Kinetics of the Uptake of Some Neutral Amino Acids by Spirodela Polyrhiza

Uptake of glycine, L-alanine, L-leucine, L-isoleucine, L-valine, and D-valine by intact fronds of the duckweed Spirodela polyrhiza was investigated. The time-course of amino acid uptake showed that the uptake rate (= net influx) was apparently not affected by accumulation of amino acid in the fronds, suggesting that there was no trans-inhibition of the influx, and that the efflux was negligible. Efflux was measured from fronds preloaded with L-(14C)leucine. When the external and internal concentration were both about 2.5 mM the efflux was ≤ 1% of the influx. Uptake rates were measured over a wide concentration range, 1 μM to 10 mM. For each amino acid the relation between uptake rate and external concentration could be described as the sum of two Michaelis-Menten processes. It is suggested that the duckweed possesses two transport systems common to the neutral amino acids tested: a low Km, low capacity system (system 1), and a high Km, high capacity system (system 2).