SPECIFICITY OF TRANSPORT 



it characteristically occupies in i>-amino acids is drawn by the ring 

 structure away from the erect position it would otherwise tend to 

 occupy, very large increases in transport affinity result. Extension 

 of such structural comparisons promises to be highly informative 

 as to the geometry of the amino acid transport sites. 1-Aminocyclo- 

 pentanecarboxvlic acid shows a high affinity for both intestinal 

 (Akedo and Christensen, 1962a) and renal transport. The renal 

 tubules resorb it so completely that, in the male rat or mouse, the 

 amino acid cycles about in the body for many months (Christensen 

 and Jones, 1962). 



Genetically determined transport systems apparently limited 

 to only one, two, or three neutral aliphatic amino acids rather 

 than the whole group, have occasionally been reported for micro- 

 organisms, a pattern departing somewhat from the broader specifi- 

 cities shown by animal cells. The solutes of a class should be studied 

 over very wide concentration ranges before concluding that no 

 competitive actions occur among them. 



Specificity of sugar transport 



A considerable number of monosaccharides are acceptable to 

 the transport system of the human red blood cell. No successful 

 generalization of the structural requirements for transport was 

 reached until LeFevre and Marshall (1958) pointed out that the 

 D-pyranoses with the highest transport affinity (D-glucose, 2-deoxy- 

 D-glucose, 3-O-methyl glucose) are the ones having the most struc- 

 tural features tending to stabilize them in the chair conformation 

 known as Cl (Figure 24). These are the sugars that appear on con- 

 formational analysis to have the highest number of (hydroxy + 

 hydroxymethyl) groups in the equatorial orientation in the Cl 

 form, but in the axial orientation in the other chair form, known as 

 1C. Passage of conformation from Cl form to 1C form is marked 

 by the passage of all equatorially orientated groups (that is, extend- 

 ing outward from the rough plane described by the ring) to an axial 

 orientation (that is, roughly perpendicular to the plane of the ring). 

 The D-pyranoses with the low est transport affinities are the ones 

 that are not so unbalanced in the number of such groups in one 

 orientation as in the other; these D-pyranoses are therefore almost as 

 stable in one conformation as they are in the other. 



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