3-Methylcrotonyl glycine (3-MCG) is a metabolite found in the urine of patients with 3-methylcrotonyl glycinuria, a metabolic disorder characterized by a deficiency in 3-methylcrotonyl-CoA carboxylase, that has diverse biological activities.[1] [2] It inhibits CO2 production and mitochondrial complex II-III and creatine kinase activity in rat cerebral cortex preparations in a concentration-dependent manner.[2] It also inhibits the Na+/K+-ATPase in purified synaptic membranes from rat cerebrum, indicating a potential role for this metabolite in the development of CNS damage associated with 3-methycrotonyl glycinuria.

Reference:
[1]. Bartlett, K., Ng, H., and Leonard, J.V. A combined defect of three mitochondrial carboxylases presenting as biotin-responsive 3-methylcrotonyl glycinuria and 3-hydroxyisovaleric aciduria. Clin. Chim. Acta. 100(2), 183-186 (1980).
[2]. Moura, A.P., Ribeiro, C.A., Zanatta, A., et al. 3-Methylcrotonylglycine disrupts mitochondrial energy homeostasis and inhibits synaptic Na+,K+-ATPase activity in brain of young rats. Cell. Mol. Neurobiol. 32(2), 297-307 (2012).