[U-C-13]GLUTAMATE METABOLISM IN RAT-BRAIN MITOCHONDRIA REVEALS MALIC ENZYME-ACTIVITY

C-13 nuclear magnetic resonance spectroscopy was used to study the act ivity of malic enzyme in isolated brain mitochondria from rat in the p resence of unlabelled malate and [U-C-13]glutamate. ADP, inorganic pho sphate, malate and [U-C-13]glutamate were added to a suspension of oxy genated mitochondria. Typical tricarboxylic acid (TCA) cycle constitue nts (malate, 2-oxoglutarate and succinate) were labelled from [U-C-13] glutamate and detected in the superfusion medium. The labelling patter ns in the different atom positions of glutamate revealed entry of both unlabelled and labelled acetyl-CoA into the TCA cycle. Unlabelled ace tyl-CoA was derived via pyruvate from exogenously applied malate by th e action of mitochondrial malic enzyme, while labelled acetyl-CoA was derived from TCA cycle intermediates, most likely by the action of mit ochondrial malic enzyme on malate produced from [U-C-13]glutamate. The results demonstrate malic enzyme activity and pyruvate recycling in i solated rat brain mitochondria.