Interrelationships of Leucine and Glutamate Metabolism in Cultured Astrocytes

Authors

  • Marc Yudkoff,

    Corresponding author
    1. Division of Metabolism, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • Yevgeny Daikhin,

    1. Division of Metabolism, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • Zhi-Ping Lin,

    1. Division of Metabolism, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • Liana Nissim,

    1. Division of Metabolism, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • Janet Stern,

    1. Division of Experimental Neurology, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • David Pleasure,

    1. Division of Experimental Neurology, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author
  • Itzhak Nissim

    1. Division of Metabolism, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, U.S.A.
    Search for more papers by this author

Address correspondence to Dr. M. Yudkoff at Division of Metabolism, Children's Hospital of Philadelphia, 1 Children's Center, Philadelphia, PA 19104, U.S.A.

Abstract

Abstract: The aim was to study the extent to which leu-cine furnishes α-NH2 groups for glutamate synthesis via branched-chain amino acid aminotransferase. The transfer of N from leucine to glutamate was determined by incubating astrocytes in a medium containing [15N]leucine and 15 unlabeled amino acids; isotopic abundance was measured with gas chromatography-mass spectrometry. The ratio of labeling in both [15N]glutamate/[15N]leucine and [2-15N]glutamine/[15N]leucine suggested that at least one-fifth of all glutamate N had been derived from leucine nitrogen. At the same time, enrichment in [15N]leucine declined, reflecting dilution of the 16N label by the unlabeled amino acids that were in the medium. Isotopic abundance in [16N]-isoleucine increased very quickly, suggesting the rapidity of transamination between these amino acids. The appearance of 15N in valine was more gradual. Measurement of branched-chain amino acid transaminase showed that the reaction from leucine to glutamate was approximately six times more active than from glutamate to leucine (8.72 vs. 1.46 nmol/min/mg of protein). However, when the medium was supplemented with α-ketoisocaproate (1 mM), the ketoacid of leucine, the reaction readily ran in the “reverse” direction and intraastrocytic [glutamate] was reduced by ∼50% in only 5 min. Extracellular concentrations of α-ketoisocaproate as low as 0.05 mM significantly lowered intracellular [glutamate]. The relative efficiency of branched-chain amino acid transamination was studied by incubating astrocytes with 15 unlabeled amino acids (0.1 mM each) and [15N]glutamate. After 45 min, the most highly labeled amino acid was [15N]alanine, which was closely followed by [15N]leucine and [15N]isoleucine. Relatively little 15N was detected in any other amino acids, except for [15N]serine. The transamination of leucine was ∼17 times greater than the rate of [1-14C]leucine oxidation. These data indicate that leucine is a major source of glutamate nitrogen. Conversely, reamination of a-ketoisocaproate, the ketoacid of leucine, affords a mechanism for the temporary “buffering” of intracellular glutamate.

Ancillary