Transport of Glutamine in Xenopus Laevis Oocytes: Relationship with Transport of Other Amino Acids

We have investigated transport of the amino acid glutamine across the surface membranes of prophase-arrested Xenopus laevis oocytes. Glutamine accumulation was linear with time for 30 min; it was stereospecific with a Km of 0.12 +/- 0.02 mM and Vmax of 0.92 +/- 0.17 pmol/oocyte.min for L-glutamine. Transport of L-glutamine was Na(+)-dependent, the cation not being replaceable with Li+, K+, choline, tris(hydroxymethyl)-aminomethane (Tris), tetramethylammonium (TMA) or N-methyl D-glucamine (NMDG); external Cl- appeared to be necessary for full activation of Na(+)-dependent glutamine transport. Two external Na+ may be required for the transport of one glutamine molecule. L-glutamine transport (at 50 microM glutamine) was inhibited by the presence of other amino acids: L-alanine, D-alanine, L-leucine, L-asparagine and L-arginine (about 60% inhibition at 1 mM); L-histidine, L-valine and glycine (25 to 40% inhibition at 1 mM); L-serine, L-lysine, L-phenylalanine and L-glutamate (45 to 55% inhibition at 10 mM). N-methylaminoisobutyric acid (MeAIB) had no effect at 10 mM, but 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) inhibited Na+/glutamine transport by about 50% at 10 mM. L-glutamine was a competitive inhibitor of the Na(+)-dependent transport of L-alanine, D-alanine and L-arginine; this evidence is consistent with the existence of a single system transporting all four amino acids. Glutamine uptake in oocytes appears to be catalyzed by a transport system distinct from the cotransport Systems A, ASC, N and Gly, although it resembles System B0,+.