3-Methylglutaric acid is a metabolite of the essential amino acid L-leucine .¹ It is produced by hydrolysis of the L-leucine catabolic intermediate 3-methylglutaconyl-CoA (3-MG-CoA), which accumulates when the activity of 3-MG-CoA hydratase, the enzyme that converts 3-MG-CoA to HMG-CoA, is deficient, in the mitochondria. 3-Methylglutaric acid (5 mM) inhibits N+/K+-ATPase activity and induces the production of reactive oxygen species (ROS) in rat cortical synaptosomes, as well as induces lipid peroxidation in rat cortical supernatants.^2,3 Urinary levels of 3-methylglutaric acid are increased in patients with 3-methylglutaconic aciduria type I, an inborn error of metabolism characterized by mutations in the gene encoding 3-MG-CoA hydratase that can lead to neurological impairments.³