The pathway of glutamate metabolism in rat brain mitochondria

Abstract

The pathway of glutamate metabolism in non-synaptic rat brain mitochondria was investigated by measuring glutamate, aspartate and NH3 concentrations and O2 uptakes in mitochondria metabolizing glutamate or glutamine under various conditions. Brain mitochondria metabolizing 10 mM glutamate in the absence of malate produced aspartate at 15 nmol/min per mg of protein, but no detectable NH3. If amino-oxyacetate were added, the aspartate production was decreased by 80% and NH3 production was observed at a rate of 6.3 nmol/min per mg of protein. Brain mitochondria metabolizing glutamate at various concentrations (0-10 mM) in the presence of 2.5 mM malate produced aspartate at rates almost stoichiometric with glutamate disappearance, with no detectable NH3 production. In the presence of amino-oxyacetate, although the rate of aspartate production was decreased by 75%, NH3 production was only just detectable (0.3 nmol/min per mg of protein). Brain mitochondria metabolizing 10 mM glutamine and 2.5 mM malate in states 3 and 4 were studied by using glutamine as a source of intramitochondrial glutamate without the involvement of mitochondrial translocases. The NH3 production due to the oxidative deamination of glutamate produced from the glutamine was estimated as 1 nmol/min per mg of protein in state 3 and 3 nmol/min per mg of protein in state 4. Brain mitochondria metabolizing 10 mM glutamine in the presence of 1 mM amino-oxyacetate under state 3 conditions in the presence or absence of 2.5 mM malate showed no detectable aspartate production. In both cases, however, over the first 5 min, NH3 production from the oxidative deamination of glutamate was 21-27 nmol/min per mg of protein, but then decreased to approximately 1-1.5 nmol/min per mg. The oxidative deamination of glutamate by glutamate dehydrogenase is apparently not a major route of metabolism of glutamate from either exogenous or endogenous (glutamine) sources in rat brain mitochondria.