Astrocyte Leucine Metabolism: Significance of Branched-Chain Amino Acid Transamination
Article first published online: 19 NOV 2002
Journal of Neurochemistry
Volume 66, Issue 1, pages 378–385, January 1996
How to Cite
Yudkoff, M., Daikhin, Y., Grunstein, L., Nissim, I., Stern, J., Pleasure, D. and Nissim, I. (1996), Astrocyte Leucine Metabolism: Significance of Branched-Chain Amino Acid Transamination. Journal of Neurochemistry, 66: 378–385. doi: 10.1046/j.1471-4159.1996.66010378.x
- Issue published online: 19 NOV 2002
- Article first published online: 19 NOV 2002
- Received April 11, 1995; final revised manuscript received July 25, 1995; accepted July 25, 1995.
- Mass spectrometry;
- Stable isotopes
Abstract: 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 µM. Virtually all leucine transport was found to be Na+ independent. Astrocytic accumulation of leucine was much greater (3×) in the presence of α-aminooxyacetic acid (5 mM), an inhibitor of transamination reactions, suggesting that the glia 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 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 µM with a Vmax of ∼2.0 nmol/mg of protein/min. The transamination of leucine is bidirectional (leucine + α-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.