Energetics of Amino Acid Uptake by Vicia faba Leaf Tissues

Abstract

The uptake of [U-14C]threonine and of (.alpha.-14C]aminoisobutyrate (.alpha.-AIB) by V. faba leaf discs is strongly pH dependent (optimum: pH 4.0) and exhibits biphasic saturation kinetics. Kinetics of .alpha.-AIB uptake at different pH values indicate that acidic pH values decrease the Km of the carriers while the maximal velocity remains nearly unaffected. Similar results were obtained for system 1 (from 0.5-5 mM) and system 2 (from 20-100 mM). After addition of amino acids to a medium containing leaf fragments, alkalinizations depending on the amino acid added and on its concentration were recorded. The effects of compounds which increase (fusicoccin) or decrease (uncouplers, ATPase inhibitors, high KCl concentrations) the protonmotive force were studied on the acidification of the medium and on amino acid uptake by the tissues. There is a close relationship between the time required for the effect of these compounds on the acidification and that needed for inhibition of uptake. Studies with thiol inhibitors show that 0.1 mM N-ethylmaleimide preferentially inhibits uptake by the mesophyll whereas 0.1 mM parachloromercuribenzenesulfonate affects uptake by the veins. Evidently, proton amino acid symport occurs in leaf tissues, particularly in the veins.