Astrocyte leucine metabolism: Significance of branched-chain amino acid transamination

We studied astrocytic metabolism of leucine, which in brain is a major donor of nitrogen for the synthesis of glutamate and glutamine. The uptake of leucine into gila was rapid, with a V(max) of 53.6 ± 3.2 nmol/mg of protein/min and a K(m) of 449.2 ± 94.9 μM. Virtually all leucine transport was found to be Na⁺ independent. Astrocytic accumulation of leucine was much greater (3X) in the presence of α-aminooxyacetic acid (5 mM), an inhibitor of transamination reactions, suggesting that the gila rapidly transaminate leucine to α-ketoisocaproic acid (KIC), which they then release into the extracellular fluid. This inference was confirmed by the direct measurement of KIC release to the medium when astrocytes were incubated with leucine. Approximately 70% of the leucine that the gila cleared from the medium was released as the keto acid. The apparent K(m) for leucine conversion to extracellular KIC was a medium [leucine] of 58 μM with a V(max) of ~2.0 nmol/mg of protein/min. The transamination of leucine is bidirectional (leucine + α-ketoglutarate mutually implies KIC + glutamate) in astrocytes, but flux from leucine → glutamate is more active than that from glutamate → leucine. These data underscore the significance of leucine handling to overall brain nitrogen metabolism. The release of KIC from gila to the extracellular fluid may afford a mechanism for the 'buffering' of glutamate in neurons, which would consume this neurotransmitter in the course of reaminating KIC to leucine.