Submitted to: Journal of Neurochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/29/2003
Publication Date: 2/1/2004
Citation: Sakai, R., Cohen, D.M., Henry, J.F., Burrin, D.G., Reeds, P.J. 2004. Leucine-nitrogen metabolism in the brain of conscious rats: its role as a nitrogen carrier in glutamate synthesis in glial and neuronal metabolic compartments. Journal of Neurochemistry. 88(3):612-622. Interpretive Summary: The network of biochemical reactions in the brain serves many functions, including the production of energy for electrical discharges and the synthesis of chemicals needed to sustain the chemical reactions themselves. We studied the pathways by which certain chemicals are synthesized in the brain of the rat by providing the rat's food that contained stable isotopes. The trace amount of nonradioactive isotopes allowed us to analyze the products of the chemical reactions in the brain of the unstressed rat. These studies provided information about the synthesis of the glutamate and glutamine in the neurons and glia (the two principal cell types in the brain), including the contributions of amino acids derived from plasma.
Technical Abstract: The source of nitrogen (N) for the de novo synthesis of brain glutamate, glutamine, and gamma-aminobutyrate (GABA)remains controversial. Because leucine is readily transported into the brain and the brain contains high activities of branched-chain aminotransferase, we hypothesized that leucine is the predominant N-precursor for brain glutamate synthesis. Conscious and unstressed rats administered with [U13C] and/or [15N]leucine as additions to the diet were killed at 0 to 9 h of continuous feeding. Plasma and brain leucine equilibrated rapidly and the brain leucine-N turnover was more than 100% per min. The isotopic dilution of [U13C]leucine (brain/plasma ratio 0.61±0.06) and [15N]leucine (0.23±0.06) differed markedly, suggesting that 15% of cerebral leucine-N turnover derives from proteolysis and 62% from leucine synthesis via reverse transamination. The rate of glutamate synthesis from leucine was 5.0 umol/g/h and at least 50% of glutamate-N originally derived from leucine. The enrichment of [5-15N]glutamine was higher than [15N]ammonia in the brain, indicating glial ammonia generation from leucine via glutamate. The enrichment of [15N]GABA, [15N]aspartate, [15N]glutamate greater than [2-15N]glutamine suggests direct incorporation of leucine-N to both glial and neuronal glutamate. These findings provide a new insight for the role of leucine as N-carrier from plasma pool and within the cerebral compartments.