Merck, 04478, <I>N</I>-(2-Methylbutyryl)glycine

Biochem/physiol Actions
2-Methylbutyrylglycine is an acyl glycine. Acyl glycines are normally minor metabolites of fatty acids. However, the excretion of certain acyl glycines 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 2.3.1.13) which is an enzyme that catalyzes the chemical reaction:. acyl-CoA + glycine ↔ CoA + N-acylglycine. The isolated excretion of high levels of 2-methylbutyrylglycine (2-MBG) is the hallmark of short/branched-chain acyl-CoA dehydrogenase deficiency or SBCADD. The disorder is also called 2-methylbutyryl-CoA dehydrogenase deficiency and has been associated with autism and mental retardation. SBCADD is a recently described autosomal recessive disorder caused by a defect in the degradation pathway of L- isoleucine leading to increased urinary excretion of 2-methylbutyryl glycine. The enzymatic defect results from disruption of the SBCAD gene. Deficiency of SBCAD leads to accumulation of its substrate, 2-methylbutyryl-CoA within the mitochondrion. This substance is transesterified with glycine by the mitochondrial enzyme acyl-CoA glycine-N-acyltransferase (glycine-N-acylase) to form 2-methylbutyryl glycine. Affected patients can be divided into two categories. The first category consists of infants detected by newborn screening programs. These infants are treated with diet and remain without clinical symptoms. In the second category affected patients are diagnosed because they presented clinically with seizures and psychomotor delay and have increased urinary excretion of 2-methylbutyryl glycine. 2-Methylbutyrylglycine has also been found in the urine of patients with propionyl-CoA carboxylase deficiency after consuming isoleucine. 2-methylbutyrylglycine is also elevated in the urine of patients with glutaric aciduria II and ethylmalonic encephalopathy.