AJP - Regulatory, Integrative and Comparative Physiology, Vol 270, Issue 5 948-R954, Copyright © 1996 by American Physiological Society
Basolateral dipeptide transport by the intestine of the teleost Oreochromis mossambicus
M. Thamotharan, V. Zonno, C. Storelli and G. A. Ahearn
Department of Physiology, University of Hawaii, Honolulu 96822, USA.
Transport characteristics of [14C]glycylsarcosine ([14C]Gly-Sar) were
measured in herbivorous tilapi (Oreochromis mossambicus) intestinal
basolateral membrane vesicles (BLMV) purified with Percoll gradient
centrifugation. Specific activity of the vesicle
Na(+)-K(+)-adenosinetriphos- phatase was increased 12-fold, whereas
specific activity of the brush-border enzyme alkaline phosphatase was
enriched only by 0.8-fold. [14C]Gly-Sar uptake was stimulated by increasing
concentrations of extravesicular protons rather than by a transmembrane
proton gradient. A transmembrane K+ diffusion potential (inside negative)
did not stimulate [14C]Gly-Sar uptake above that observed with
short-circuited vesicles. An inwardly directed Na+ gradient had no effect
on peptide uptake. Kinetic analysis of basolateral transport rate revealed
that the transport occurred by a saturable process conforming to
Michaelis-Menten kinetics [Kt [concentration of [14C]Gly-Sar that yielded
one-half of maximal influx (Jmax)] = 13.27 +/- 3.80 mM, Jmax = 15,155 +/-
3,096 pmol.mg protein-1.6 s-1]. The basolateral transporter was insensitive
to diethylpyrocarbonate (DEP), a specific inhibitor of proton-coupled
peptide transport systems. [14C]Gly-Sar influx into tilapia BLMV showed
cis-inhibition by several other dipeptides, suggesting that the
[14C]Gly-Sar transporter was shared by other peptides too. These
observations strongly suggest that the basolateral intestinal dipeptide
transporter in herbivorous fishes is distinctly different from either the
high- or low-affinity brush-border transporter. It is proton dependent,
electroneutral, sodium independent and accepts a wide variety of
dipeptides.
