Biochemistry and bioenergetics of glutaryl-CoA dehydrogenase deficiency

Glutaryl-CoA dehydrogenase (GCDH) is a central enzyme in the catabolic pathway of L-tryptophan, L-lysine, and L-hydroxylysine which catalyses the oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA and CO2. Glutaryl-CoA dehydrogenase deficiency (GDD) is an autosomal recessive disease characterized by the accumulation of glutaric and 3-hydroxyglutaric acids in tissues and body fluids. Untreated patients commonly present with severe striatal degeneration during encephalopathic crises. Previous studies have highlighted primary excitotoxicity as a trigger of striatal degeneration. The aim of this PhD study was to investigate in detail tissue-specific bioenergetic and biochemical parameters of GDD in vitro, post mortem, and in Gcdh-/- mice. The major bioenergetic finding was uncompetitive inhibition of alpha-ketoglutarate dehydrogenase complex by glutaryl-CoA. It is suggested that a synergism of primary and secondary excitotoxic effects in concert with age-related physiological changes in the developing brain underlie acute and chronic neurodegenerative changes in GDD patients. The major biochemical findings were highly elevated cerebral concentrations of glutaric and 3-hydroxyglutaric acid despite low permeability of the blood-brain barrier for these dicarboxylic acids. It can be postulated that glutaric and 3-hydroxyglutaric acids are synthesized de novo and subsequently trapped in the brain. In this light, neurological disease in GDD is not 'transported' to the brain in analogy with phenylketonuria or hepatic encephalopathy as suggested previously but is more likely to be induced by the intrinsic biochemical properties of the cerebral tissue and the blood-brain barrier.