BIOLOGICAL TRANSPORT 



In this connection, we may recall again that the erythrocyte 

 concentrates glycine strongly at the reticulocyte stage (Riggs et ctl., 

 1952) but, at the adult stage, shows only a facilitated diffusion with 

 high affinity for leucine and valine but with almost no affinity for 

 glycine (Winter, 1962). This behavior suggests that the ability to 

 render the carrier site reactive is largely lost on maturation of the 

 erythrocyte as the energy supply available to the cell decreases, and 

 perhaps argues also in favor of a generic relationship between the 

 two mediating structures, such as that shown in Figure 22. 



The foregoing examples show that a quantitative and not merely 

 a qualitative demonstration is needed to establish that two solutes 

 actually use exclusively the same mediation. 



The transport of a-aminoisobutyric acid illustrates that the 

 presence of an alpha-hvdrogen atom is not necessary to the trans- 

 port of the neutral amino acids. Since the a-hydrogen is necessary 

 to many catabolic attacks on amino acids, many species do not 

 metabolize such an amino acid, and it can therefore be used to 

 study transport uncomplicated by irrelevant metabolic modifica- 

 tions. This approach has permitted the study of nutritional and 

 endocrine influences (to be discussed in Chapter 8) on distribu- 

 tion and transport, without the possibility that these factors have 

 primarily slowed or accelerated the formation or destruction of the 

 amino acid, rather than transport. 



In writing the following sentence in 1902, Rudolf Hober was 

 supporting the use of organic compounds presenting a wide range of 

 physical properties for the study of permeability: "When the 

 foreign substances do not destroy the normal course of life processes, 

 but when they instead pass through the organism uninjurious or 

 unmodified, then their unusualness represents a distinct advantage 

 for experimental study." In all fairness we must recognize that, in 

 the cited discussion, Hober had not anticipated the use of structural 

 analogs able to compete for a chemical mediation of transport. 



A larger hydrocarbon structure, again occupying both the a x 

 and a 2 positions in 1-aminocyclopentanecarboxylic acid, leads to 

 much higher transport affinity than is shown for a-aminoisobutyric 

 acid. It also competes strongly with L-valine for the dissimilar site 

 by which that amino acid enters the Ehrlich cell. If the cyclopentane 

 ring is not closed between ring carbons numbered 3 and 4, that is, 

 as in the analog a-diethylglvcine, we observe very little affinity 

 for either site. Apparently, if the sidechain group at the position 



64 



