Phenylpropionylglycine is an acylglycine. Acylglycines are normally minor metabolites of fatty acids. However, the excretion of certain acylglycines is increased in several inborn errors of metabolism. In certain cases the measurement of these metabolites in body fluids can be used to diagnose disorders associated with mitochondrial fatty acid beta-oxidation. Acyl glycines are produced through the action of glycine N-acyltransferase (EC 22.214.171.124) which is an enzyme that catalyzes the chemical reaction: acyl-CoA + glycine ↔ CoA + N-acylglycine. The detection of phenylpropionylglycine in urine after an oral load of phenylpropionic acid can be used to diagnose deficiency of medium-chain acyl-CoA dehydrogenase, a frequent and treatable metabolic defect.