KINETICS OF THE CHLORIDE-ANION EXCHANGER OF BRUSH-BORDER MEMBRANE-VESICLES ISOLATED FROM CHICKEN JEJUNUM

The kinetic parameters of the apical Cl--anion exchanger of chicken jejunum have been evaluated using brush-border membrane vesicles. SITS inhibited pH gradient-driven Cl- uptake in a dose-dependent manner with a IC50 of 700 mu M. In the presence of a pH gradient (pH 7.7 inside, 5.5 outside), Cl- uptake tends to saturate with increasing external Cl- concentration. With 5 mM SITS the relationship between Cl-36(-) uptake and external Cl- concentration was linear. SITS-insensitive Cl- uptake may represent a diffusion component and has an apparent diffusion constant, K-d, of 0.068 +/- 0.002 nmol/mg protein per 15 s per mM. Eadie-Hofstee analysis of the data obtained by subtracting the SITS-insensitive component from the total uptake indicates a single transport system with a K-m of 5 +/- 0.9 mM. A 50 mM outwardly directed bicarbonate gradient increased the V-max from 3.63 +/- 0.33 to 6.40 +/- 0.54 nmol/mg protein per 15 s. The initial rate of H-2-DIDS-sensitive Cl- uptake increased as the intravesicular pH increased, with a Hill coefficient greater than one. An intra- to extravesicular gradient of Cl-, I- and HCO3- stimulated Cl- uptake in the absence of a pH gradient.