Characteristics of glutamine transport in dog jejunal brush-border membrane vesicles

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 K(m) of 0.97 ± 0.58 mM and a V(max) 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 K(m) of 3.70 ± 0.66 mM and a V(max) 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.