Aspartate aminotransferase isotope exchange reactions: implications for glutamate/glutamine shuttle hypothesis

Abstract

Aspartate aminotransferase (AAT) catalyzes amino group transfer from glutamate (Glu) or aspartate (Asp) to a keto acid acceptor-oxaloacetate (OA) or alpha-ketoglutarate (KG), respectively. Data presented here show that AAT catalyzes two partial reactions resulting in isotope exchange between 3H-labeled Glu or 3H-labeled Asp and the cognate keto acid in the absence of the keto acid acceptor required for the net reaction. Tritiated keto acid product was detected by release of 3H2O from C-3 during base-induced enolization. Tritium released directly from C-2 (or C-3) by the enzyme was also evaluated and is a small fraction of that released because of exchange to the keto acid pool. Exchange is dependent on AAT concentration, time-dependent, proportional to the amino-to-keto acid ratio, and blocked by aminooxyacetate (AOA), an AAT inhibitor. Enzymatic conversion of [3H]KG to Glu by glutamic dehydrogenase (GDH) or of [3H]OA to malate by malic dehydrogenase (MDH) "protects" the label from release by base, showing that base-induced isotope release is from keto acid rather than a result of release during the exchange process. AAT isotope exchange is discussed in the context of the glutamate/glutamine shuttle hypothesis for astrocyte/neuron carbon cycling.

Regulation of glial metabolism studied by 13C‐NMR

“…120 While the mitochondrial isozyme is expressed in neurons and synaptic terminals 121,122 it is widely accepted that cME is preferentially, if not exclusively, a glial enzyme. 118,120,200 Although cME is reversible, more than 95% of the malate present is converted to pyruvate at physiological concentrations of malate and pyruvate, 201 which is consistent with the cytosolic isozyme accounting for as much as 95% of total ME activity in astrocytes. 121 Nevertheless, the presence of neuronal mME in vivo 64 suggests the potential operation of the pyruvate recycling pathways also in neurons.…”

Remodeling of Hepatic Metabolism and Hyperaminoacidemia in Mice Deficient in Proglucagon-Derived Peptides

“…AGXT is also involved in the metabolism of pyruvate and serine (15). GOT1 is involved in the malate-aspartate shuttle and thereby in the production of oxaloacetate, which serve as substrates for gluconeogenesis via a PEPCK-mediated reaction (16). SDS , AGXT , and GOT1 mRNA were expressed at significantly lower levels in the GCGKO livers than in the control livers, especially when the mice were fed ad libitum.…”

Vaccination with autoantigen protects against aggregated β‐amyloid and glutamate toxicity by controlling microglia: effect of CD4+CD25+ T cells

“…as a result of synaptic activity, it is restored to normal by local mechanisms [25]. These include transporters that help to remove excessive glutamate, and enzymes that convert it to glutamine [26,27]. Glutamate is normally buffered via its active transport from the extracellular milieu, an activity carried out mainly by astrocytes [25] but also by neurons and microglia [28].…”