CEREBRAL ASPARTATE UTILIZATION - NEAR-EQUILIBRIUM RELATIONSHIPS IN ASPARTATE-AMINOTRANSFERASE REACTION

The pathways of nitrogen transfer from 50 muM [N-15]aspartate were stu died in rat brain synaptosomes and cultured primary rat astrocytes by using gas chromatography-mass spectrometry technique. Aspartate was ta ken up rapidly by both preparations, but the rates of transport were f aster in astrocytes than in synaptosomes. In synaptosomes, N-15 was in corporated predominantly into glutamate, whereas in glial cells, gluta mine and other N-15-amino acids were also produced. In both preparatio ns, the initial rate of N transfer from aspartate to glutamate was wit hin a factor of 2-3 of that in the opposite direction. The rates of tr ansamination were greater in synaptosomes than in astrocytes. Omission of glucose increased the formation of [N-15]-glutamate in synaptosome s, but not in astrocytes. Rotenone substantially decreased the rate of transamination. There was no detectable incorporation of N-15 from la beled aspartate to 6-amino-N-15-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 N-15-labeled adenine nucleotides was marginally increased by the prese nce of 1 mM aminooxyactate, but was unaffected by pretreatment with 1 mM 5-amino-4-imidazolecarboxamide ribose. It is concluded that (1) asp artate aminotransferase is near equilibrium in both synaptosomes and a strocytes under cellular conditions, but the rates of transamination a re faster in the nerve endings; (2) in the absence of glucose, use of amino acids for the purpose of energy production increases in synaptos omes, but may not do so in glial cells because the latter possess larg er glycogen stores; and (3) nerve endings have a very limited capacity for salvage of the adenine nucleotides via the purine nucleotide cycl e.