxal as substrate. kcat is 2200 min(-1) with glutathione as substrate. Optimum pH is 6.8. Optimum temperature is 45 degrees Celsius. Homotrimer. Cys-120 and Cys-313 are oxidized to sulfinic acid. Belongs to the peptidase C56 family.