Cerebral aspartate utilization: Near-equilibrium relationships in aspartate aminotransferase reaction

Maria Erecińska, David E Pleasure, David Nelson, Itzhak Nissim, Marc Yudkoff

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Abstract

The pathways of nitrogen transfer from 50 μM [15N]aspartate were studied in rat brain synaptosomes and cultured primary rat astrocytes by using gas chromatography-mass spectrometry technique. Aspartate was taken up rapidly by both preparations, but the rates of transport were faster in astrocytes than in synaptosomes. In synaptosomes, 15N was incorporated predominantly into glutamate, whereas in glial cells, glutamine and other 15N-amino acids were also produced. In both preparations, the initial rate of N transfer from aspartate to glutamate was within a factor of 2-3 of that in the opposite direction. The rates of transamination were greater in synaptosomes than in astrocytes. Omission of glucose increased the formation of [15N]-glutamate in synaptosomes, but not in astrocytes. Rotenone substantially decreased the rate of transamination. There was no detectable incorporation of 15N from labeled aspartate to 6-amino-15N-labeled adenine nucleotides during 60-min incubation of synaptosomes under a variety of conditions; however, such activity could be demonstrated in glial cells. The formation of 15N-labeled adenine nucleotides was marginally increased by the presence of 1 mM aminooxyacetate, but was unaffected by pretreatment with 1 mM 5-amino-4-imidazolecarboxamide ribose. It is concluded that (1) aspartate aminotransferase is near equilibrium in both synaptosomes and astrocytes under cellular conditions, but the rates of transamination are faster in the nerve endings; (2) in the absence of glucose, use of amino acids for the purpose of energy production increases in synaptosomes, but may not do so in glial cells because the latter possess larger glycogen stores; and (3) nerve endings have a very limited capacity for salvage of the adenine nucleotides via the purine nucleotide cycle.

Original languageEnglish (US)
Pages (from-to)1696-1706
Number of pages11
JournalJournal of Neurochemistry
Volume60
Issue number5
StatePublished - May 1993
Externally publishedYes

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Keywords

  • Aspartate aminotransferase
  • Aspartate transport
  • Astrocyte
  • Gas chromatography
  • Mass spectrometry
  • Purine nucleotide cycle
  • Synaptosome

ASJC Scopus subject areas

  • Biochemistry
  • Cellular and Molecular Neuroscience

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