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

Frederick L. Weber, University of Kentucky College of Medicine
Susan B. Deak, University of Kentucky College of Medicine
Roger A. Laine, University of Kentucky College of Medicine


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. © 1979, American Gastroenterological Association. All rights reserved.