Formation of 3-hydroxyglutaric acid in glutaric aciduria type I: in vitro participation of medium chain acyl-CoA dehydrogenase

3-Hydroxyglutaric acid (3-OH-GA) in urine has been identified as the most reliable diagnostic marker for glutaric aciduria type I (GA I). We showed that hydratation of glutaconyl-CoA to 3-hydroxyglutaryl-CoA, which is subsequently hydrolyzed to 3-OH-GA, is efficiently catalyzed by 3-methylglutaconyl-CoA hydratase (3-MGH). We have now investigated whether mitochondrial acyl-CoA-dehydrogenases can convert glutaryl-CoA to glutaconyl-CoA. Short-chain acyl-CoA dehydrogenase (SCAD), medium-chain acyl-CoA dehydrogenase (MCAD), and long-chain acyl-CoA dehydrogenase (LCAD) accepted glutaryl-CoA as a substrate. The highest k _cat of glutaryl-CoA was found for MCAD (0.12 ± 0.01 second^-1) and was about 26-fold and 52-fold higher than those of LCAD and SCAD, respectively. The turnover of MCAD for glutaryl-CoA was about 1.5% of that of its natural substrate octanoyl-CoA. Despite high K _m (above 600 μM) and low turnover rate, the oxidation of glutaryl-CoA by MCAD in combination with 3-MGH could explain the urinary concentration of 3-OH-GA in GA I patients.