5 



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Site isolation? 

 Membrane separation 



Our main objective in the chemical approach to transport 

 should be the identification of the reactive sites that mediate trans- 

 ports and the demonstration of their action in an isolated state. A 

 very small number of transport-mediating groups appear to be able, 

 in analogy to enzyme action, to transfer enormous numbers of sol- 

 ute molecules. Up to the present time, no procedures have succeeded 

 in concentrating transport sites; or perhaps we should say more 

 accurately, no efficient way is available for recognizing procedures 

 able to concentrate transport sites or structures. Once the mem- 

 brane is broken, unfortunately, we can expect to find at most only 

 stoichiometric binding of the solute by the site, rather than the 

 more sensitive indicator— mediation of transport, by which one site 

 can operate for thousands of solute molecules. Therefore the sensi- 

 tivity with which the site can be recognized probably falls, follow- 

 ing cell breakage, by many orders of magnitude. The natural low 

 concentration of such sites can be illustrated with the potassium 

 transport site on the red blood cell estimated at one per every 1,000,- 

 000 square Angstroms (Solomon et al., 1956) or per every 10,000,- 

 000 square Angstroms (Glvnn, 1957b) of surface area. The latter 

 value represents about 1,000 per cell. LeFevre set an upper limit to 

 the number of glucose-transferring sites on the red blood cell ghost 



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