
Full text loading...
Multiple transport systems for l-aspartic acid exist in Streptomyces hydrogenans. The intracellular accumulation of l-aspartate against a concentration gradient was immediately inhibited by proton conductors, such as carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone, 2,4-dinitrophenol or nigericin. Transport activity was gradually lost when inhibitors of protein synthesis were added. l-Aspartate transport had two pH optima at 6·5 and 4·5. At pH 6·5, two saturable transport components with different K m and V max values could be resolved by kinetic studies. A high-affinity system (system I) preferred the l-isomers of the anionic forms of aspartic and glutamic acid. At the same pH, a second, low-affinity system (system II) operated, which was presumably less specific than system I and also able to accept, at high concentrations, neutral amino acids. At pH 4·5, the Lineweaver-Burk plot revealed only a single catalytic component, with K m and V max values similar to those of system II. Again, in contrast to system I, this component showed high affinity for neutral amino acids. The data suggest that l-aspartic acid and l-glutamic acid are transported by this system as neutral zwitterionic molecules.
Article metrics loading...
Full text loading...
References
Data & Media loading...