The protocol was approved by the ethics committee of the "Fundación para el Estudio de las Enfermedades Neurometabólicas" where all the studies were performed. An informed consent was obtained from the parents prior to each test. Both patients were in stable condition, without any sign of intercurrent illness and had normal liver enzymes and creatine kinase when the tests were performed. A heparin lock was maintained throughout each test and blood was obtained for AC, non-esteritied fatty acids, insulin, glucose, p-hydroxybutyrate, amino acids, carnitine (total and free) and uric acid levels. For acylcarnitine analysis, blood samples were spotted in a filter paper (Schleicher & Schuell 903) and allowed to dry at room temperature. Sample preparation and analysis were performed as described11,12 with a VG Quattro II, triple quadrupole mass spectrometer (Micromass, UK) with electrospray injection, an HPLC pump and autosampler (Hewlett Packard series 1050). Acquisition was performed scanning for ions parent of m/z 85. The signals in the profiles correspond to the [M+] ions of the acylcarnitine butyl esters. Free, acetyl, octanoyl and hexadecanoylcarnitines were quantitated with their corresponding stable isotopes. Labelled internal standards were obtained from Dr. Herman J. Ten Brink (Academic Hospital V. U. Amsterdam). Normal values were obtained from 58 children in whom fatty acid oxidation defects were excluded by our usual protocol of amino acids, AC and carnitine in blood, and organic acids in urine. None of the controls received a special diet and their blood samples were obtained after an overnight fast (children) or at the end of their usual fasting period (infants). When enough sample was available, levels of octanoic acid were measured in plasma by GC/MS, as described.13 Mutation analysis was performed by Drs. B. Storstein Andersen and N. Gregersen at the Centre for Medical Molecular Biology, University of Aarhus, Denmark.

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