504 10. EFFECTS OF MORE THAN ONE INHIBITOR 



One Inhibitor Alters the Metabolism so That the Response to a Second 

 Inhibitor Is Changed 



The two inliibitors sometimes act on metabolic pathways that are not 

 directly related but because of the complex interrelationships within the 

 living cell their actions will not be independent. A change of intracellular 

 pH brought about by an inhibitor may modify the effect of another inhi- 

 bitor, and such i^H changes are by no means uncommon. Likewise, any 

 metabolically dependent components of the cell, such as substrates or in- 

 termediates, can rise or fall in response to inhibition, thus altering the 

 conditions upon which the other inhibitor must act. It has been shown in 

 lymph node cells that iodoacetate blocks glucose metabolism and as a con- 

 sequence the intracellular glucose concentration rises to approximately 

 that in the medium, whereas 2,4:-dinitrophenol accelerates glucose utiliza- 

 tion and causes a fall in the glucose concentration (Helmreich and Eisen, 

 1958). A second inhibitor acting on some phase of carbohydrate metabolism 

 might well have its effect modified by these fluctuations in glucose level. 

 Intracellular substrate concentration changes can also be produced by the 

 inhibition of active transport of substrate; depression of amino acid uptake 

 in nerve cells or certain bacteria would create disturbances in metabolism 

 that could increase or decrease the sensitivity to a second inhibitor. Such 

 relationships may be thought of as sequential inhibitions, but truly the 

 actual systems affected can be separated both spatially and functionally 

 in the cell 



It has been postulated that proper concentrations of certain inhibitors 

 can reduce the ATP concentration in the cell to levels just sufficient to 

 support growth or function. Under these conditions another inhibitor that 

 also produces a decline in ATP, even though it acts on an entirely different 

 metabolic pathway, would inhibit those processes that depend on ATP 

 at concentrations much lower than if it were acting alone. The excellent 

 analysis of the inhibition of gastric secretion by Davenport is one of the 

 few competent treatments of a phase of multiple inhibition. As the con- 

 centration of 2,4-dinitrophenol was increased, gastric acid secretion began 

 to be inhibited around 0.01 niM. It was reasoned that the application of 

 another inhibitor would shift this point of minimal 2,4-dinitrophenol in- 

 hibition depending on whether this second inhibitor interfered with ATP 

 generation or with ATP utilization (Davenport et al., 1955). iV-Ethylmal- 

 eimide shifted the minimal 2,4-dinitrophenol concentration for inhibition 

 to lower concentrations and iodoacetamide shifted it to higher concentra- 

 tions (Fig. 10-6), from which different sites of action of these two inhibitors 

 were made likely. This type of inhibition analysis could be applied profitably 

 to many other situations and the technique of plotting concentration-in- 

 hibition curves for one inhibitor in the absence and presence of another 

 inhibitor is a convenient and often illuminating procedure. 



