Transport of asparagine by rat brain synaptosomes: an approach to evaluate glutamine accumulation.
Isolated rat brain synaptosomes accumulated L-asparagine with a Km value of 348 microM and a Vmax value of 3.7 nmol/mg of protein/min at 28 degrees C. Uptake of L-asparagine was inhibited by the presence of L-glutamine, whereas transport of L-glutamine was blocked by L-asparagine. Alanine, serine, cysteine, threonine, and, in particular, leucine were also inhibitory whereas alpha-(methylamino)isobutyrate, ornithine, lysine, arginine, and glutamate were much less effective blockers. Transport of L-asparagine had a substantial sodium-dependent component, whereas that of the D-stereoisomer was almost unaffected by the presence or absence of the cation. L-Asparagine was accumulated to a maximal gradient, [L-Asn]i/[L-Asn]o, of 20-30, and this value was reduced to 5-6 by withdrawal of sodium or addition of high [KCI]. A plot of log [Na+]o/[Na+]i against the log [L-Asn]i/[L-Asn]o had a slope close to I, which indicates that a single sodium ion is transported inward with each asparagine molecule. It is postulated that uptake of L-asparagine occurs, to a large extent, in cotransport with Na+ and that it utilizes the sodium chemical gradient and the membrane electrical potential as the source of energy. The similarity between the L-asparagine and L-glutamine transport systems and the reciprocal inhibition of influx of the two amino acids suggest that the same mechanism is responsible for glutamine accumulation. This could explain the high [Gln]i maintained by the brain in vivo.
Erecińska
M
M
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
Zaleska
M M
MM
Chiu
L
L
Nelson
D
D
D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D001216:Asparagine / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013997:Time Factors
D000596:Amino Acids; D005973:Glutamine; D001216:Asparagine