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First published online on June 24, 2004.
 Copyright © 2004 by The Physiological Society
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jphysiol.2003.060293v1
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Received December 23, 2003
Revised February 9, 2004
Accepted after revision June 18, 2004

Bidirectional substrate fluxes through the System N (SNAT5) glutamine transporter may determine net glutamine flux in rat liver

Fiona Elizabeth Baird1, Kevin Beattie1, Russell Hyde1, Vadivel Ganapathy2, Michael J Rennie3, and Peter M. Taylor1*

1 University of Dundee
2 Medical College of Georgia
3 University of Nottingham Graduate Medical School

* To whom correspondence should be addressed. E-mail: p.m.taylor{at}dundee.ac.uk.

System N (SNAT3, 5) amino acid transporters are key mediators of glutamine transport across the plasma membrane of mammalian cell types including hepatocytes and astrocytes. We demonstrate that SNAT5 shows simultaneous bidirectional glutamine fluxes when overexpressed in Xenopus oocytes. Influx and efflux are both apparently Na+-dependent but, as they are not directly coupled, the carrier is capable of mediating net amino acid movement across the cell membrane. The apparent Km values for glutamine influx and efflux are similar (~ 1mM) and the transporter behaviour is consistent with a kinetic model in which re-orientation of the carrier from outside- to inside-facing conformations (either empty or substrate-loaded) is the limiting step in the transport cycle. In perfused rat liver, the observed relationship between influent (portal) glutamine concentration and net hepatic glutamine flux may be described by a simple kinetic model assuming the balance between influx and efflux through System N determines net flux, where under physiological conditions efflux is generally saturated due to high intracellular glutamine concentration. SNAT5 shows a more periportal mRNA distribution than SNAT3 in rat liver, indicating that SNAT5 may have particular importance for modulation of net hepatic glutamine flux.


Key words: Amino acid transport • Liver • Metabolic regulation




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