440 9. INHIBITION IN CELLS AND TISSUES 



other hand, the other possible mechanisms enumerated above will generally 

 lead to very complex kinetic behavior, although superficially it may ap- 

 pear to be simple. 



A discrepancy between the inhibition on a metabolic process in intact 

 cells and the same series of reactions when isolated from the cell may be 

 an indication that transport systems are being affected, but is not proof. 

 The only generally reliable method for demonstrating a selective inhibition 

 on uptake is analysis for the substrate in the cell, and this must usually 

 be done with labeled material since the intracellular concentration may in 

 any case be quite low. If the transport is inhibited, the intracellular con- 

 centration of substrate will usually fall, but if an enzyme in the cell is in- 

 hibited, the substrate concentration may rise. Mechanisms (3) and (4) 

 above are not likely to produce a specific inhibition of transport because 

 the same systems for formation of energy and synthesis of proteins presum- 

 ably are used throughout the cell. 



Inhibition of Enzyme Synthesis 



Many types of inhibitor can depress the synthesis of enzymes. This is 

 particularly well seen in the case of adaptive or inducible enzymes. Protein 

 synthesis is particularly susceptible to inhibition because it depends on 

 so many phases of cell metabolism. Aside from the complex series of steps 

 that are directly related to formation of protein — the activation of amino 

 acids, the transfer to soluble RNA, the transfer to ribonucleoprotein, and 

 the linking of peptide bonds — there are the ATP-supplying reactions of 

 oxidative phosphorylation, and in some cases the reactions forming one 

 or more of the amino acids required. Whenever an experiment is run over 

 an interval of time during which enzyme synthesis is important, the sec- 

 ondary inhibition of this process should be taken into account, although 

 it seldom is. To take one example, the uncoupling of oxidative phosphory- 

 lation by 2,4-dinitrophen'Ol will not only produce immediate effects on the 

 metabolism but will-seCondarily interfere with protein synthesis and even- 

 tually disturb various enzyme systems upon which it has no direct action. 

 Indeed, a "vicious cycle" may be readily set up, since failure in the synthe- 

 sis of transport proteins or enzymes can further limit the amount of energy 

 available for this synthesis. 



Enzyme synthesis may be modified by inhibitors in other ways. An 

 amino acid analog may be incorporated into an enzyme as it is synthetized 

 and, as a result, the catalytic activity may be lost. This has been demon- 

 strated with j9-fluorophenylalanine and tryptazan. The analog here is not 

 truly an enzyme inhibitor but its introduction can lead to a decrease in 

 enzyme activity. Inasmuch as enzymes, like other proteins, are i^resumably 

 formed on a template, the binding or interaction of a substance with this 



