AJP - Gastrointestinal and Liver Physiology, Vol 257, Issue 1 80-G85, Copyright © 1989 by American Physiological Society
Characteristics of glutamine transport in dog jejunal brush-border membrane vesicles
N. M. Bulus, N. N. Abumrad and F. K. Ghishan
Department of Surgery, Vanderbilt University, Nashville, Tennessee 37232.
The present study characterizes glutamine transport across brush-border
membrane vesicles (BBMV) prepared from dog jejunum. The purity of these
vesicles was demonstrated by a 20-fold enrichment of leucine
aminopeptidase, a marker for BBM. Glutamine uptake was found to occur into
an osmotically active space with no membrane binding and to exhibit
temperature and pH dependence (optimal uptake at pH 7-7.5). Glutamine
uptake was driven by an inwardly directed Na+ gradient with a distinct
overshoot not observed under K+ gradient. Lithium could not substitute for
Na+ as a stimulator of glutamine uptake. Na+-dependent glutamine uptake was
not inhibited by methylaminoisobutyric acid, a typical substrate for system
A, and was found to be electrogenic and saturable with a Km of 0.97 +/-
0.58 mM and a Vmax of 3.93 +/- 0.99 nmol.mg protein-1.10 s-1. A
Na+-glutamine coupling ratio of 1:1 could be demonstrated by a plot of Hill
transformation. Na+-independent glutamine uptake was found to be
electroneutral and saturable with a Km of 3.70 +/- 0.66 mM and a Vmax of
2.70 +/- 1.55 nmol.mg protein-1.10 s-1. Inhibition studies confirmed the
presence of a Na+-dependent as well as a Na+-independent carrier for
glutamine uptake. We conclude that glutamine uptake across dog BBMV occurs
via two transport systems: a Na+-dependent high-affinity system similar to
the neutral brush-border system and a Na+-independent lower-affinity system
similar to system L.
