An L-proline-dependent proton flux is located at the apical membrane levelof the eel enterocytes

This study has demonstrated the existence of an L-proline-dependent (Na ind ependent) proton flux at the apical membrane level of the eel intestinal ab sorbing cells. Using isolated eel enterocytes and the pH-sensitive fluoresc ent dye 2', 7'-bis-(2-carboxyethyl)-5-(and-6)-carboxy-fluorescein, acetoxym ethyl ester (BCECF), it was shown that a 20 mM concentration of the imino a cid L-proline in the extracellular medium determined an intracellular acidi fication of similar to0.28 pH units. However, neither sucrose nor other ami no acids were able to significantly acidify the resting intracellular pH. A hyperbolic relationship between extracellular proline concentration and in tracellular proton accumulation was observed. Using both isolated brush-bor der and basolateral membrane vesicles, it was demonstrated that this prolin e-proton cotransport mechanism was located at the apical membrane level onl y. In addition, the existence of a coupling mechanism between proline and p roton fluxes was demonstrated by the observation that, in brush-border memb rane vesicles, the presence of a pH gradient (pH(in) > pH(out)) stimulated the uptake of L-proline.