DEMONSTRATION AND CHARACTERIZATION OF A TRANSPORT-SYSTEM CAPABLE OF LYSINE AND LEUCINE ABSORPTION THAT IS ENCODED FOR IN PORCINE JEJUNAL EPITHELIUM BY EXPRESSION OF MESSENGER-RNA IN XENOPUS-LAEVIS OOCYTES

Defolliculated Xenopus laevis oocytes were injected with size-fraction ated poly(A)(+) RNA (RNA) isolated from the jejunal epithelium of grow ing pigs (average BW 33.8 kg) to identify proteins capable of Na+-inde pendent amino acid transport. The ability of oocytes to absorb L-lysin e (lysine) or L-leucine (leucine) from Na+-free media was quantified i n oocytes after injection of RNA fractions or water. Specific RNA frac tions were identified that induced saturable uptake of lysine (K-t = 5 2 mu M) and leucine (K-t = 97 mu M), whereas endogenous oocyte uptake was not saturable. Induced uptake of .05 mM lysine by was inhibited (P < .05) 68.1% by 5 mM and 38.9% by .2 mM L-cystine (cystine). Induced uptake of .05 mM leucine was inhibited (P < .05) 83.1% by 5 mM lysine and 23.2% by .2 mM cystine. Although not significant (P > .05), 5 mM L -glutamate (glutamate) quantitatively stimulated the induced uptake of .05 mM lysine by 18.8% and the induced uptake of .05 mM leucine by 60 %. To identify mRNA species responsible for this b(o,+) transporter-li ke activity, oocytes were co-injected with the RNA fractions and degen erate DNA oligomers complementary (antisense) to the cloned human kidn ey b(o,+) amino acid transporter, or (as a negative control) with a DN A oligomer complementary to the rabbit intestinal Na+/glucose cotransp orter, or with water. Only those oocytes injected with two specific RN A fractions and the antisense DNA oligomer complementary to the b(o,+) transporter displayed reduced (P < .05) uptake of lysine (45.7, 55.4% ) and leucine (44.1, 65.9%). These results indicate that messenger RNA encoding for a protein capable of stimulating the competitive absorpt ion of lysine and leucine is expressed by the jejunal epithelia of gro wing pigs.