Glutamine, Glutamate, and Other Possible Regulators of α-Ketoglutarate and Malate Uptake by Synaptic Terminals

Abstract: The uptake of a-ketoglutarate and malate by rat brain synaptosomal preparations was found to be affected by a variety of substances at physiologically relevant concentrations. Glutamine altered the uptake of γ-ketoglutarate by causing an apparent reduction in the substrate-carrier affinity and an increase in V_max. In contrast, glutamine did not appear to affect the V_max of malate uptake, but it did increase markedly the uptake velocity at low concentrations of malate. L-Glutamate and L-as-partate were comparatively strong inhibitors of γ-keto-glutarate and malate uptake. N-Acetylaspartate was a weak inhibitor of γ-ketoglutarate uptake, a finding that contrasts with our previous observation that this compound potently inhibited γ-ketoglutarate uptake by synaptosomes obtained from the cerebellum of 8- to 14-day-old mice. Ca²⁺ exhibited a variable effect but usually enhanced the uptake of γ-ketoglutarate. The addition of small amounts of postmicrosomal supernatant to the incubation medium enhanced the uptake of γ-ketoglutarate by low-density synaptosomes. By comparison, the uptake of glutamate, glutamine, γ-aminobutyric acid, and several other amino acids was not affected. The enhancement of γ-ketoglutarate uptake by the supernatant was due to a heat labile substance that was retained by dialysis tubing (MW cutoff = 8,000) and Amicon filter cones (CF 25), and was precipitated by ammonium sulfate at 60% saturation. In experiments in which the metabolic conversion of [U-^-14C] γ-ketoglutarate to glutamate, as-partate, glutamine, and aminobutyric acid was determined, the presence of glutamine and glutamate in the incubation medium did not affect the pattern of labelling appreciably.