SPECIFICITY OF TRANSPORT 



sidechains enter and reach a steady state of distribution, whereas 

 that marked Glycine shows the much lower initial rates for this 

 amino acid. Despite this difference, however, glycine in the course 

 of time reaches a much higher distribution ratio in favor of the cell 

 interior than do L-leucine and L-valine, as has long been known 

 (Christensen et ah, 1952a). The same is true for «-aminoisobutyric 

 acid. Even though the entry rate for these two is relatively slow, 

 their rate of exodus can be shown to be even slower, so that they 

 eventually reach cellular levels over twenty times as high as the 

 extracellular levels. In contrast, leucine and valine both enter and 

 leave rapidly, so that high accumulations are never reached. Obvi- 

 ously these results cannot be explained by the action of a single 

 mediating site; specifically, they require that the mediating structure 

 by which amino acids escape have a selectivity among the amino 

 acids different from the selectivity of the mediation by which they 

 enter. 



A similar difficulty is presented by comparisons between en- 

 antiomorphic pairs of amino acids. Ordinarily the l isomer, in terms 

 of the initial rate of entry, has an affinity about ten times that 

 of the d isomer. But gradually the d isomer reaches a distribution 

 ratio at least half as high as that attained by the l isomer. Accord- 

 ingly, the d isomer not only enters more slowly, but also leaves the 

 cell more slowly than its antipode. Such results have been obtained 

 with several enantiomorphic pairs. One cannot readily conceive 

 that two optical antipodes would diffuse out at different rates. Thus, 

 even if all the D-isomer escape occurred by simple diffusion, clearly 

 a major fraction of the escape of the l isomers must be mediated 

 to explain their much faster exodus. At least some D-alanine escape, 

 however, appears also to be mediated, since it shows competitive 

 inhibition of L-alanine exit (Oxender, 1962b). 



But if the cell has a separate passive mediator by which the 

 neutral amino acids escape from its interior, why should this media- 

 tor not work in the opposite direction and participate also in their 

 entry? Accordingly, one should be able to detect entry by two 

 mediators with different affinity patterns. By observing the extent to 

 which each of a number of neutral amino acids inhibits the uptake 

 of others, Oxender (1962a, 1962b) has shown that their entry occurs 

 by two different sites with overlapping affinities. The first of these 

 has already been described in part; it shows high selectivity for 



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