2-ethylhydracrylic aciduria in short/branched-chain acyl-CoA dehydrogenase deficiency: application to diagnosis and implications for the R-pathway of isoleucine oxidation

Clin Chem. 2005 Mar;51(3):610-7. doi: 10.1373/clinchem.2004.043265. Epub 2004 Dec 22.


Background: Isolated excretion of 2-methylbutyrylglycine (2-MBG) is the hallmark of short/branched-chain acyl-CoA dehydrogenase deficiency (SBCADD), a recently identified defect in the proximal pathway of L-isoleucine oxidation. SBCADD might be underdiagnosed because detection and recognition of urine acylglycines is problematic. Excretion of 2-ethylhydracrylic acid (2-EHA), an intermediate formed in the normally minor R-pathway of L-isoleucine oxidation, has not previously been described in SBCADD.

Methods: Samples from four patients with 2-MBG excretion were analyzed by gas chromatography-mass spectrometry for urine organic acids, quantification of 2-MBG, and chiral determination of 2-methylbutyric acid. Blood-spot acylcarnitines were measured by electrospray-tandem mass spectrometry. Mutations in the ACADSB gene encoding SBCAD were identified by direct sequencing.

Results: SBCADD was confirmed in each patient by demonstration of different ACADSB gene mutations. In multiple urine samples, organic acid analysis revealed a prominent 2-EHA peak usually exceeding the size of the 2-MBG peak. Approximately 40-46% of total 2-methylbutyric acid conjugates were in the form of the R-isomer, indicating significant metabolism via the R-pathway.

Conclusions: If, as generally believed, SBCAD is responsible for R-2-MBG dehydrogenation in the R-pathway, 2-EHA would not be produced in SBCADD. Our observation of 2-ethylhydracrylic aciduria in SBCADD implies that a different or alternative enzyme serves this function. Increased flux through the R-pathway may act as a safety valve for overflow of accumulating S-pathway metabolites and thereby mitigate the severity of SBCADD. Awareness of 2-ethylhydracrylic aciduria as a diagnostic marker could lead to increased detection of SBCADD and improved definition of its clinical phenotype.

MeSH terms

  • Biomarkers / urine
  • Butyrates / chemistry
  • Butyrates / urine
  • Butyryl-CoA Dehydrogenase / deficiency*
  • Butyryl-CoA Dehydrogenase / genetics
  • Gas Chromatography-Mass Spectrometry
  • Glycine / analogs & derivatives*
  • Glycine / urine
  • Humans
  • Infant, Newborn
  • Isoleucine / metabolism*
  • Mutation
  • Oxidation-Reduction
  • Stereoisomerism
  • Valerates / urine*


  • Biomarkers
  • Butyrates
  • Valerates
  • Isoleucine
  • 2-ethylhydracrylic acid
  • 2-methylbutyrylglycine
  • Butyryl-CoA Dehydrogenase
  • 2-methylbutanoic acid
  • Glycine