Obligatory amino acid exchange via systems b(o,+)-like and y(+)L-like - A tertiary active transport mechanism for renal reabsorption of cystine and dibasic amino acids

Mutations in the rBAT gene cause type I cystinuria, a common inherited aminoaciduria of cystine and dibasic amino acids due to their defective renal and intestinal reabsorption (Calonge, M. J., Gasparini, P., Chillaron, J., Chillon, M., Gallucci, M., Rousaud, F., Zelante, L., Testar, X., Dallapiccola, B., Di Silverio, F., Barcelo, P., Estivill, X., Zorzano, A., Nunes, V., and Palacin, M. (1994) Not. Genet. 6, 420-426; Calonge, M. J., Volipini, V., Bisceglia, L., Rousaud, F., De Sanctis, L., Beccia, E., Zelante, L., Testar, X., Zorzano, A., Estivill, X., Gasparini, P., Nunes, V., and Palacin, M. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9667-9671). One important question that remains to be clarified is how the apparently non-concentrative system b(o,+)-like, associated with rBAT expression, participates in the active renal reabsorption of these amino acids. Several studies have demonstrated exchange of amino acids induced by rBAT in Xenopus oocytes. Here we offer evidence that system b(o,+)-like is an obligatory amino acid exchanger in oocytes and in the ''renal proximal tubular'' cell line OK. System b(o,+)-like showed a 1:1 stoichiometry of exchange, and the hetero-exchange dibasic (inward) with neutral (outward) amino acids were favored in oocytes. Obligatory exchange of amino acids via system b(o,+)-like fully explained the amino acid-induced current in rBAT-injected oocytes. Exchange via system b(o,+)-like is coupled enough to ensure a specific accumulation of substrates until the complete replacement of the internal oocyte substrates. Due to structural and functional analogies of the cell surface antigen 4F2hc to rBAT, we tested for amino acid exchange via system y(+)L-like. 4F2hc-injected oocytes accumulated substrates to a level higher than CAT1-injected oocytes (i.e. oocytes expressing system y(+)) and showed exchange of amino acids with the substrate specificity of system y(+)L and L-leucine-induced outward currents in the absence of extracellular sodium. In contrast to L-arginine, system y(+)L-like did not mediate measurable L-leucine efflux from the oocyte. We propose a role of systems b(o,+)-like and y(+)L-like in the renal reabsorption of cystine and dibasic amino acids that is based on their active tertiary transport mechanism and on the apical and basolateral localization of rBAT and 4F2hc, respectively, in the epithelial cells of the proximal tubule of the nephron.