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

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Abstract
The uptake of .alpha.-ketoglutarate and malate by rat brain synaptosomal preparations was affected by a variety of substances at physiologically relevant concentrations. Glutamine altered the uptake of .alpha.-ketoglutarate by causing an apparent reduction in the substrate-carrier affinity and an increase in Vmax. Glutamine did not appear to affect the Vmax of malate uptake, but it did increase markedly the uptake velocity at low concentrations of malate. L-Glutamate and L-aspartate were comparatively strong inhibitors of .alpha.-ketoglutarate and malate uptake. N-Acetylaspartate was a weak inhibitor of .alpha.-ketoglutarate uptake, a finding that contrasts with previous observation that this compound potently inhibited .alpha.-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 .alpha.-ketoglutarate. The addition of small amounts of postmicrosomal supernatant to the incubation medium enhanced the uptake of .alpha.-ketoglutarate by low-density synaptosomes. By comparison, the uptake of glutamate, glutamine, GABA and several other amino acids was not affected. The enhancement of .alpha.-ketoglutarate uptake by the supernatant was due to a heat labile substance that was retained by dialysis tubing (MW cutoff = 8000) 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] .alpha.-ketoglutarate to glutamate, aspartate, glutamine and GABA was determined, the presence of glutamine and glutamate in the incubation medium did not affect the pattern of labeling appreciably.