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 defecti ve renal and intestinal reabsorption (Calonge, M. J., Gasparini, P., C hillaron, J., Chillon, M., Gallucci, M., Rousaud, F., Zelante, L., Tes tar, 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 cla rified is how the apparently non-concentrative system b(o,+)-like, ass ociated with rBAT expression, participates in the active renal reabsor ption of these amino acids. Several studies have demonstrated exchange of amino acids induced by rBAT in Xenopus oocytes. Here we offer evid ence 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-exchan ge dibasic (inward) with neutral (outward) amino acids were favored in oocytes. Obligatory exchange of amino acids via system b(o,+)-like fu lly explained the amino acid-induced current in rBAT-injected oocytes. Exchange via system b(o,+)-like is coupled enough to ensure a specifi c accumulation of substrates until the complete replacement of the int ernal oocyte substrates. Due to structural and functional analogies of the cell surface antigen 4F2hc to rBAT, we tested for amino acid exch ange via system y(+)L-like. 4F2hc-injected oocytes accumulated substra tes to a level higher than CAT1-injected oocytes (i.e. oocytes express ing 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, syste m y(+)L-like did not mediate measurable L-leucine efflux from the oocy te. We propose a role of systems b(o,+)-like and y(+)L-like in the ren al reabsorption of cystine and dibasic amino acids that is based on th eir active tertiary transport mechanism and on the apical and basolate ral localization of rBAT and 4F2hc, respectively, in the epithelial ce lls of the proximal tubule of the nephron.