Abstract— NADP‐dependent isocitrate dehydrogenase is localized in both external and internal membranes and in the mitochondrial matrix from brains of adult and newborn animals. Electrophoretic studies indicate that the mitochondrial enzyme occurs in the form of two isoenzymes; the isoenzyme of the external membranes behaves similarly to that of the cytoplasm. Ischaemia in adult animals results in a decrease in enzyme specific activity, particularly in external membranes, while in the mitochondrial fraction of the newborn the activity of the enzyme increased.

The cytoplasmic fraction derived from rat brain was shown to possess the ability to oxidize citrate in the presence of NADP, but not in that of NAD. The rate of citrate oxidation is limited by the rate of the aconitase-catalysed isomerization. The dependence of the reaction rate on the protein, citrate and NADP concentrations, and on reaction time, was determined. It was found that 2-oxoglutarate and NADPH, both formed in the citrate oxidation reaction, do not appear in amounts equivalent to the losses of citrate. The possible ways of utilization of the above products in the rat brain cytoplasmic fraction are discussed.It is suggested that the oxidation of citrate in rat brain cytoplasm may proceed-among other metabolic routes-through its conversion into isocitrate (aconitase) and then 2-oxoglutarate (NADP dehydrogenase) and oxaloacetate (aspartate aminotransferase). In addition it has been shown that hypoxia may markedly effect the activity of the cytoplasmic citrateoxidizing system.

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