Ascorbic acid transport by intestinal brush-border membrane vesicles of the teleost Anguilla anguilla

Transport of L-ascorbate by intestinal brush-border membrane vesicles of European eel Anguilla anguilla was stimulated by a transmembrane Na gradient (out > in) but not by a similarly directed gradient of K. Under short-circuited membrane potential conditions, a kinetic analysis of L-ascorbate influx indicated the presence of a single Na-dependent carrier process (Kapp = 0.75 +/- 0.07 mM and Jmax = 0.33 +/- 0.03 nmol.mg protein-1.min-1) and a nonsaturable transfer component with an apparent diffusional permeability (P) of 0.27 +/- 0.02 microliter.mg protein-1.min-1. D-Isoascorbate was a competitive inhibitor of L-ascorbate influx exhibiting a Ki of 8.21 +/- 0.63 mM. The electrogenic nature of +Na-L-ascorbate cotransport was confirmed by a stimulatory effect of an inside-negative membrane potential on vitamin uptake. Hill analysis of L-ascorbate influx over a wide range of external Na concentrations suggested a 2 Na-to-1 L-ascorbate binding ratio. Results indicate that the vitamin L-ascorbate is transported across fish intestinal brush-border membranes by an electrogenic Na-dependent carrier process in conjunction with more than one sodium ion.