Transport characteristics of glutamine in human intestinal brush-border membrane vesicles

Transport of glutamine (Gln) across the brush-border membrane of the human intestine was examined using the brush-border membrane vesicle (BBMV) technique. Osmolarity and temperature studies indicate that the uptake of Gln by BBMV is mostly the result of transport of the substrate into the intravesicular space. Transport of Gln was Na⁺ gradient dependent (out > in) with a distinct 'overshoot' phenomenon. Initial rate of transport of Gln as a function of concentration was saturable both in the presence and absence of a Na⁺ gradient (out > in). Apparent K(m) of 1.86 and 1.36 mM and V(max) of 1,906 and 637 pmol·mg protein^-1·7 s^-1 were calculated for the Na⁺-dependent and the Na⁺-independent transport processes of Gln, respectively. The transport of [³H]Gln (0.58 mM) by the Na⁺-dependent and the Na⁺-independent processes was severely inhibited by the addition to the incubation medium of serine, asparagine, and unlabeled Gln. Inducing a relatively negative intravesicular compartment with the use of valinomycin and an outwardly directed K⁺ gradient or with the use of anions of different lipid permeabilities indicates that Gln transport by the Na⁺-dependent process is electrogenic in nature. Transport of Gln by the Na⁺-independent process, however, appeared to be electroneutral in nature. These results demonstrate the existence of two carrier-mediated transport processes for Gln in the human intestinal BBMV, one is Na⁺ dependent and the other is Na⁺ independent. Furthermore, the results suggest that Gln transport by the Na⁺-dependent process probably occurs by a Gln-Na⁺ cotransport mechanism.