Potential transition-state inhibitors of glyoxalase. I

In the 1960s Szent-Györgyi and his co-workers reported the inhibition of tumor growth with extracts from normal tissues. Characterization using derivatization and spectral analysis demonstrated that the inhibitor was an α, β dicarbonyl. Methylglyoxal is the lowest class member of the ketoaldehydes. A biological system which catalyses the formation of lactate from methylglyoxal was reported early in this century. The formation of D-lactate (not the L-lactate of glycolysis) is catalyzed by the mammalian enzymes, glyoxalase I (S-lactoyl-glutathione methylglyoxal-lyase, isomerizing; EC 4.4.1.5) and glyoxalase II (S-2-hydroxyacylglutathione hydrolase: EC 3.1.2.6), with glutathione (GSH) as a coenzyme. There have been a number of reports on the formation of methylglyoxal in mammalian tissue, and experimental support for the inhibition of tumor growth by inhibition of glyoxalase I has been published. However, the rapid catabolism of analogues of methylglyoxal or glutathione suggests that inhibitors that resemble the transition state may more effectively bind significantly to glyoxalase I. The transition state in the formation of S-lactoylglutathione from the hemimercaptal of methylglyoxal is an enediol. Compounds such as 3,4-dihydroxycyclobutene 1,2-dione (squaric acid) (a coplanar compound) significantly inhibit yeast glyoxalase I. Here the inhibition in vitro of human red blood cell glyoxalase I with a number of compounds that resemble the transition state of methylglyoxal hemimercaptal is reported. These include the following with I50 in mM: 3,4-dihydroxy benzoic acid (0.32 mM), 3,4-dihydroxy benzohydroxamic acid (0.38 mM), 4-methyl-6,7-dihydroxy coumarin (0.03 mM), and 6,7-dihydroxy coumarin (0.03 mM). Coumarin was not inhibitory. Calculations based on the in vitro data and these structures may be predictive of their growth inhibitory nature in such model animal tumors as L1210 leukemia.