The glycine conjugates of isocaproic, 4-methylhexanoic, 7-hydroxyoctanoic and 8-hydroxyoctanoic acids have been identified in the urine of children with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency using gas chromatography-mass spectrometry of the trimethylsilyl derivatives. A quantitative study showed that the glycine conjugates of isocaproic and 4-methylhexanoics acids were excreted during acute episodes and in smaller amounts when subjects were asymptomatic. The glycine conjugates of 7-hydroxyoctanoic and 8-hydroxyoctanoic acids were detectable during acute episodes. None of the conjugates was detected in controls or controls receiving a diet containing medium-chain triglycerides. It is suggested that the glycine conjugates of isocaproic acid and 4-methylhexanoic acid are metabolites of branched-chain fatty acids and that they are specific for MCAD deficiency.
