[U-13C]glutamate metabolism in rat brain mitochondria reveals malic enzyme activity

13C nuclear magnetic resonance spectroscopy was used to study the activity of malic enzyme in isolated brain mitochondria from rat in the presence of unlabelled malate and [U-13C]glutamate. ADP, inorganic phosphate, malate and [U-13C]glutamate were added to a suspension of oxygenated mitochondria. Typical tricarboxylic acid (TCA) cycle constituents (malate, 2-oxoglutarate and succinate) were labelled from [U-13C]glutamate and detected in the superfusion medium. The labelling patterns in the different atom positions of glutamate revealed entry of both unlabelled and labelled acetyl-CoA into the TCA cycle. Unlabelled acetyl-CoA was derived via pyruvate from exogenously applied malate by the action of mitochondrial malic enzyme, while labelled acetyl-CoA was derived from TCA cycle intermediates, most likely by the action of mitochondrial malic enzyme on malate produced from [U-13C]glutamate. The results demonstrate malic enzyme activity and pyruvate recycling in isolated rat brain mitochondria.