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 glia was rapid, with a Vmax of 53.6 +/- 3.2 nmol/mg of protein/min and a Km of 449.2 +/- 94.9 microM. Virtually all leucine transport was found to be Na+ independent. Astrocytic accumulation of leucine was much greater (3x) in the presence of alpha-aminooxyacetic acid (5 mM), an inhibitor of transamination reactions, suggesting that the glia rapidly transaminate leucine to alpha-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 glia cleared from the medium was released as the keto acid. The apparent Km for leucine conversion to extracellular KIC was a medium [leucine] of 58 microM with a Vmax of approximately 2.0 nmol/mg of protein/min. The transamination of leucine is bidirectional (leucine+alpha-ketoglutarate<==>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 glia 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.