436 2. ANALOGS OF ENZYME REACTION COMPONENTS 



potently and competes with lactate rather than pyruvate. Tartronate and 

 malonate also inhibit competitively with lactate and noncompetitively with 

 respect to pyruvate, although by no means as strongly as oxalate, and 

 Ottolenghi and Denstedt (1958) concluded that pyruvate and lactate react 

 with different sites on the enzyme surface, but this is not necessary, as 

 Novoa et al. (1959) have shown, since the configurations of the active sites 

 on E-NAD and E-NADH are different. The five lactate dehydrogenase 

 (LD) isoenzymes from human tissues are inhibited to different degrees by 

 oxalate at 0.02 vaM (see accompanying tabulation) (Emerson et al., 1964). 



There are A and B monomers, the B monomer being more sensitive to 

 oxalate. LD^ is a pure B tetramer, LD5 is a pure A tetramer, and the others 

 are intermediate in the proportion of A to B. This is an illuminating example 

 of how enzymes may respond differently to inhibitors by reason of varied 

 composition. 



There are several interesting studies on lactate dehydrogenases from 

 which deductions on the nature of the active site and certain interaction 

 energies may be derived. Dikstein (1959) found that yeast lactate dehydro- 

 genase is not inhibited potently by monocarboxylates: the concentrations 

 for 50% inhibition are 6000 vaM for formate, 2300 mM for acetate, 800 milf 

 for propionate, 300 mM for butyrate, 120 mM for valerate, and 10 mM for 

 caprylate. By plotting log (1)50 against the number of carbon atoms in the 

 aliphatic chains he obtained a straight line, from the slope of which it was 

 possible to calculate that the transfer of a methylene group from the solvent 

 to the enzyme surface involves an over-all energy change of 0.5 kcal/mole. 

 It was assumed that the interaction energy between the COO" group and 

 an enzyme cationic group could be calculated from the intercept of this 

 line, but it is doubtful that at the high concentrations used the residual 

 energy can be attributed with certainty to ion-ion interactions, since non- 

 specific effects cannot be excluded. 



The D-of-hydroxy acid dehydrogenase of yeast is competitively inhibited 

 by several dicarboxylates (see accompanying tabulation) (Cremona, 1964). 

 Monocarboxylates inhibit much more weakly and the inhibitions are usually 

 not purely competitive. Oxalate is bound approximately 3.7 kcal/mole more 



