Amino acid transport by the small intestine of the rat. The transintestinal transport of tryptophan in relation to the transport of neutral and basic amino acids.

Abstract

The transintestinal transport of tryptophan by everted sacs of the rat smal intestine was studied. It was shown that: 1. 1. The transport of trytophan was enhanced by methionine, leucine and sarcocine; 2. 2. Trytophan was a potent inhibitor of the transport of leucine; 3. 3. The transport of trytophan was inhibited by higher concentrations of methionine; 4. 4. Tryptophan was a potent competitive inhibitor of the transport of lysine; 5. 5. Lysine inhibited the transport of tryptophan when a part of the intestinal capacity for transport of neutral amino acids was occupied by methionine; 6. 6. The effect of increasing the initial mucosal concentrations of tryptophan or lysine, keeping the initial serosal fluids free of amino acids, was to saturate the capacity for transport of these amino acids; 7. 7. The net transport of tryptophan and lysine from the mucosal to the serosal fluids was reduced if these amino acids were initially present in the serosal fluids. It is concluded that: 8. (i) tryptophan is a substrate for the carrier of the diamino acids as well as for the carrier of the neutral amino acids; 9. (ii) The enhancement by methionine, leucine and sarcosine of the transintestinal tryptophan transport represents a counterflow effect of these amino acids on the transport of tryptophan by diamino acid carrier; 10. (iii) The reduction caused to the transport of tryptophan and lysine by their presence in the initial serosal fluids may be explained as a kinetic consequence of the transport of these amino acids by a mobile carrier.