Characterization of glutamine transport into resting and concanavalin A‐stimulated peripheral human lymphocytes

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Abstract
Characteristics of glutamine transport, its substrate specificity, and its pattern of competitive and non‐competitive inhibition in response to amino acid analogues were determined in peripheral human lymphocytes, incubated with or without concanavalin A (Con A). Maximum capacity of transport (Vmax) at 37°C and 136.9 mM Na+ was 30 pmol/106 cells/30 seconds, while the apparent Km was 142 μM. In cells exposed to 10 mM histidine, asparagine, serine, or leucine transport of glutamine declined to 28%, 15%, 17%, and 21%, respectively, of the rates in controls. Inhibition by histidine (Ki = 0.58 mM) and serine (Ki = 0.25 mM) was competitive, by leucine was non‐competitive (Ki = 0.64), while α‐methylamino‐isobutyric acid and 2‐amino carboxy‐bicyclo (2.2.1)‐heptane had no effect. In cells cultured for 24 hours with or without 10 μg/ml Con A, the apparent Km was 70 μM vs. 89 μM and Vmax 73 vs. 26 pmol/106 cells/30 seconds. Sodium depletion (9.0 mM NaCl) greatly diminished glutamine transport in resting and stimulated cells. Inhibition of glutamine transport by serine was sodium sensitive, while inhibition by histdine and asparagine was not. Serine had no competitive effect in sodium‐depleted media. The data demonstrate what appear to be two carrier systems for glutamine, sodium sensitive and sodium insensitive. It is suggested that glutamine transport into lymphocytes occurs via processes similar to System N and System ASC described in other cells, with System ASC as the sodium‐sensitive component. Con A augments the capacity rather than the affinity of glutamine transporting systems.

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