SPECIFICITY OF TRANSPORT 



tively slow entry of a-aminoisobutyric acid and glycine (as illus- 

 trated in Figure 20), and rather little of the alanine entry, re- 

 main for assignment to the va line-preferring site. When this cor- 

 rection is made, the fall-off in relative affinity of this site in the 

 series valine, alanine, and glycine is essentially as steep as it is 

 in the mature human erythrocyte (Table 1, page 39). The aber- 

 rant affinity for alanine uptake in the Ehrlich cell shown in this 

 table, compared with intestinal transport and uptake by the red 

 blood cell, appears to be explained by the dominance of the A 

 site in the Ehrlich cell. At the reticulocyte stage rabbit erythrocyte 

 concentrates glycine actively (Riggs et al., 1952); hence a mediation 

 resembling the A site must be present at that stage; but the amino 

 acid transport shown by the mature erythrocyte corresponds very 

 closely to the affinity of the valine-preferring mediator. The be- 

 havior of glycine, alanine, and a-aminoisobutyric acid in intestinal 

 transport and in their uptake by the isolated diaphragm of the rat 

 (Akedo and Christensen, 1962a; 1962b), as well as their uptake by 

 bacteria (Marquis, 1961; Mora, 1961), suggests that this dichotomy 

 in the transport of the neutral amino acids may be very widespread. 

 It will be interesting to know how the two mediations are distrib- 

 uted between the opposite faces of secretory cells, as in the in- 

 testinal mucosa. 



Whereas most of the entry of glycine and a-aminoisobutyric 

 acid into the Ehrlich cell occurs by the apparently oneway A site, 

 their exodus appears to occur by the other, valine-preferring site, 

 despite the unfavorable affinity (Figure 21a). That is, these amino 

 acids appear to accumulate in the cell until their concentrations 

 there are high enough to compensate for their unfavorable affinities 

 for this mediator. 



Although alanine has the better affinity for the A site, a-amino- 

 isobutyric acid is a better solute for investigation of this mediator, 

 because its selectivity between the two sites is almost completely in 

 favor of this one (A). Such a dual-affinity amino acid as methionine 

 could conceivably play a special role in transport by being strongly 

 accumulated by the A site, the accumulations then serving in turn 

 for exchange in the valine-preferring mediation to cause consider- 

 able uphill transport of such amino acids as leucine and valine (Fig- 

 ure 21b). As shown in Figure 20, 1-aminocyclopentanecarboxylic 

 acid has a course of uptake like that for methionine, and similar 

 affinity for the two sites. This situation makes it an interesting trans- 



59 



