AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 6 1248-R1253, Copyright © 1993 by American Physiological Society
Ascorbic acid transport by intestinal brush-border membrane vesicles of the teleost Anguilla anguilla
M. Maffia, G. A. Ahearn, S. Vilella, V. Zonno and C. Storelli
Dipartimento di Biologia, Universita di Lecce, Italy.
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.
