The subcellular localizations of γ-aminobutyrate transaminase (EC and glutamate dehydrogenase (EC in brain tissue of adult rats were compared with each other and with those of NAD+-isocitrate dehydrogenase (EC 1.1.41) and monoamine oxidase (EC; kynuramine as substrate). Crude mitochondrial fractions from brain tissue were centrifuged in continuous sucrose density gradients. γ-Aminobutyrate transaminase and glutamate dehydrogenase were always found at a higher density than NAD+-isocitrate dehydrogenase and monoamine oxidase. When centrifuged for 1 h at 53 000gav., there was a slight difference between the distribution profiles of glutamate dehydrogenase and γ-aminobutyrate transaminase. This difference was larger when the centrifugation time was only 15 min. It is concluded that there are subpopulations of brain mitochondria with differing proportions of γ-aminobutyrate transaminase and glutamate dehydrogenase. The results are discussed in relation to evidence obtained with labelled precursors in vivo that there are at least two small glutamate compartments in adult brain.

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