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.