Absorption of glycine and L-alanine by the human jejunum.

Abstract

Glycine and L-alanine absorption from the human jejumum was studied by a "steady-state" perfusion technique. Perfusates contained either glycine or L-alanine, or both, the nonabsorbed indicator, polyethylene glycol, and sufficient NaCl so that osmolality approximated 300 mOsm/kg. Aspirates were obtained continuously by siphonage from a site 15 cm distal to the perfusing site. [alpha]-Amino N content of the aspirates was accounted for by the amounts of perfused amino acids recovered, indicating that the test segment contained little endogenous amino acid. Amino acid absorption was not impaired either by deletion of Na from the perfusates or the substitution of mannitol for NaCl. A sodium compound, presumably NaCl, replaced absorbed amino acid on a mole-tor-mole basis, maintaining the isosmolallty of the lumenal contents. Absorption of either glycine or L-alanine increased as its concentration in the perfusate was raised, but the increase was not directly proportional over the range tested and approached a limiting value. Kinetic constants were calculated by the Linewearer-Burk method. In 2 subjects, apparent Michaelis constants were 75 and 77 m[image]/L for glycine and 34 and 38 m[image]/L for L-alanine. Corresponding apparent maximal velocities, (m[image]/15 min./15-cm gut segment) were 13 and 16 for glycine and 12 and 18 for L-alanine. At equimolar concentrations ranging from 50 to 300 m[image]/L, L-alanine absorption always exceeded that of glycine when the two were perfused together. When varying concentrations of glycine were perfused with L-alanine, 150 m[image]/L, glycine absorption decreased in a manner suggestive of competitive inhibition. These findings in humans are consistent with published data derived from in vitro studies which indicate that glycine and L-alanine are absorbed by active processes and share a rate-limiting step for which L-alanine has greater affinity.