BIOLOGICAL TRANSPORT 



measure the rate of transport we may get a different comparison be- 

 tween solutes than if we use stoichiometric binding to a receptor 

 site. 



The comparison between the active sites of enzymes and those 

 of transport mediators needs to be made cautiously until the opera- 

 tion of each is understood. 



If we consider, in turn, various groups of solutes undergoing 

 transport, the metal ions offer us very limited opportunities to use 

 comparison among analogs to study the nature of binding sites in- 

 volved, because usually only one analog even approaches a fit 

 (rubidium for potassium ion, strontium for calcium ion, manganese 

 for magnesium ion, lithium, in some cases, for sodium ion). Ap- 

 parently in some cases, the ammonia ion, amines, and even amino 

 acids can compete with the alkali metals for transport (cf. Conway 

 and Duggan, 1958). These structures are so diverse that one wonders 

 whether they actually compete for the same chemical site. 



Specificity of glycerol transport 



The site serving as the portal of entry for glycerol into the 

 human red blood cell operates also for 1,2,4-trihydroxybutane, but 

 not for erythritol. The tetrahydroxy compound apparently belongs 

 to the domain of monosaccharide transport. The propanediols, espe- 

 cially 1,3-propanediol, are inhibitors of glycerol transport, although 

 they are so lipophilic as to enter the red blood cells without obvious 

 mediation. A small slowing of 1,2-propanediol entry by cupric ion 

 has been observed, indicating that a small part of its entry probably 

 is mediated. Ethylene glycol and even ordinary alcohols show some 

 ability to inhibit glycerol transport, more than would be expected 

 nonspecifically from their entry into lipid phases. 



Specificity of amino acid transport 



The amino acids probably offer the largest series of analogous 

 structures for study, particularly since almost all the "neutral" or 

 monoamino, monocarboxylic members seem to show some affinity 

 for a common site or sites. Given that the product contains the 

 a-amino and a-carboxyl group, it has been difficult to discover struc- 

 tural alterations on the amino acid sidechain that abolish transport, 



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