CHARACTERIZATION OF LEUCINE-LEUCINE TRANSPORT IN LEAF TISSUES

Uptake of the dipeptide [H-3]Leu-Leu into leaf discs from mature broad bean (Vicia fabaL.) was characterized. Uptake was maximal at pH 6.0 a nd appeared to be mediated by three systems with apparent K-m values o f 20 mu M, 350 mu M and 43 mM, respectively. Leu-Leu uptake was sensit ive to N-ethylmaleimide, p-chloromercuribenzenesulphonic acid, diethyl pyrocarbonate, and carbonyl-cyanide-m-chlorophenylhydrazone. Nitrate d id not compete with peptide uptake, although the peptide transporter a nd the nitrate transporter have been reported to be homologous. The ab ility of leaf tissues to take up peptides strongly decreased with leaf age, and the phloem export of peptides as measured by exudation exper iments was very low. It is concluded that the leaf tissues contain a p eptide transporter that may take up some peptides with a high affinity , but that this transporter is not involved in the long-distance trans port of nitrogen under the form of di- or tri-peptides.