Modulation of Amino Acid Uptake by Euglena Cells after Preincubation with Substrates

Abstract

Kinetics, capacity and specificity of amino acid uptake by light-induced autotrophic Euglena gracilis cells have been studied with special reference to regulation (modulation) of amino acid transport. Both proteinogenous L-leucine and non-proteinogenous DL-α-aminobutyric acid uptake can be stimulated (2- to 4-fold) in cells preincubated for 1 to 2 h with 10 to 50 mM D-glucose, L-alanine and other acidic or neutral L-amino acids. Basic L-amino acids, which are not taken up by E. gracilis cells, or D-amino acids are unable to stimulate the amino acid transport system; D-alanine was the most inhibitory substrate. Cells grown in heterotrophic media in darkness or light do not respond with uptake modulation after substrate preincubation. The modulation seems to be a transient phenomenon; its intensity depends on preincubation time and substrate concentration. By using low concentrations of inhibitors with different mode of actions, cycloheximide and CCCP, our results suggest that the observed modulation of amino acrid uptake is rather due to reversible changes in the amino acid — proton symport system than a result of de novo synthesis of polypeptides involved in the amino acid uptake process. However, it is difficult to exclude entirely this possibility.