D. C. TOSTESON I51 



reactions involving ATP are activated by K and inhibited by Na (2). Although 

 the stability constants of Na ATP and K ATP are the same, INIeichior proposes 

 that differences in the molecular shape and volume of the comi)lexes may 

 render combination of the K ATP with an enzyme possible, whereas the Na 

 compound cannot react. He suggests that such a system would have the 

 properties necessary for selective K transport. 



From this brief survey of model systems, it is clear that many avenues of 

 promising investigation exist. At present, however, no adequate model which 

 can account for selective K and Na transport in red cells, or, for that matter, 

 in any animal cell, exists. jMany important questions remain unanswered. 

 Does diffusion of cations in the red cell membrane occur through aqueous 

 channels analagous to the pores in an ion exchanger as suggested here, or 

 through the lipid phase of the membrane as suggested by Maizels (60)? If the 

 former, do the catioiis and anions both diffuse through the same positively 

 charged pores, or does the slow cation diffusion occur through negative pores 

 of very small area? Does transport of K and Na involving chemical reactions 

 occur in the lipid or aqueous phase? What is the relation between the dif- 

 ferential effect of K and Na on certain enzymes, and selective ion transport? 

 What are the specific chemical reactions coupling ion transport to metabolism? 

 How are these reactions integrated with the diffusion characteristics of the 

 cell membrane so as to control the cation composition of the cell? 



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