Amino Acid Transport in Photoautotrophic Suspension Cells of Chenopodium rubrum L.: Stereospecificity and Interaction with Potassium Ions

Abstract

Summary On photoautotrophically grown suspension cells of Chenopodium rubrum L. uptake of 14 C-labeled amino acids was studied as a function of amino acid and K + concentration, and uptake of ( 86 Rb + )K + in the presence of different amino acids. (1) A stereospecific amino acid uptake mechanism was halfsaturated at 4 μM L-alanine and 20 μM D-alanine, measured by the mean rate of uptake between 30 and 90 s after addition of the transportee. This high-affinity mechanism accumulated L-alanine 112-fold in the cytoplasm and is attributed to an electrogenic carrier. (2) Low affinity uptake, presumably facilitated diffusion by the uncharged carrier, governs uptake at concentrations > 100 μM and periods > 5 min. The parallel operation of both uptake systems lowers the accumulation ratio during prolonged uptake, e.g. to 4.2 after a 40 min-uptake from 200 μM L-alanine. (3) Whereas K + stimulates the high-affinity uptake of L-serine at 0.4-1.1 mM K + compared to 0.001-0.01 mM K + by a factor of 1.3, L-alanine (0.2 mM), α-amino-isobutyric acid (0.2 mM) and L-serine (1 mM) reduce the apparent influx of ( 86 Rb + ) K + by 34, 64, and 62%, respectively. The causes of these interactions are unknown. A K + -dependent amino acid cotransport is unlikely.