LACTATE METABOLISM 433 



wick, 1957; Novoa et al., 1959). The substrate and inhibitor constants have 

 been summarized by Papaconstantinou and Colo wick (1961 a), as shown 

 in the accompanying tabulation; if the KJs, are dissociation constants, 



Source 



A',„ (pyruvate) K^ (oxamate) 

 {mM) (mif) 



Beef heart 0.137 0.0374 



Ascites carcinoma 0.212 0.0563 



Rabbit muscle 0.302 0.10 



oxamate is bound around 0.77 kcal/mole more tightly than pyruvate. The 

 difficulty in interpreting K^ is that oxamate complexes with the apode- 

 hydrogenase, E, with E-NAD, and with E-NADH, the dissociation con- 

 stants being different for each (Novoa et al., 1959). The values for three 

 different pH's are shown in the accompanying tabulation. Such ternary 



complexes have recently been demonstrated by ultracentrifugal separation, 

 1 mole of oxamate being bound for each mole of NAD or NADH (Novoa 

 and Schwert, 1961). The dissociation constant for the E-NADH-oxamate 

 complex determined ultracentrifu gaily is 0.011 mM at pH 7.4. No evidence 

 could be found for a complex with the apoenzyme alone; whether such a 

 complex occurs or not, the inhibition is mainly due to the ternary complexes. 

 The lactate dehydrogenase from human liver and heart is also inhibited by 

 oxamate (Plummer and Wilkinson, 1963). The reduction of 2-ketobutyrate 

 is inhibited more strongly than pyruvate reduction, presumably because 

 2-ketobutyrate is bound to the enzyme less tightly, but the reduction of 

 succinic semialdehyde by a rat brain lactate dehydrogenase is inhibited 

 to the same degree as the reduction of pyruvate, namely, 88% by 0.1 mM 

 (Fishbein and Bessman, 1964). Not all lactate dehydrogenases are sensitive 

 to oxamate; that from L. mesenteroides is inhibited only 50% by 7 mM 

 (Papaconstantinou and Colowick, 1961 a). Oxamate specifically inhibits the 

 L( + )-lactate dehydrogenase and does not affect d( — )-lactate dehydroge- 



