THEORETICAL AND EXPERIMENTAL DISCRIMINATION BETWEEN CL--H-( SYMPORTERS AND CL)OH- ANTIPORTERS/

A Cl--H+ symport and a Cl-/OH- antiport cannot be readily distinguishe d physicochemically, but a kinetic distinction is theoretically possib le, because a Cl--H+ symporter involves a two-site carrier whereas a C l-/OH- antiporter involves a single-site carrier. Accordingly, we have developed kinetic models and equations that we have tested by studyin g Cl- uptake by isolated guinea pig ileal brush-border membrane vesicl es as a funct tion of Cl- or H+ concentration. We conclude that a two- site Cl--H+ symporter with a 1:1 stoichiometry explains the pH-depende nt Cl- uptake and Cl-/Cl- exchange activities of the brush-border memb rane in terms of a single random nonobligatory mobile carrier where ex change occurs by counterflow. This symport, probably involving an anio n exchanger (AE2) protein, differs, therefore, functionally from the e rythrocyte's band 3 AE1, which involves an antiport. The question is w hether members of the AE gene family can be functionally diverse, even when their primary structures exhibit up to 50% overall homologies.