| Abstract: | The mechanisms involved in active transport and intracellular accumulation of amino acids have been reviewed. In particular, the frequently observed interrelationship between the transport of acidic amino acids and potassium ions was discussed. Kinetic studies on the uptake of radioactive L-aspartate and K+ in the microorganism Streptomyces hydrogenans were performed. The following results were obtained:1. L-Aspartate was actively transported into the cells. However, only a part of the aspartate taken up from the medium remained in the pool as free amino acid. Within 60 min, up to 35% of the label was incorporated into protein. By thin-layer chromatography of cell extracts several radioactive metabolites of aspartate were detected. 2. Aspartate was transported by a t least two different uptake systems exhibiting moderate specificity. At neutral pH , the amino acid was transported as anion; its uptake was inhibited by L-glutamate as well as by dicarboxylic acids, whereas neutral amino acids did not have a significant effect. 3. The influx of aspartate into K+-rich cells was stimulated specifically by extracellular Rb+ and K+, whereas Ki+ and Na+ inhibited aspartate transport. 4. Kinetic analysis of the aspartate influx showed that extracellular K+ increased the affinity of the transport systems for aspartate by a factor of three. These results suggest that K+ is bound by the aspartate carrier and is cotransported together with the amino acid across the membrane. 5. Kinetic measurements of the uptake of 42K+ revealed that the influx of K+ as well was stimulated by extracellular aspartate. Likewise the rate of 28Mg2+ uptake was increased by aspartate. |
