Kinetic analysis of L-glutamine transport into porcine jejunal enterocyte brush-border membrane vesicles

L-Glutamine transport into porcine jejunal enterocyte brush border membrane vesicles was studied. Uptake was mediated by a Na+-dependent and a Na+-independent pathway as well as by diffusion. The initial rates of glutamine uptake over a range of concentrations in both Na+-independent and Na+-free conditions were analyzed and kinetic parameters were obtained. Na+-dependent glutamine transport had a K-m of 0.77 +/- 0.16 mM and a J(max) of 70.7 +/- 5.8 pmol mg protein(-1) s(-1); Na+-independent glutamine transport had a K-m of 3.55 +/- 0.78 mM and a J(max) of 55.1 +/- 6.6 pmol mg protein(-1) s(-1). The non-saturable component measured with HgCl2-poisoned brush border membrane vesicles in the Na+-free condition contained passive diffusion and non-specific membrane binding and was defined to be apparent glutamine diffusion and the glutamine permeability coefficient (K-diff) was estimated to be K-diff = 3.78 +/- 0.06 pmol 1 mg protein(-1) mmol(-1) s(-1). Results of inhibition experiments showed that Na+-dependent glutamine uptake occurred primarily through the brush border system-B-o transporters, whereas Na+-independent glutamine uptake occurred via the system-L transporters. Furthermore, the kinetics of L-leucine and L-cysteine inhibition of L-glutamine uptake demonstrated that neutral amino acids sharing the same brush border transporters can effectively inhibit each other in their transport. (C) 1998 Elsevier Science Inc. All rights reserved.