Differences of affinity and energetics of the active transport systems for amino acids in Chang liver cells

The plasma membrane of Chang liver cells was shown to have at least two distinct active transport systems, one with preferential affinity for glycine and one for leucine. The uptakes of glycine and leucine were specificially inhibited by Me-AIB and b-BCH, respectively. The uptake of glycine decreased remarkably within 10 min on incubation with DNP (2 mM), KCN (5 mM), and malonate (20 mM) under aerobic conditions, along with a decrease of cellular ATP concentration to as low as 1/4 of normal, while the uptake of leucine was not depressed under these conditions. Leucine uptake was, however, greatly reduced within 10 min on incubation with DNP plus ICH2CONH2 (5 mM), when the cellular ATP was estimated at about 0.066 mM. The active transport of leucine, but not that of glycine, was accompanied by further acidification of the intracellular fluid, which was lower in pH than the extracellular fluid by approximately 0.3 unit without addition of amino acid to the medium.