LACTATE METABOLISM 435 



1963). Inasmuch as the conditions were aerobic here, it is difficult to un- 

 derstand how an inhibition of lactate dehydrogenase would account for the 

 marked effects observed, unless there is an elevation of the NADH/NAD 

 ratio due to the prevention of pyruvate reduction, this slowing the oxidation 

 of glyceraldehyde-3-P and reducing the generation of ATP. This does not 

 seem to occur in Ehrlich ascites carcinoma cells inasmuch as oxamate stim- 

 ulates the formation of C^^Og from glucose-6-C^* without affecting that 

 from glucose- 1-C^'* (Christensen and Wick, 1963). The stimulation is thus 

 associated only with oxidation through the cycle and there is no effect on 

 the fraction going through the pentose-P pathway. More pyruvate enters 

 the cycle since less goes to lactate. The effects of oxamate on glucose utiliza- 

 tion will depend for one thing on how rapidly pyruvate can be oxidized. 

 The Crabtree effect is abolished almost completely by 40 uiM oxamate, i.e., 

 in the presence of glucose, oxamate stimulates the respiration in ascites 

 cells (Papaconstantinou and Colowick, 1961 a). Simultaneously, glucose up- 

 take is depressed 40% and lactate formation 70%. In view of the conclusion 

 about the nature of the Crabtree effect in the section on 2-DG, it would 

 seem that inhibition of lactate dehydrogenase could not be responsible for 

 this effect of oxamate. It is possible that oxamate diverts more pyruvate 

 into the cycle and hence stimulates respiration under these conditions, but 

 this would not be a true abolition of the Crabtree effect. One would not 

 expect glucose respiration to be depressed by oxamate in any case if the 

 only action is on lactate dehydrogenase, but 31% respiratory depression 

 is produced by 10 mM oxamate in guinea pig alveolar macrophages (Oren 

 et at., 1963). Despite the statements relative to the specificity of oxamate, 

 it must be admitted that very few enzymes or metabolic pathways have 

 been studied. It is quite likely that certain phases of amino acid metabolism 

 might also be inhibited, since oxamate could be considered as an amino 

 acid analog. It will also be noted that all the effects discussed in this para- 

 graph were produced by oxamate at the high concentration of 10 raM 

 or above. 



Oxalate often inhibits lactate-metabolizing enzymes as potently as does 

 oxamate and similarly forms ternary complexes with lactate dehydrogenase 

 and NAD. However, it differs from oxamate in being competitive with 

 lactate instead of pyruvate; this has been shown on beef heart lactate 

 dehydrogenase (^, = 0.015 mM at pH 6.7) (Novoa et al, 1959), yeast 

 D-lactate dehydrogenase (^, = 0.007) (Labeyrie and Stachiewicz, 1961), 

 yeast D-lactate cytochrome c reductase {K^ = 0.0016 mM) (Nygaard, 1961 

 b), and yeast D-hydroxy acid dehydrogenase (/if, = 0.0025 mM) (Boeri et 

 al., 1960), although the inhibition seems to be uncompetitive on the lactate 

 dehydrogenase oi Propionihacterium pentosaceum (Molinari and Lara, 1960). 

 The complex kinetics have been treated in detail by Novoa et al. (1959), 

 but it is still rather puzzling that oxalate inhibits all these enzymes so 



