184 1- lODOACETATE AND lODOACETAMIDE 



is variable a direct action on transport mechanisms is more likely. One 

 difficulty in using this technique is that the appropriate conditions must be 

 selected, since otherwise a consistent correlation may be merely fortuitous. 

 Actually, the inhibitions of H+ transport and of lactate formation from 

 glucose in gastric mucosa by iodoacetamide show a fairly good and con- 

 sistent correlation under different conditions; this leads to the conclusion 

 that the EM pathway is the primary site of action. However, when aceto- 

 acetate is used as the substrate, little lactate is formed and this is not affect- 

 ed by iodoacetamide, although the inhibition of H+ transport is identical 

 to that when glucose is the substrate. These results might be interpreted in 

 different ways. The EM pathway could still be operative in the presence 

 of acetoacetate and necessary for transport, or the iodoacetamide could be 

 blocking pyruvate utilization, or it could be acting on the transport system 

 itself. 



The second procedure involves a determination of the effect of the in- 

 hibitor on the inhibition-concentration curve for 2,4-dinitrophenol and is 

 perhaps more reliable than the previous method. It was described briefly 

 on page 1-504 and illustrated in Fig. 1-10-6. One assumes that 2,4-dinitro- 

 phenol depresses cell function by uncoupling oxidative phosphorylation and 

 reducing the supply of ATP. If the inhibitor being studied also inhibits ATP 

 formation, it should require less 2,4-dinitrophenol to initiate inhibition of 

 function, whereas if the inhibitor acts on the functional system, more 2,4- 

 dinitrophenol may be required. Iodoacetamide shifts the inflection point 

 of the 2,4-dinitrophenol curve to the right, i.e., to higher concentrations of 

 the uncoupler, and thus appears to exert its primary action on the ender- 

 gonic or transport phase. Such an approach could be profitably combined 

 with analyses for ATP. 



When the transport of a particular substance is fovmd to be inhibited, 

 care must be exercised in immediately attributing the site of the action to 

 a system responsible for the movement of this substance specifically. The 

 transport of one ion must involve the movement of another ion to main- 

 tain electrical neutrality. In many situations the transports of Na+ and K+ 

 are linked and it is very difficult to determine whether one or the other or 

 both are actively transported, and similarly in the kidney there has been 

 prolonged argument as to the primary ion resorbed in the proximal tubules 

 (Na+ or CI"). The transport of water in many tissues (e.g., the kidney or 

 the ciliary body of the eye) is for the most part secondary to ion transport. 

 If an inhibitor causes a cell to swell, one is not justified in locating the action 

 on a water-transport system, nor in concluding that normally water is ac- 

 tively pumped out of the cell, since the water may merely follow changes 

 in ion distribution. There are examples where Na+ and K+ transports seem 

 to be linked and yet a certain inhibitor will exhibit a selective effect. The 

 Na+ extrusion and K+ uptake in E. coli are affected similarly by iodo- 



