A D-mannose transport system in renal brush-border membranes

Abstract

Dog renal brush-border membrane vesicles (BBMV) from whole kidney cortex contain both low-affinity, high-capacity and high-affinity, low-capacity Na-dependent D-glucose cotransporters. D-Mannose is an epimer of D-glucose, differing in structure only in the axial, rather than equatorial, orientation of the hydroxyl group at the C-2 position of the pyranose ring. Uptake experiments of radioactive sugars into BBMV by standard Millipore filtration were performed to determine whether D-mannose shares either, or both, of the D-glucose carriers or if it is transported by an independent system. Transport of D-mannose occurs into an osmotically active space and is saturable and sodium dependent with a 1:1 Na:D-mannose stoichiometry, Km of 0.063 mM, Vmax of 3.6 nmol.mg-1.min-1, 25 degrees C, and pH 7.4. When an NaSCN electrochemical gradient was present, an “overshoot” was present, indicating active cotransport. Up to 50 mM D-mannose did not inhibit sodium-dependent D-glucose or alpha-methylglucoside uptake (0.01–20 mM). Sodium-dependent D-mannose uptake was inhibited by the following compounds in order of decreasing effectiveness: fructose greater than mannoheptulose greater than 2-deoxy-D-glucose greater than 2-fluoro-2-deoxy-D-glucose much greater than phloretin, cytochalasin B, galactose, 3-O-methyl-D-glucose, and L-mannose. Phlorizin also inhibited D-mannose uptake, but the high concentration required and the fact that a competitive pattern of inhibition could not be demonstrated contrasted with its effect on D-glucose transport.(ABSTRACT TRUNCATED AT 250 WORDS)