BIOLOGICAL TRANSPORT 



amino acids with large hydrocarbon sidechains. Its approximate 

 properties are tentatively summarized in Table 2. It is also only mod- 

 erately stereospecific. It operates rapidly in both directions; at the 

 present we do not know whether it can operate uphill or represents 

 only a facilitated diffusion. 



The other site, tentatively designated A, apparently operates 

 for entry only and shows good affinity for L-alanine, a-aminoiso- 

 butyric acid, and glycine, decreasing in that order. When the hydro- 

 carbon chain is lengthened, and particularly when it is branched 

 at C 3 as in valine, affinity for this site is rapidly lost; the entry of 

 valine at a 5-mM level appears to occur only to a minor extent 

 by this site. If the apolar sidechain is not branched, with further 

 lengthening a secondary increase in affinity for the A site occurs, 

 as can be illustrated with norleucine and methionine. Apparently 

 the ethereal sulfur in methionine is roughly equivalent to a methyl- 

 Table 2 Tentative Summary of Characteristics of Two Transport Mediators 

 for Neutral Amino Acids as Identified in the Ehrlich Cell 



ene group in this matter. This relationship appears to explain why 

 methionine has good affinity for both sites, and why in Figure 20 

 it shows not only a high initial rate but also high accumulation into 

 the cell. It shows strong competitive action for both sites, a factor 

 that may explain the unusual toxicity of elevated plasma levels of 

 this amino acid. 



When allowance is made for entry through the A site by 

 glycine, a-aminoisobutyric acid, and alanine, very little of the rela- 



58 



