Role of Branched‐Chain Aminotransferase Isoenzymes and Gabapentin in Neurotransmitter Metabolism

Abstract

Abstract:Because it is well known that excess branchedchain amino acids (BCAAs) have a profound influence on neurological function, studies were conducted to determine the impact of BCAAs on neuronal and astrocytic metabolism and on trafficking between neurons and astrocytes. The first step in the metabolism of BCAAs is transamination with a-ketoglutarate to form the branched-chain a-keto acids (BCKAs). The brain is unique in that it expresses two separate branched-chain aminotransferase (BOAT) isoenzymes. One is the common peripheral form [mitochondrial (BCATm)], and the other [cytosolic(BCATc)] is unique to cerebral tissue, placenta, and ovaries. Therefore, attempts were made to define the isoenzymes' spatial distribution and whether they might play separate metabolic roles. Studies were conducted on primary rat brain cell cultures enriched in either astroglia or neurons. The data show that over time BCATm becomes the predominant isoenzyme in astrocyte cultures and that BCATc is prominent in early neuronal cultures. The data also show that gabapentin, a structural analogue of leucine with anticonvulsant properties, is a competitive inhibitor of BCATc but that it does not inhibit BCATm. Metabolic studies indicated that BCAAs promote the efflux of glutamine from astrocytes and that gabapentin can replace leucine as an exchange substrate. Studying astrocyte-enriched cultures in the presence of [U-14C]glutamate we found that BOKAs, but not BCAAs, stimulate glutamate transamination to a-ketoglutarate and thus irreversible decarboxylation of glutamate to pyruvate and lactate, thereby promoting glutamate oxidative breakdown. Oxidation of glutamate appeared to be largely dependent on the presence of an a-keto acid acceptor for transamination in astrocyte cultures and independent of astrocytic glutamate dehydrogenase activity. The data are discussed in terms of a putative BCAAJBCKA shuttle, where BOATs and BCAAs provide the amino group for glutamate synthesis from a-ketoglutarate via BCATm in astrocytes and thereby promote glutamine transfer to neurons, whereas BCATc reaminates the amino acids in neurons for another cycle.

Cytosolic Branched Chain Aminotransferase (BCATc) Regulates mTORC1 Signaling and Glycolytic Metabolism in CD4+ T Cells

“…The assay was carried out in a 20-ml scintillation vial, and cells were gassed with a 95% O 2 , 5% CO 2 mixture for 20 s, sealed with a rubber stopper, and incubated in Dubnoff metabolic shaker at 37°C for 120 min. The reaction was stopped by addition of 3% perchloric acid (final concentration); 14 CO 2 production and formation of [ 14 C]KIC were quantified as described previously (7,30). To measure KIC in and outside the cells, at the end of the oxidation assay, T cell suspensions were centrifuged for 10 s at 13,200 ϫ g to separate the supernatant (representing the T cell medium) from the T cells.…”

“…To measure KIC in and outside the cells, at the end of the oxidation assay, T cell suspensions were centrifuged for 10 s at 13,200 ϫ g to separate the supernatant (representing the T cell medium) from the T cells. The T cell pellets were re-suspended in fresh Krebs buffer and KIC production in each was determined as described previously (30).…”

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“…Glutamate is an important metabolic currency that is used as a neurotransmitter as well as a source of energy. Transaminases in brain tissue normally convert leucine to α-ketoglutarate to supply nitrogen to the cerebral glutamate pool (6,7). Elevated aKIC -the ketoacid derivative of leucine -reverses the net direction of nitrogen flow and thus depletes the brain of glutamate (6,7).…”

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“…Another target of gabapentin is the cytosolic isoform of branched-chain aminotransferase (Hutson et al, 1998). However, this potential target is not inhibited by pregabalin (T.-Z.…”