Abstract
The presence of a Na+/D-mannose cotransporter in brush-border membrane vesicles (BBMV) isolated from chicken small intestine was examined. In the presence of an electrochemical gradient for Na+, but not in its absence, D-mannose was accumulated transiently by the BBMV. D-Mannose uptake into the BBMV was energized by both the membrane potential and the chemical gradient for Na+. The relationship between D-mannose transport and external D-mannose concentration was described by an equation that represented the superposition of a saturable component (Michaelis-Menten constant Km 12.5 microM) and another component unsaturatable up to 80 microM D-mannose. D-Mannose uptake was inhibited by various substances in the following order of potency: D-mannose>>D-glucose>phlorizin>phloretin>D-fructose. For the uptake of alpha-methyl-glucopyranoside the order was D-glucose=phlorizin>>phloretin=D-fructose=D-mannose. The initial rate of D-mannose uptake increased as the extravesicular [Na+] increased, with a Hill coefficient of 1, suggesting that the Na+:D-mannose cotransport stoichiometry is 1:1. It is concluded that the intestinal apical membrane has a saturable, electrogenic and concentration- and Na+-dependent mannose transport mechanism that differs from the sodium-dependent glucose transporter SGLT1.
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