MITOCHONDRIA, CELLS, AND TISSUES 579 



holism occur during the development of the inhibition. Such adaptations 

 are usually not those involved in true tolerance but are only relatively rap- 

 id adjustments in the cellular metabolism to the altered enzyme activity. 

 The immediate dependence of electron-transport rates upon phosphate ac- 

 ceptors (such as ADP) would, under certain circumstances, allow for read- 

 justments in respiratory rate in response to an inhibitor. A shift in metabolic 

 pathways to the utilization of another type of substrate could conceivably 

 occur when the pathway of one substrate is blocked. The buffering capacity 

 of many multienzyme systems towards inhibition has been discussed in 

 Chapter 7 and it is evident that such phenomena, usually involving changes 

 in the concentrations of the intermediates, could readily slow down the 

 rate of inhibition. Of course, when the inhibition develops slowly over 

 several hours or longer, other types of adaptation, such as enzyme induction, 

 may occur. The rates at which inhibitors kill microorganisms could well be 

 influenced by this sort of adaptation. 



{G) Secondary changes occurring in cells. The primary inhibition on an en- 

 zyme may initiate processes that lead to depression of the metabolism or 

 function secondarily. It is known that the inhibition of metabolism or func- 

 tion continues to increase progressively after the inhibitor is removed from 

 the external medium in some cases. This could only be due to the continued 

 development of the secondary changes induced by the inhibitor that is 

 bound within the cells. A good illustration of this is provided by the action 

 of iodoacetate on the heart, where both the respiration (Webb et al., 1949 b) 

 and function (Webb, 1950 a) continue to decline after the iodoacetate is 

 washed out. The action of phenols upon bacteria has been shown (Cooper, 

 1912) to occur in two phases: an initial absorption or fixation, which is 

 followed by a slower protein denaturation leading to the death of the cells. 

 Since the cell is a closely organized unit, it is quite possible with a strategic 

 interruption of enzyme activity to initiate a sequence of events that will 

 eventually produce total dissolution of the cell. 



(//) Multiple actions of the inhibitor. Since inhibitors are unfortunately 

 seldom specific in their actions, the kinetics are often complicated by the 

 simultaneous occurrence of two or more inhibitory processes developing at 

 different rates. To use iodoacetate again as an example, it is clear that 

 this inhibitor can affect various components involved in the total respira- 

 tory rate, not only the sulfhydryl enzymes (e.g., phosphoglyceraldehyde 

 dehydrogenase, 6-phosphofructokinase, pyruvate and «-ketoglutarate de- 

 hydrogenases, isocitric dehydrogenase, succinic dehydrogenase, and others) 

 but possibly also nonenz\Tiiic units functioning structurally in the cell. 

 In such cases, the kinetic data are simply not interpretable in terms of 

 any single reaction. 



The study of cellular inhibition kinetics has scarcely begun and a great 

 deal more accurate data must be available before a logical attack can be 



