GEORGE T. SCOTT AND HUGH R. HAVVVOOl) 55 



experiment is more easily controlled in the dark, since darkness alone causes 

 some ion shifts. 



Protection by ATP Against Ion Shifts Caused by lodoacetate. Since DNC 

 caused a loss of potassium and a gain of sodium by L'lva, presumably by dis- 

 sociating respiration from the production of energy-rich phosphate bonds, the 

 possibilit}' of some degree of protection by exogenous ATP against the ion 

 shifts caused by iodoacetate was investigated. In these experiments ATP was 

 added in small amounts at 2- to 3-hour intervals to samples maintained in 

 iodoacetate in the dark. The data in table 6 represent one such experiment. 

 Thus, over a 26-hour period the higher concentration of ATP afforded about 

 25% protection against the potassium loss by iodoacetate. However, the ATP 

 offered no protection against the sodium influx resulting from iodoacetate, but 

 rather the iodoacetate- ATP samples showed a higher sodium than the iodo- 

 acetate samples. 



Table 6. Protective inpluence of atp against potassium loss observed with io~^ 

 m/l. iodoacetate in the dark 



Control 



io~' m/I. iodoacetate 



io~' m/1. iodoacetate plus 3 X io~^m/1. ATP 



io~' m/1. iodoacetate plus io~^ m/1. ATP 



The experimental period covered here is 26 hr. The ATP was added in the concentrations 

 shown at 2 to 3 hr. intervals throughout this period {Viva). 



The difficulties described below in experiments with exogenous ATP may 

 well apply to experiments with any exogenous metabolite. Because of the 

 metabolic complexities of any biological system, any attempt to localize 

 precisely the sites of coupling between ion transports and metabolism is beset 

 with difficulties. Nonetheless, these experiments do show that while added 

 phosphoglycerate is more effective in protecting against potassium loss with 

 iodoacetate, pyruvate effectively maintains the sodium-pumping mechanism 

 in the presence of the inhibitor. 



The experiments with ATP, while they did not show a complete protection 

 by ATP against the potassium disturbances caused by iodoacetate, did never- 

 theless demonstrate a significant protection for potassium. It would probably 

 be an unjustified extrapolation to conclude on the basis of these few experi- 

 ments that the sodium-pumping mechanism is not energized at one stage or 

 another by ATP. Certainly the decoupling agent employed, DNC, caused a 

 markedly rapid increase in cellular sodium. 



There are certain inherent difficulties in experiments with exogenous ATP. 

 One problem is the penetration of this relatively large molecule into the cell. 



