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Valproate Increases Glutaminase and Decreases Glutamine Synthetase Activities in Primary Cultures of Rat Brain Astrocytes
Author(s) -
Collins Roger M.,
Zielke H. Ronald,
Woody Robert C.
Publication year - 1994
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1994.62031137.x
Subject(s) - glutaminase , glutamine synthetase , glutamine , glutamate receptor , hyperammonemia , medicine , metabolism , enzyme , endocrinology , biochemistry , chemistry , biology , amino acid , receptor
It has been proposed that hyperammonemia may be associated with valproate therapy. As astrocytes are the primary site of ammonia detoxification in brain, the effects of valproate on glutamate and glutamine metabolism in astrocytes were studied. It is well established that, because of compartmentation of glutamine synthetase, astrocytes are the site of synthesis of glutamine from glutamate and ammonia. The reverse reaction is catalyzed by the ubiquitous enzyme glutaminase, which is present in both neurons and astrocytes. In astrocytes exposed to 1.2 m M valproate, glutaminase activity increased 80% by day 2 and remained elevated at day 4; glutamine synthetase activity was decreased 30%. Direct addition of valproate to assay tubes with enzyme extracts from untreated astrocytes had significant effects only at concentrations of 10 and 20 m M , When astrocytes were exposed for 4 days to 0.3, 0.6, or 1.2 m M valproate and subsequently incubated with l ‐[U‐ 14 C]glutamate, label incorporation into [ 14 C]glutamine was decreased by 11, 25, and 48%, respectively, and is consistent with a reduction in glutamine synthetase activity. Label incorporation from l ‐[U‐ 14 C]glutamate into [ 14 C]aspartate also decreased with increasing concentrations of valproate. Following a 4‐day exposure to 0.6 m M valproate, the glutamine levels increased 40% and the glutamate levels 100%. These effects were not directly proportional to valproate concentration, because exposure to 1.2 m M valproate resulted in a 15% decrease in glutamine levels and a 25% increase in glutamate levels compared with control cultures. Intracellular aspartate was inversely proportional to all concentrations of extracellular valproate, decreasing 60% with exposure to 1.2 m M valproate. These results indicate that valproate increases glutaminase activity, decreases glutamine synthetase activity, and alters Krebs‐cycle activity in astrocytes, suggesting a possible mechanism for hyperammonemia in brain during valproate therapy.