title: Transport characteristics of L-glutamate in human jejunal brush-border membrane vesicles.
abstract: Previous work using human jejunal brush-border membrane vesicles has demonstrated the existence of a distinct transport system in man for acidic amino acids. This system is energized by an inwardly directed Na+ gradient and an outwardly directed K+ gradient. These studies further characterize the transport of L-glutamate in the human jejunal brush-border membrane vesicles. Efflux studies were performed by loading the brush-border membrane vesicles with radiolabeled L-glutamate and sodium chloride. Extravesicular K+ accelerated the efflux of L-glutamate when compared to extravesicular Na+ or choline, indicating that potassium serves to recycle the carrier. Unlabeled extravesicular L-glutamate (but not D-glutamate) also enhanced the efflux of radiolabeled L-glutamate demonstrating that there is a bidirectional similarity to the transport system. The effect of pH on the transport system was also investigated by varying the intravesicular and extravesicular pH from 5.5 to 9. A pH environment of 6.5 produced the highest initial uptake rates as well as the greatest overshoots for transport of L-glutamate into brush-border membrane vesicles. The imposition of an inwardly directed pH gradient (5.5 outside, 7.5 inside) accelerated both the influx and efflux of L-glutamate. These results demonstrate that the L-glutamate carrier system in human jejunum appears to have similar energizing characteristics in either direction across the brush-border membrane. In addition, the system operates at an optimal pH of 6.5 and protonation of the system may enhance its mobility.
mesh_terms: D001693:Biological Transport, Active / Q000187:drug effects; D002412:Cations; D002794:Choline / Q000494:pharmacology; D005971:Glutamates / Q000378:metabolism* / Q000494:pharmacology; D018698:Glutamic Acid; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007583:Jejunum / Q000378:metabolism*; D008871:Microvilli / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
chemical_list: D002412:Cations; D005971:Glutamates; D018698:Glutamic Acid; D012964:Sodium; D002794:Choline; D011188:Potassium