Cerebral metabolic disturbances in the brain during acute liver failure: from hyperammonemia to energy failure and proteolysis

Several observations suggest that patients with fulminant hepatic failure may suffer from disturbances in cerebral metabolism that can be related to elevated levels of arterial ammonia. One effect of ammonia is the inhibition of the rate limiting TCA cycle enzyme alpha-ketoglutarate dehydrogenase (alphaKGDH) and possibly also pyruvate dehydrogenase, but this has been regarded to be of no quantitative importance. However, recent studies justify a revision of this point of view. Based on published data, the following sequence of events is proposed. Inhibition of alphaKGDH both enhances the detoxification of ammonia by formation of glutamine from alpha-ketoglutarate and reduces the rate of NADH and oxidative ATP production in astrocytic mitochondria. In the astrocytic cytosol this will lead to formation of lactate even in the presence of sufficient oxygen supply. Since the aspartate-malate shuttle is compromised, there is a risk of depletion of mitochondrial NADH and ATP unless compensatory mechanisms are recruited. One likely compensatory mechanism is the use of amino acids for energy production. Branched chain amino acids, like isoleucine and valine can supply carbon skeletons that bypass the alphaKGDH inhibition and maintain TCA cycle activity. Large-scale consumption of certain amino acids can only be maintained by cerebral proteolysis, as has been observed in these patients. This hypothesis provides a link between hyperammonemia, ammonia detoxification by glutamine production, cerebral lactate production, and cerebral catabolic proteolysis in patients with FHF.