^- 



434 2. ANALOGS OF ENZYME REACTION COMPONENTS 



nase up to 8 raM (Dennis and Kaplan, 1960). The active site of the l( + )- 

 lactate dehydrogenase was represented as containing a cationic group for 

 electrostatic interaction with the C00~ group, and a hydrogen bonding 

 group which interacts with the OH group of lactate and the NHg group of 

 oxamate (it might also bond to the CO or enolized COH group of oxamate). 

 The role of the amino group of oxamate on the binding is not known, nor 

 can the importance of the enolic tautomer of oxamate be evaluated. The 

 marked decrease in inhibition between pH 8.45 and 9.70 (see tabulation 

 above) could indicate the deprotonation of an amino group, but it could 

 be on the enzyme as well as the inhibitor. 



The aerobic lactate production in human leucocytes is inhibited less than 

 25% by 10 mM oxamate, even in broken cell suspensions, suggesting that 

 lactate dehydrogenase is not solely responsible for lactate formation, al- 

 though the sensitivity of the leucocytic enz^^me to oxamate has not been 

 examined (McKinney et al., 1955). The effects of oxamate on tumor cell 

 metabolism and growth have been studied thoroughly by Papaconstantinou 

 and Colowick (1957, 1961 a, b). Anaerobic glycolysis in ascites carcinoma 

 cells is inhibited 50% by 8 mM oxamate and aerobic glycolysis is similarly 

 depressed. This may indicate that oxamate does not penetrate into cells 

 readily, since the K^ of 0.0563 mM for ascites cell lactate dehydrogenase 

 would lead one to expect a greater effect at this concentration. The inhi- 

 bition of anaerobic glycolysis decreases with time due to the accumulation 

 of pyruvate, whereas no accumulation of pyruvate occurs aerobically, in- 

 dicating that oxamate has little effect on pyruvate oxidase (an 18% inhi- 

 bition of pyruvate oxidation by 10 mM oxamate was observed). A decrease 

 in the inhibition anaerobically with time was also noted in Tetrahymena 

 pyriformis (Warnock and van Eys, 1963). The growth of HeLa cells is 

 completely inhibited by 40-80 mM oxamate and this is paralleled by de- 

 creases in glucose uptake and lactate formation, so that lactate dehydro- 

 genase appears in some manner to be essential for the growth of these cells 

 (assuming that oxamate acts specifically on lactate dehydrogenase). It was 

 proposed that oxamate might be a useful inhibitor for selectively blocking 

 glycolysis in mammalian cells. Mice can tolerate quite large doses (1 g/kg), 

 however. A block of glycolysis, of course, refers here only to an inhibition 

 of lactate formation, and the formation or utilization of pyruvate should 

 not be significantly affected, so it would seem that aerobic glucose meta- 

 bolism, at least with respect to the generation of energy, would be resistant 

 to oxamate. 



A number of disturbing observations have appeared which cast some 

 doubt on the simple concept that oxamate specifically inhibits lactate de- 

 hydrogenase. Leached HeLa cells restored to normal conditions actively 

 extrude Na+ and accumulate K+; these processes are inhibited 50% and 

 77%, respectively, by 38 mM oxamate (Wickson-Ginzburg and Solomon, 



