ELECTROGENIC L-HISTIDINE TRANSPORT IN NEUTRAL AND BASIC-AMINO-ACID TRANSPORTER (NBAT)-EXPRESSING XENOPUS-LAEVIS OOCYTES - EVIDENCE FOR 2 FUNCTIONALLY DISTINCT TRANSPORT MECHANISMS INDUCED BY NBAT EXPRESSION

We have investigated the neutral and basic amino acid transporter (NBA T)-induced transport of L-histidine in Xenopus laevis oocytes. Transpo rt of L-histidine (pH 7.5) was electrogenic and Na+-dependent with a 1 4-fold increase in L-histidine- (1 mM) evoked current (I-His = -14.7 /- 1.5 nA) in NBAT-expressing oocytes compared with native (water-inje cted or uninjected) oocytes (-1.0 +/- 0.2 nA); the Na+-dependent histi dine transport showed a stoichiometry of 1:1 (histidine:sodium). I-His was stereospecific at pH 7.5 and saturable in both NaCl and tetrameth ylammonium chloride media. L-Histidine (1 mM) at pH 8.5, at which hist idine is uncharged, evoked an Na+-independent outward current (11 +/- 1.2 nA) in NBAT-expressing oocytes. The total inward 0.1 mM I-His incr eased from -9 +/- 0.8 nA at pH 7.5 to -19 +/- 2.6 nA at pH 6.5, at whi ch histidine is predominantly cationic. The increase in I-His from pH 7.5 to 6.5 was found to be almost entirely due to the Na+-independent component. At pH 7.5, L-histidine weakly inhibited the Na+-independent L-arginine uptake; however, this inhibition was much stronger (>90%) at pH 6.5, L-Histidine transport, at pH 7.5, is stimulated by NBAT exp ression, but unlike L-phenylalanine or L-arginine transport, L-histidi ne transport is Na+-dependent and stereoselective. The induction of Na +-dependent L-histidine transport in NBAT-expressing oocytes provides new evidence that NBAT stimulates functionally distinct amino acid tra nsporters including Na+-dependent L-histidine and Na+-independent L-ar ginine and L-phenylalanine transporters. The parallel induction of two different mechanisms argues that NBAT is not an amino acid transporte r itself but, instead, is a transport-activating protein for a range o f amino acid translocases.