BASOLATERAL DIPEPTIDE TRANSPORT BY THE INTESTINE OF THE TELEOST OREOCHROMIS-MOSSAMBICUS

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..