Neurotoxicity of ammonia and fatty acids: Differential inhibition of mitochondrial dehydrogenases by ammonia and fatty acyl coenzyme a derivatives

In several metabolic encephalopathies, hyperammonemia and organic acidemia are consistently found. Ammonia and fatty acids (FAs) are neurotoxic: previous workers have shown that ammonia and FAs can act singly, in combination, or synergistically, in inducing coma in experimental animals. However, the biochemical mechanisms underlying the neurotoxicity of ammonia and FAs have not been fully elucidated. FAs are normally converted to their corresponding CoA derivatives (CoAs) once they enter cells and it is known that these fatty acyl CoAs can alter intermediary metabolism. The present study was initiated to determine the effects of ammonia and fatty acyl CoAs on brain mitochondrial dehydrogenases. At a pathophysiological level (2 mM), ammonia is a potent inhibitor of brain mitochondrial -ketoglutarate dehydrogenase complex (KGDHC). Only at toxicological levels (10–20 mM) does ammonia inhibit brain mitochondrial NAD⁺- and NADP⁺-linked isocitrate dehydrogenase (NAD-ICDH, NADP-ICDH), and NAD⁺-linked malate dehydrogenase (MDH) and liver mitochondrial NAD-ICDH. Butyryl- (BCoA), octanoyl- (OCoA), and palmitoyl (PCoA) CoA were potent inhibitors of brain mitochondrial KGDHC, with IC₅₀ values of 11, 20, and 25 M, respectively; moreover, the inhibitory effect of fatty acyl CoAs and ammonia were additive. At levels of 250 M or higher, both OCoA (IC₅₀=1.15 mM) and PCoA (IC₅₀=470 M) inhibit brain mitochondrial NADP-ICDH; only at higher levels (0.5–1 mM) does BCoA inhibit this enzyme (by 30–45%). Much less sensitive than KGDHC and NADP-ICDH, brain mitochondrial NAD-ICDH is only inhibited by 1 mM BCoA, OCoA, and PCoA by 22%, 35%, and 44%, respectively. Even at 1 mM, OCoA and PCoA (but not BCoA) only slightly inhibited brain mitochondrial MDH (by 23%). In the presence of toxicological levels of ammonia (20 mM) and fatty acyl CoAs (1 mM), the inhibitory effect of fatty acyl CoAs and ammonia on brain mitochondrial NAD-ICDH, NADP-ICDH, and MDH are only partially additive. These results provide some support for our hypothesis that selective inhibition of a rate-limiting and regulated enzymatic step (e.g., KGDHC) by ammonia and fatty acyl CoAs may be one of the major mechanisms underlying the neurotoxicity of ammonia and FAs. The data also suggest that the same mechanism may acocunt for the synergistic effect of ammonia and FAs in inducing coma. Since the inhibition of KGDHC by ammonia and fatty acyl CoAs occurs at pathophysiological levels, the results may assume some pathophysiological and/or pathogenetic importance in metabolic encephalopathies in which hyperammonemia and organic acidemia are persistent features.We dedicate this paper to Dr. Santiago Grisolia. Dr. Grisolia has carried out many pioneering studies in urea metabolism and ammonia toxicity. His interesting ideas have been influential in these and related fields of research. He continues to contribute significantly in unravelling the mechanisms of ammonia toxicity.