Intracellular Distribution of Free Amino Acids between the Vacuolar and Extravacuolar Compartments in Internodal Cells of Chara australis

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Abstract
Distribution of free amino acids between the vacuolar and extra-vacuolar (cytoplasmic) compartments in internodal cells of Chara australis was studied. Under the control conditions (14-h light : 10-h dark), most (90%) of the cellular free amino acids were found in the extra-vacuolar compartment. The reverse was true for ammonia. The major amino acids were isoasparagine, alanine, glutamic acid, aspartic acid, serine and glycine. The contents of hydrophobic and basic amino acids were minor and relatively greater proportions were found in the vacuole except when their contents were extremely low. When cells were kept for 3 days under continuous light or in continuous darkness, the total free amino acid content increased to about 120% (light) and about 150% (dark) that of the control. These increases mainly took place in the vacuole, but the amino acid species responsible for the increments differed with the light conditions. In contrast, the cytoplasmic content was relatively constant (50–60 mM) even under continuous light or darkness. The results suggest that the vacuole acts in the homeostasis of the cytoplasmic amino acid content. As anion, amino acids in the cytoplasm compensated for about 10–20% of the reported “anion deficiency” in the cytoplasm.