DEMONSTRATION AND CHARACTERIZATION OF DIPEPTIDE TRANSPORT-SYSTEM ACTIVITY IN SHEEP OMASAL EPITHELIUM BY EXPRESSION OF MESSENGER-RNA IN XENOPUS-LAEVIS OOCYTES
Jc. Matthews et al., DEMONSTRATION AND CHARACTERIZATION OF DIPEPTIDE TRANSPORT-SYSTEM ACTIVITY IN SHEEP OMASAL EPITHELIUM BY EXPRESSION OF MESSENGER-RNA IN XENOPUS-LAEVIS OOCYTES, Journal of animal science, 74(7), 1996, pp. 1720-1727
Research from this laboratory has recently demonstrated that the omasa
l epithelium of sheep is capable of absorbing dipeptides. In order to
express proteins potentially responsible for the mediated absorption o
f small peptides, size-fractionated poly(A)(+)RNA (RNA) isolated from
omasal epithelial tissue of sheep (average BW 67.5 kg) were injected i
nto defolliculated Xenopus Laevis oocytes. The ability of oocytes inje
cted with RNA or water to absorb [C-14]glycyl-L-sarcosine (Gly-Sar) fr
om media (usually pH 5.5) was compared. After 4 d (P <.02) of culture,
specific RNA fractions induced an increased (P <.02) rate of Gly-Sar
absorption, as compared with water-injected oocytes. The dependency of
Gly-Sar uptake on the presence of a pH gradient was evaluated at pH 5
.0, 5.5, 6.0, 6.5, and 7.5. Inducible uptake increased (P <.001) in th
e presence of increasing proton concentrations, whereas endogenous upt
ake of Gly-Sar decreased (P <.001). At pH 5.5, induced Gly-Sar uptake
was saturable (K-t=.4 mM), but endogenous uptake was not. The specific
ity of Gly Sar absorption was studied by the co-incubation of .1 mM Gl
y-Sar with 5 mM levels of competing substrates (pH 5.5). Induced uptak
e was inhibited (P <.05) 44% by carnosine, 94% by methionylglycine, an
d 91% by glycylleucine, but not by glycine. Incubation of RNA with DNA
oligomers that were complementary to the rabbit intestinal transporte
r completely inhibited (P <.05) induced Gly-Sar uptake. These results
indicate that sheep omasal epithelial cells express messenger RNA that
encode for proteins that are capable of H+-dependent dipeptide transp
ort activity.