ANTIBODY AS A SPECIFIC ENZYME INHIBITOR 169 



prepared from pigeon breast muscle, pig kidney and pig liver. He 

 found that the "maximum concentration" (critical concentration)* of 

 lactic acid is about five to eight times greater than the maximum con- 

 centration of malic acid, thus showing that the affinity of the enzyme is 

 much higher towards malic acid than towards lactic acid. 



The question of whether the oxidation product of one substrate 

 inhibits the oxidation of the other substrate was also studied: i.e., 

 does oxalacetic acid, the oxidation product of malic acid, inhibit the 

 dehydrogenation of lactic acid, and conversely does pyruvic acid, the 

 oxidation product of lactic acid, inhibit the dehydrogenation of malic 

 acid? In the former case the lactic acid dehydrogenation was inhibited 

 to an extent of 50 to 60 per cent (by 1 mg. of oxalacetic acid), using 

 the maximum concentration of lactic acid, while with the same con- 

 centration of oxalacetic acid the dehydrogenation of malic acid was 

 also inhibited to the same extent. But pyruvic acid only inhibited the 

 dehydrogenation of lactic acid and not the dehydrogenation of malic 

 acid. This inability of pyruvic acid to inhibit the dehydrogenation of 

 malic acid was explained by the fact that the affinity of the enzyme is 

 much higher for malic acid than for lactic acid. 



Das in a subsequent study (1937b) investigated the mechanism 

 of inhibition of reversible dehydrogenation-hydrogenation systems by 

 their respective reaction products. The experimental conditions were 

 arranged so as to obtain 50 per cent inhibition. He found, for example, 

 that the dehydrogenation of lactic acid to pyruvic acid was inhibited 

 50 per cent by 0.3 mg. of pyruvic acid. Conversely the hydrogenation 

 of pyruvic acid to lactic acid was inhibited 50 per cent by 3.0 mg. 

 of lactic acid. The reaction was presented as follows: 



0.3 mg. of pyruvic acid (50%) 



t 

 Lactic acid ^ pyruvic acid 



i 

 3.0 mg. of lactic acid (50%) 



In the same way the dehydrogenation of malic acid to oxalacetic acid 

 was inhibited 50 per cent by 0.02 mg. of oxalacetic acid, and the hy- 

 drogenation of the latter to malic acid was inhibited 50 per cent by 



* "Maximum concentration" is the concentration at which the enzyme is most active. 



