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 u ptake of leucine into glia was rapid, with a V-max of 53.6 +/- 3.2 nmo l/mg of protein/min and a k(m) of 449.2 +/- 94.9 mu M. Virtually all l eucine transport was found to be Na+ independent, Astrocytic accumulat ion of leucine was much greater (3x) in the presence of alpha-aminooxy acetic acid (5 mM), an inhibitor of transamination reactions, suggesti ng 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 t he medium when astrocytes were incubated with leucine. Approximately 7 0% of the leucine that the glia cleared from the medium was released a s the keto acid. The apparent K-m for leucine conversion to extracellu lar KIC was a medium [leucine] of 58 mu M with a V-max of similar to 2 .0 nmol/mg of protein/min. The transamination of leucine is bidirectio nal (leucine + alpha-ketoglutarate <-> KIC + glutamate) in astrocytes, but flux from leucine --> glutamate is more active than that from glu tamale --> 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 ''buff ering'' of glutamate in neurons, which would consume this neurotransmi tter in the course of reaminating KIC to leucine.