Characterization of leucine-leucine transport in leaf tissues

Uptake of the dipeptide [(3)H]Leu-Leu into leaf discs from mature broad bean (Vicia faba L.) was characterized. Uptake was maximal at pH 6.0 and appeared to be mediated by three systems with apparent K(m) values of 20 µM, 350 µM and 43 mM, respectively. Leu-Leu uptake was sensitive to N-ethylmaleimide, p-chloromercuribenzenesulphonic acid, diethylpyrocarbonate, and carbonyl-cyanide-m-chlorophenylhydrazone. Nitrate did not compete with peptide uptake, although the peptide transporter and the nitrate transporter have been reported to be homologous. The ability of leaf tissues to take up peptides strongly decreased with leaf age, and the phloem export of peptides as measured by exudation experiments was very low. It is concluded that the leaf tissues contain a peptide transporter that may take up some peptides with a high affinity, but that this transporter is not involved in the long-distance transport of nitrogen under the form of di- or tri-peptides.