LACTATE METABOLISM 437 



tightly than malonate, suggesting that there are two cationic groups quite 

 close on the enzyme. 



Inhibitor Ki (mil/) 



The D-lactate dehydrogenase of yeast studied by Boeri et al. (1960) is 

 inhibited potently by oxalate {K^ = 0.0025 mM), moderately by malonate 

 (^ . = 0.9 mM), and not at all by 16 mM succinate or fumarate, indicating 

 the importance of the position of the second C00~ group. The enzyme is 

 inactivated gradually by EDTA and reactivated with Zn++. Although this 

 does not prove that Zn++ is the normal metal ion involved, it points to the 

 possibility of chelation between a-hydroxy carboxylates, or dicarboxylates, 

 with an enzyme-bound metal ion. The configuration around the a-carbon 

 atom is important since L-lactate binds very weakly to the enzyme {K^ = 

 = 62 mM). 



The lactate cytochrome c reductases of yeast are flavoproteins and are 

 competitively inhibited by a variety of lactate analogs (Nygaard, 1961 a, 

 b, c). Fatty acid inhibitions lead to the calculation of 0.37 kcal/mole for 

 the interaction of methylene groups with the enzyme and 2.80 kcal/mole 

 for the C00~ group in the case of the D-lactate cytochrome c reductase, 

 and of 0.37 kcal/mole and 0.88 kcal/mole, respectively, for the L-lactate 

 cytochrome c reductase. Neither eftzyme is inhibited by dicarboxylates, 

 with the exception of oxalate, and from the differences in inhibition patterns 

 it is probably safe to assume that the active sites of the two enzymes are 

 quite different. 



Some analog inhibitors of lactate dehydrogenases need only be listed for 

 reference: pyruvate (Green and Brosteaux, 1936; Das, 1937 b; Neilands, 

 1952; Labeyrie and Stachiewicz, 1961), bromopyruvate, chloropyruvate, 

 hydroxypyruvate (Busch and Nair, 1957), phenylpyruvate (Dikstein, 1959), 

 a-ketoglutarate (Boeri et al., 1960), malate (Boeri et al., 1960; Busch and 

 Nair, 1957), tartronate (Green and Brosteaux, 1936; Lehmann, 1938), tar- 

 trate (Labeyrie and Stachiewicz, 1961), mandelate (Lehmann, 1938), a, 

 y-diketovalerate (Meister, 1950), benzenesuLfonate (Baptist and Vestling, 

 1957), mercaptoacetate, mercaptosuccinate, or-mercaptopropionate, of-mer- 

 captobutyrate, and a-mercaptovalerate (Chaffee and Bartlett, 1960). The 

 benzenesulfonates, where the COO" group is replaced by a SO3" group. 



