SPECIFICITY OF TRANSPORT 



as long as a new positive or negative charge is not introduced. Fur- 

 thermore, the inhibitory action of a variety of amino acids of ab- 

 normal structure indicates that they use the same transport sites as 

 the normal amino acids do. Accordingly, we shall note some strange 

 amino acids in use for the study of normal amino acid transport. 



The anionic amino acids, on the one hand, and the cationic 

 amino acids, on the other, appear to represent transport families 

 distinct from the neutral amino acids in that inhibitions occur only 

 within each group and not between the three groups. Conceivably, 

 sites of a similar nature could serve for all three families, except 

 that some might be shielded from approach of an anionic or a 

 cationic amino acid by the presence of a nearby charged group. 

 This simple view is doubted for the dicarboxylic amino acids, how- 

 ever, since, for the Ehrlich cell, the presence of an a-methyl group 

 on the amino acid blocks the transport of the dicarboxylic but not 

 of the neutral amino acids (Christensen, 1955). The presence of the 

 y-amide group, as in glutamine, makes an a-methyl group again ac- 

 ceptable. 



Among the diamino acids, a very interesting, intense augmenta- 

 tion of transport occurs when the omega amino group is brought 

 close enough to the a-amino group. Whereas ornithine undergoes 

 only a mildly concentrative uptake, a,y-diaminobutyric acid is ac- 

 cumulated to an extent that can be destructive to the cell (Christen- 

 sen et al., 1952b). 



A clear affinity sequence for the rates of uptake of the neutral 

 amino acids having aliphatic hydrocarbon sidechains and for their 

 inhibitory action has been observed in the human erythrocyte 

 (where the uptake apparently is not concentrative), also in the 

 Ehrlich ascites tumor cell, and in the everted sac of the rat intestine, 

 a useful experimental preparation introduced by Wilson and Wise- 

 man (1954). In this sequence glycine has the least affinity, alanine 

 next, followed by valine and leucine in order. The same sequence 

 of affinities is shown in the strength of their ability to drive the 

 exchange uptake of susceptible amino acids. 



These results imply that the hydrocarbon sidechain of these 

 amino acids may be involved as a third point of binding, in addition 

 to the amino and carboxyl groups, which have so far proved spe- 

 cifically necessary for the characteristic interaction. Presumably the 

 hydrocarbon sidechains could join to a third locus at the transport 



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