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

Abstract

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 +-gradient 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 HgCl 2-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 l 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° 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.