Ammonia is a major toxin in the CNS and interferes with cerebral energy metabolism. Ammonia may exert its deleterious effect by interfering with the transport of """"""""reducing equivalents"""""""" between cytosol and mitochondria. With the use of a new inhibitor of aspartate aminotransferase, i.e. Beta-methyleneaspartate (BetaMA), we have shown that the malate-aspartate shuttle (MAS) operates in brain for the transport of reducing equivalents. The data strongly suggest that the MAS and tricarboxylic acid cycle are tightly linked. We wish to extend these studies to determine whether excess ammonia produces the same biochemical effects (e.g. decreased 02 consumption and decreased ATP) as does BetaMA, and to determine whether the effects are additive. The astrocytes in the brains of liver disease patients and in the brains of portacaval-shunted rats are morphologically abnormal (Alzheimer type II changes). This abnormality may be due to a greater susceptibility to ammonia-induced metabolic impairment in astrocytes than in neurons. To test this hypothesis, we will investigate the biochemical effects on cultures of neurons and astrocytes of acute and chronic exposure to excess ammonia and BetaMA. We will also use [13N]ammonia (13N, positron-emitter; t1/2=9.96 min) to determine to what extent normal metabolic compartmentation is disrupted in the hyperammonemic rat brain. The metabolism of 13N-labeled amino acids will be investigated in the brains of normal and hyperammonemic rats. Evidence will be sought that branched-chain amino acid mixtures exert their beneficial effects by acting to replenish small compartment (astrocytic) glutamate, thereby stimulating the MAS and improving the cerebral energy balance. With an understanding of a) how ammonia interferes with cerebral energy metabolism and b) how the brain attempts to maintain nitrogen homeostasis it may be possible to suggest improved therapeutic interventions in patients with liver disease.
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