Intestinal transport of amino acids studied with l-valine

Abstract

L-Valine (0.003 m) was transported against a concentration gradient by everted rat intestinal sacs. After 5 min a mucosa-rich fraction had near-maximum valine concentration. There was a 45-min lag before the valine concentration in the serosal medium became equal to that in the tissue water. Placing valine either in the mucosal or serosal medium resulted in the same final ratio of valine between them (8/1, serosal/mucosal after 2 hr). Net transport was inhibited by substrate concentrations greater than 0.01 m, whereas tissue uptake was proportional to the initial substrate concentration through the range 0–0.05 m. The apparent Michaelis constant for valine net transport was 2.89 x 10^–3 m and the maximal velocity of the system was 16.3 µmoles valine/500 mg tissue per hr. Potent inhibitors of tissue uptake and net transport were anaerobiosis, cyanide (0.002 m), p-chloromercuribenzoate (0.0004 m), 2,4-dinitrophenol (0.0002 m) (noncompetitively), and l-leucine (0.005 m) (competitively). Neutral l-amino acids having lipophilic side chains inhibited valine translocation in a predictable manner on the basis of competitive inhibition for an acceptor site on the tissue.