DEMONSTRATION AND CHARACTERIZATION OF DIPEPTIDE TRANSPORT-SYSTEM ACTIVITY IN SHEEP OMASAL EPITHELIUM BY EXPRESSION OF MESSENGER-RNA IN XENOPUS-LAEVIS OOCYTES

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.