Aspartate aminotransferase isotope exchange reactions: implications for glutamate/glutamine shuttle hypothesis.
Aspartate aminotransferase (AAT) catalyzes amino group transfer from glutamate (Glu) or aspartate (Asp) to a keto acid acceptor-oxaloacetate (OA) or alpha-ketoglutarate (KG), respectively. Data presented here show that AAT catalyzes two partial reactions resulting in isotope exchange between 3H-labeled Glu or 3H-labeled Asp and the cognate keto acid in the absence of the keto acid acceptor required for the net reaction. Tritiated keto acid product was detected by release of 3H2O from C-3 during base-induced enolization. Tritium released directly from C-2 (or C-3) by the enzyme was also evaluated and is a small fraction of that released because of exchange to the keto acid pool. Exchange is dependent on AAT concentration, time-dependent, proportional to the amino-to-keto acid ratio, and blocked by aminooxyacetate (AOA), an AAT inhibitor. Enzymatic conversion of [3H]KG to Glu by glutamic dehydrogenase (GDH) or of [3H]OA to malate by malic dehydrogenase (MDH) "protects" the label from release by base, showing that base-induced isotope release is from keto acid rather than a result of release during the exchange process. AAT isotope exchange is discussed in the context of the glutamate/glutamine shuttle hypothesis for astrocyte/neuron carbon cycling.
Kimmich
George A
GA
Department of Biochemistry and Biophysics, School of Medicine and Dentistry, University of Rochester, Rochester, New York 14642, USA. george_kimmich@urmc.rochester.edu
Roussie
James A
JA
Randles
Joan
J
D000468:Alkalies / Q000737:chemistry; D001219:Aspartate Aminotransferases / Q000737:chemistry*; D001224:Aspartic Acid / Q000737:chemistry; D002384:Catalysis; D002952:Citric Acid Cycle / Q000502:physiology; D018698:Glutamic Acid / Q000737:chemistry* / Q000378:metabolism*; D005973:Glutamine / Q000737:chemistry / Q000378:metabolism*; D007656:Ketoglutaric Acids / Q000737:chemistry; D008291:Malate Dehydrogenase / Q000378:metabolism; D062907:Oxaloacetic Acid / Q000737:chemistry; D014316:Tritium / Q000737:chemistry
D000468:Alkalies; D007656:Ketoglutaric Acids; D005973:Glutamine; D014316:Tritium; D062907:Oxaloacetic Acid; D001224:Aspartic Acid; D018698:Glutamic Acid; D008291:Malate Dehydrogenase; D001219:Aspartate Aminotransferases