| Popis: |
A transport system for cationic amino acids has been described for the rabbit reticulocyte. This system appears neither concentrative nor energy-dependent. At high levels, several neutral amino acids with large apolar side chains inhibit uptake by it; furthermore, the presence of external leucine accelerates lysine exodus from the cell. These effects do not, however, arise from an interaction between the new system and the dominant Na+-independent system by which these amino acids are transported, because 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid, a new specific substrate for the latter system, shows no inhibition of diamino acid uptake. Furthermore, lysine is devoid of inhibitory action on leucine uptake. Although the relations described above can be shown either in the presence or absence of Na+, the basic amino acids tested are taken up from 10 to 50% more rapidly when Na+ is present at about 0.14 m. Unequivocal evidence that the Na+-stimulated portion represents an independent system has not been obtained. In the presence of Na+, cysteine and homoserine inhibit a substantially larger portion than the Na+-stimulated portion of the uptake of diamino acids whereas no inhibition is seen in the absence of Na+. The result is that, on the addition of one of these two amino acids in excess, Na+ is changed from a stimulant to an inhibitor of diamino acid transport. |
