M. Thamotharan et al., BASOLATERAL DIPEPTIDE TRANSPORT BY THE INTESTINE OF THE TELEOST OREOCHROMIS-MOSSAMBICUS, American journal of physiology. Regulatory, integrative and comparative physiology, 39(5), 1996, pp. 948-954
Transport characteristics of [C-14]glycylsarcosine ([C-14]Gly-Sar) wer
e measured in herbivorous tilapia (Oreochromis mossambicus) intestinal
basolateral membrane vesicles (BLMV) purified with Percoll gradient c
entrifugation. Specific activity of the vesicle Na+-K+-adenosinetripho
sphatase was increased 12-fold, whereas specific activity of the brush
-border enzyme alkaline phosphatase was enriched only by 0.8-fold. [C-
14]Gly-Sar uptake was stimulated by increasing concentrations of extra
vesicular protons rather than by a transmembrane proton gradient. A tr
ansmembrane K+ diffusion potential (inside negative) did not stimulate
[C-14]Gly-Sar uptake above that observed with short-circuited vesicle
s. An inwardly directed Na+ gradient had no effect on peptide uptake.
Kinetic analysis of basolateral transport rate revealed that the trans
port occurred by a saturable process conforming to Michaelis-Menten ki
netics [K-t{concentration of [C-14]Gly-Sar that yielded one-half of ma
ximal influx (J(max))} = 13.27 +/- 3.80 mM, J(max) = 15,155 +/- 3,096
pmol . mg protein(-1). 6 s(-1)]. The basolateral transporter was insen
sitive to diethylpyrocarbonate (DEP), a specific inhibitor of proton-c
oupled peptide transport systems. [C-14]Gly-Sar influx into tilapia BL
MV showed cis-inhibition by several other dipeptides, suggesting that
the [C-14]Gly-Sar transporter was shared by other peptides too. These
observations strongly suggest that the basolateral intestinal dipeptid
e transporter in herbivorous fishes is distinctly different from eithe
r the high- or low-affinity brush-border transporter. It is proton dep
endent, electroneutral, sodium independent and accepts a wide variety
of dipeptides..