INHIBITION OF ENZYMES 775 



viously described (page 1-569), since this technique will often provide in- 

 formation on complexes formed with components other than the apoenzyme. 

 Whatever the mechanism or formal type of inhibition by mercurials, it 

 is certain that many systems must be represented by mutual depletion 

 kinetics. This is clearly seen in many of the enzyme titrations (page 804), 

 inhibition being produced by mercurials at roughly equimolar concentra- 

 tions relative to the enzymes, but at this point the problem will be treated 

 in a more general manner. Mutual depletion behavior implies that the inhi- 

 bition will depend on the concentration of the enzyme. This is seen with 

 yeast pyruvate decarboxylase in the work of Stoppani et al. (1953) (see 

 accompanying tabulation), and even more markedly with pig heart suc- 



Pyruvate % Inhibition by: 



cinate oxidase, which is inhibited 89% by 0.01 mM p-MB when the enzyme 

 concentration is 0.15 mg/ml but only 59% by 0.76 mM p-MB when the 

 enzyme concentration is 30 mg/ml (Stoppani and Brignone, 1957). Another 

 example is muscle p>Tuvate oxidase (see accompanying tabulation) (Onrust 



Enzyme extract „^ inhibition by p-MB 0.11 mM 

 (ml) 



0.4 82 



0.8 56 



1.2 34 



1.5 33 



et al., 1954). These few examples well illustrate the importance of this factor 

 and very clearly demonstrate the quantitative meaninglessness of most 

 reported inhibitions if the relative enzyme concentration is not known or 

 stated. Impurities also may contribute to the depletion of the mercurial. 

 The crude bacterial enzyme for converting histidinol to histidine is not 

 inhibited by 0.02 mM p-MB, but the partially purified enzyme is inhibited 

 50% (Adams, 1954), and it is likely that the pure enzyme would be inhib- 



