THE PROBLEM OF ENDOGENOUS METABOLISM 



441 



template may distm'b the synthetic process. Some polypeptide antibiotics 

 may possibly inhibit bacterial growth by this mechanism. With respect to 

 enzyme inhibitors, particular attention must be given to those capable of 

 chemically reacting with protein or nucleic acids (such as the arsenicals, 

 mercurials, heavy metals, alkylating agents, and phosphor ylating agents). 

 Our ignorance of the exact nature of the template surface prevents more 

 rational predictions as to the compounds most likely to react there. 



THE PROBLEM OF ENDOGENOUS METABOLISM 



When an unpurified enzyme is being examined, it is common to find that 

 some enzymic or metabolic activity is exhibited in the absence of added sub- 

 strate. If the effect of an inhibitor on the metabolism of the added substrate 

 is desired, some correction must be applied to the inhibition observed on the 

 total system. If the respiration of a tissue slice preparation, for example, 

 is being determined in the presence of an added substrate, the inhibition 

 observed upon adding some substance cannot be quantitatively equat3d with 

 the inhibition on the oxidation of this substrate, unless the proper endoge- 

 nous controls are also run. In many reports this correction has not been 

 made and frequently one finds no mention of endogenous metabolism in 

 situations where it would be expected to occur. Let us consider three exam- 

 ples to illustrate the errors in interpretation that are possible (see tabulation). 



In the first case, if the endogenous contribution were neglected, the inhibi- 

 tion would be calculated as 46.7%. However, the addition of substrate in- 

 creases the metabolism by 24 in the absence of the inhibitor and by 10 when 

 the inhibitor is present; thus the inhibition of substrate metabolism is 

 actually 58.4%. This example also shows that even though there is no in- 

 hibition of the endogenous metabolism, a correction is necessary. In the 

 second case, the uncorrected inhibition is 44.4%. However, addition of 

 substrate increases the rate by 12 whether inhibitor is present or not; thus 

 there is actually no inhibition of the metabolism of the added substrate. 

 In the third case, an inhibition of around 26% would be calculated, and yet 

 the substrate produces a greater rise in rate when the inhibitor is present 

 than in the uninhibited preparation. 



