Abstract

The research of Szent-Györgyi and others has suggested that the three-carbon ketoaldehyde methylglyoxal has a potential role in the control of cell growth. Its metabolism to D-lactate (not the L-lactate of glycolysis) is catalysed by the mammalian enzymes glyoxalase I (S-lactoyl-glutathione methylglyoxal-lyase, isomerizing; EC 4.4.1.5) and glyoxalase II (S-2-hydroxyacylglutathione hydrolase; 3.1.2.6), with glutathione as a coenzyme. Direct determination of methylglyoxal in biological tissues is difficult because of the active glyoxalase system. However, the product of the glyoxalase system, D-lactate, should indicate formed or added methylglyoxal. A stereospecific assay was used to measure D-lactate in human plasma; it involved the spectrophotometric analysis of NADH at 340 nm catalysed by D-lactate dehydrogenase (D-lactate:NAD+ oxidoreductase; EC 1.1.1.28) from Lactobacillus leichmannii. Blood collected by venepuncture was used for the determination of the plasma concentration of D-lactate. The mean concentration for seven normal subjects was 0.023 mM +/- 0.002 S.E.M. When the glycolytic pathway in whole blood was inhibited in vitro with fluoride, a significant increase in D-lactate was found (about 0.15 mM/hour at 37 degrees C). Added methylglyoxal also produced an increase in D-lactate formation. Some specific precursors of L-lactate (dihydroxyacetone phosphate, for example) added to whole blood produced an increased concentration of D-lactate, even when glycolysis was not inhibited. This finding indicates that catabolites of glucose lead to methylglyoxal synthesis and suggest a control function for the glyoxalase enzyme system in glycolysis that could be exploited for cancer therapy.