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Volume 272, Number 1, Issue of January 3, 1997 pp. 125-130
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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Electrogenic L-Histidine Transport in Neutral and Basic Amino Acid Transporter (NBAT)-expressing Xenopus laevis Oocytes
EVIDENCE FOR TWO FUNCTIONALLY DISTINCT TRANSPORT MECHANISMS INDUCED BY NBAT EXPRESSION

(Received for publication, April 16, 1996, and in revised form, October 14, 1996)

From the Department of Anatomy and Physiology, University of Dundee, Dundee, DD1 4HN, Scotland, United Kingdom

We have investigated the neutral and basic amino acid transporter (NBAT)-induced transport of L-histidine in Xenopus laevis oocytes. Transport of L-histidine (pH 7.5) was electrogenic and Na⁺-dependent with a 14-fold increase in L-histidine- (1 mM) evoked current (I_His = 14.7 ± 1.5 nA) in NBAT-expressing oocytes compared with native (water-injected or uninjected) oocytes (1.0 ± 0.2 nA); the Na⁺-dependent histidine transport showed a stoichiometry of 1:1 (histidine:sodium). I_His was stereospecific at pH 7.5 and saturable in both NaCl and tetramethylammonium chloride media. L-Histidine (1 mM) at pH 8.5, at which histidine is uncharged, evoked an Na⁺-independent outward current (11 ± 1.2 nA) in NBAT-expressing oocytes. The total inward 0.1 mM I_His increased from 9 ± 0.8 nA at pH 7.5 to 19 ± 2.6 nA at pH 6.5, at which 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 expression, but unlike L-phenylalanine or L-arginine transport, L-histidine 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 transporters including Na⁺-dependent L-histidine and Na⁺-independent L-arginine and L-phenylalanine transporters. The parallel induction of two different mechanisms argues that NBAT is not an amino acid transporter itself but, instead, is a transport-activating protein for a range of amino acid translocases.

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