Multiphasic Uptake of Amino Acids by Barley Roots

Concentration-dependence and other characteristics of uptake of ³H-labeled l -lysine, l -methionine and l -proline by excised roots of barley (Hordeum vulgare L.) were studied. Use of relatively short uptake and wash periods and low solute concentrations ensured good estimates of influx across the plasmalemma. Uptake in the range of 10^?7M– 6.3 × 10^?3M can be precisely represented by four or five phases of single, multiphasic mechanisms. The mechanisms appear to be relatively specific as judged from the competition by unlabeled analogues. Structural requirements for interaction of a compound with the uptake site for methionine are given, as are the effects of analogues on the phase pattern for this amino acid. There is no indication of separate uptake and transition sites for methionine or lysine. i.e. phase transitions seem in this case to be caused by binding of molecule(s) to the uptake site. Uptake, but not phase patterns, was highly pH-dependent. The optima were pH 5 for lysine, pH 3–5 (a broad peak) for methionine and about pH 5.5 for proline. Uptake of the three amino acids was strongly inhibited by 2,4-dinitrophenol. sulfhydryl reagents and deoxycholate.