Absorption of Keto-Analogues of Branched-Chain Amino Acids from Rat Small Intestine

Abstract

The keto-analogues of essential amino acids provide a source of essential amino acids that contain no nitrogen, but little is known about the mechanism of keto-analogue intestinal absorption. Absorption of the keto-analogue of valine was compared with that of the corresponding amino acid valine by perfusing rat jejunum and ileum in vivo. In both ileum and jejunum, keto-valine was absorbed at rates only moderately below those of the amino acid valine when perfused at 1–50 mM. In the jejunum, absorption rates for keto-valine relative to valine ranged from 73% to 39% at perfusate concentrations of 1 mM and 25 mM, respectively. Similar results were obtained in the ileum. The absorption curve of keto-valine was nonlinear at low perfusate concentrations and linear at higher concentrations, consistent with both saturable transport and passive diffusion. In further studies, 3 mM keto-valine was perfused into jejunal segments. Twenty-five millimolar of the keto-analogues of isoleucine and phenylalanine inhibited keto-valine absorption by 43% and 50%, respectively, whereas 3 mM of the keto analogue of isoleucine did not. Sixty-seven millimolar valine and 56 mM glucose significantly increased keto-valine absorption by 30% and 57% respectively; there were corresponding fourfold and eightfold increments in water transport. The keto analogues of leucine and isoleucine were absorbed at the same rates as keto-valine when perfused at 3 mM and 25 mM. Arterial and mesentric venous blood was sampled after perfusing 25 mM of keto-leucine and keto-isoleucine. There was a significant release of the corresponding amino acids into mesentric venous blood compared with controls, which suggest partial transamination during absorption. The data indicate: (a) Keto-analogues of the branched-chain amino acids are absorbed at rates moderately below those of the corresponding amino acids; (b) keto-valine absorption is in part mediated throught a saturable mechanism; (c) keto-valine absorption is influenced by solvent drag; and (d) partial transamination occurs during keto-analogue absorption in the rat jejunum.