Glutamate pays its own way in astrocytes

Abstract

In vitro and in vivo studies have shown that glutamate can be oxidized for energy by brain astrocytes. The ability to harvest the energy from glutamate provides astrocytes with a mechanism to offset the high ATP cost of the uptake of glutamate from the synaptic cleft. This brief review focuses on oxidative metabolism of glutamate by astrocytes, the specific pathways involved in the complete oxidation of glutamate and the energy provided by each reaction.

Keywords: astrocytes; glutamate; glutamate dehydrogenase; oxidative metabolism; pyruvate recycling pathway.

Figure 1A

Schematic diagram indicating the pathways of metabolism of exogenous glutamate taken up by astrocytes. The portion of the figure outlined with a dashed line shows transport of glutamate from the extracellular milieu. The oval depicts the astrocytic glutamate transporter GLT1/EAAT2 or GLAST; rectangle depicts Na⁺, K⁺-ATPase. Astrocytic glutamate can be converted to α-ketoglutarate by the energy producing enzyme glutamate dehydrogenase or via one of the transaminase enzymes (primarily via aspartate aminotransferase but also by BCAT and ALAT). α-Ketoglutarate formed from glutamate is metabolized via a partial TCA cycle to either malate or oxaloacetate. For the complete oxidation of glutamate to occur malate (or OAA) must leave the TCA cycle, be converted to pyruvate by malic enzyme, and then converted to acetyl CoA by pyruvate dehydrogenase. The acetyl CoA formed by these pyruvate recycling reactions re-enters the TCA cycle and is subsequently oxidized for energy. The dotted in the TCA cycle indicates that the NADH generated from the isocitrate dehydrogenase reaction would only be formed when the acetyl CoA formed from the pyruvate recycling pathway is metabolized via the TCA cycle because the initial entry of carbons α-KG at is subsequent to this step. The complete oxidation of one glutamate molecule can yield 24–27 ATP depending on whether the initial step in metabolism is via GDH or AAT. The ATP is estimated at ∼20 since the maximum theoretical yield of ATP is never recovered due to mitochondrial proton leak. [See Table 1 for reaction details and (B) for labeling pattern of metabolites labeled from the initial metabolism of [U-¹³C]glutamate and the labeling pattern from subsequent metabolism via the pyruvate recycling pathway and re-entry into the TCA cycle]. Abbreviations: Glu, glutamate; α-KG, α-ketoglutarate OAA, oxaloacetate; Asp, aspartate; GDH, glutamate dehydrogenase; AAT aspartate aminotransferase; BCAT, branched-chain aminotransferase; ALAT, alanine aminotransferase; KGDH, α-ketoglutarate dehydrogenase; ME, malic enzyme; PDH, pyruvate dehydrogenase; GS, glutamine synthetase. BCAA, branched chain amino acid; BCKA, branched chain ketoacid.

Figure 1B

Labeling pattern from the initial metabolism of [U-¹³C]glutamate and the labeling pattern from the complete oxidation of these glutamate carbons via the pyruvate recycling pathway and re-entry into the TCA cycle. Labeling from the initial entry of [U-¹³C]glutamate into the TCA cycle is shown inside the dotted line. Note that metabolites generated are also uniformly labeled in all carbons. [Note that any glutamine formed in the cytosol directly from [U-¹³C]glutamate would also be uniformly labeled [U-¹³C]glutamine; reaction not shown in this figure]. [U-¹³C]malate or OAA leaving the TCA cycle would be converted to [U-¹³C]pyruvate by malic enzyme or the combined action of PEPCK and pyruvate kinase, and would also give rise to [U-¹³C]lactate. The [U-¹³C]pyruvate can be converted to [1,2-¹³C]acetyl CoA which enters the TCA cycle for further oxidation. Any citrate formed from the condensation of the [1,2-¹³C]acetyl CoA with unlabeled oxaloacetate in the cycle would give rise to [4,5-¹³C]glutamate and glutamine, and also to both [1,2-¹³C]aspartate and [3,4-¹³C]aspartate. These partially labeled glutamate, glutamine, and aspartate molecules can be readily distinguished from the [U-¹³C]glutamate, glutamine, and aspartate by ¹³C-NMR spectroscopy. It is very likely that studies of pyruvate recycling from [U-¹³C]glutamate underestimate the amount of recycling taking place since any citrate formed from the condensation of [1,2-¹³C]acetyl CoA with [U-¹³C]OAA formed from the initial entry and metabolism of the glutamate into the TCA cycle give rise to [U-¹³C]glutamate which can not be distinguished from the precursor. Abbreviations: Glu, glutamate; Gln, glutamine; OAA, oxaloacetate; Asp, aspartate; Lac, lactate; Pyr, pyruvate; ME, malic enzyme; PEPCK, phosphoenolpyruvate carboxykinase.