Biochemistry of glutaric aciduria type I: activities of in vitro expressed wild-type and mutant cDNA encoding human glutaryl-CoA dehydrogenase.

Abstract

Glutaric aciduria type I (GA I; McKusick 231670) is caused by deficiency of glutaryl-CoA dehydrogenase activity (EC 1.3.99.7), a key enzyme in the catabolism of the amino acids lysine, hydroxylysine and tryptophan (Besrat et al 1969; Goodman et al 1975). The failure to metabolize glutaryl-CoA may give rise to elevated levels of glutarate, 3-hydroxyglutarate and glutaconate in the body fluids of the patient. Recent studies showed the leading role of 3-hydroxyglutarate in the pathogenesis of GA I (Baric et al, unpublished). Yet the source of this dicarboxylic acid is not well understood. Glutaryl-CoA dehydrogenase catalyses the reaction in two steps: dehydrogenation of glutaryl-CoA to glutaconyl-CoA and decarboxylation of glutaconyl-CoA to crotonyl-CoA, which can be measured separately (Hartel et al 1993). Whereas the hydration of glutaconyl-CoA to 3-hydroxyglutarylCoA is catalysed by methylglutaconase (EC 4.2.1.18; Liesert and Buckel, unpublished), the formation of glutaconyl-CoA is less clear. It could arise from glutaryl-CoA dehydrogenase mutants in which the decarboxylase activity is more affected than the dehydrogenase activity. Another possibility would be a nonspecific oxidation of glutaryl-CoA by medium-chain acyl-CoA dehydrogenase (MCAD, EC 1.3.99.3) or similar enzymes. In order to address this question, mechanistically and clinically interesting mutants were introduced into a modified human glutaryl-CoA dehydrogenase cDNA that can be expressed in Escherichia coli.