Carrier-mediated uptake and phloem systemy of a 350-dalton chlorinated xenobiotic with an alpha-amino acid function

In a previous paper we have shown that epsilon-(phenoxyalkanecarboxylyl)-L- Lys conjugates are potent inhibitors of amino acid transport systems and th at it is possible to modulate the uptake inhibition by hydrophobic or hydro philic additions in the 4-position of the aromatic ring (J.F. Chollet, C. D eletage, M. Faucher, L. Miginiac, J.L. Bonnemain [1997] Biochem Biophys Act a 1336: 331-341). In this report we demonstrate that epsilon-(2,4-dichlorop henoxyacetyl)-L-Lys (2,4D-Lys), one of the largest molecules of the series and one of the most potent inhibitors, is a highly permeant conjugate. Upta ke of 2,4D-Lys by broad bean (Vicia faba) leaf discs is mediated by an acti ve carrier system (K(m)1 = 0.2 mM; V(max)1 = 2.4 nmol cm(-2) h(-1) at pH 5. 0) complemented by an important diffusive component. Among the compounds te sted (neutral, basic, and acidic amino acids, auxin, glutathione, and sugar s), only the aromatic amino acids clearly compete with 2,4D-Lys. The conjug ate accumulates in the vein network, is exported toward the growing organs, and exhibits a distribution pattern different from that of the herbicide m oiety. However, over time 2,4D-Lys progressively splits into 2,4D and lysin e. Analyses by highperformance liquid chromatography and liquid scintillati on spectrometry of the phloem sap collected from the castor bean system, us ed as a systemy test, indicate decreasing capacities of 2,4D, 2,4D-Lys, and glyphosate, respectively, to move from the epidermis cell wall to the siev e element. Our results show that it is possible to design synthesis of larg e-size xenobiotics (approximately 350 D) with a lipophilic pole, exhibiting high mobility within the vascular system.