In any patient suspected to suffer from a mitochondrial beta-oxidation defect, enzymatic analysis needs to be done to pinpoint the true underlying defect. In principle, there are two scenarios: the first scenario is identification of the patient by an abnormal acylcarnitine profile in neonatal screening, the second is identification based on clinical signs and symptoms in combination with a characteristic acylcarnitine profile. Indeed, most of the currently identified mitochondrial beta-oxidation deficiencies are associated with distinct acylcarnitine profiles in plasma, which are different for each of the individual beta-oxidation defects (Fig. ). The only exception to this rule is CACT- versus CPT2-deficiency, which have identical acylcarnitine profiles. In all other cases, the acylcarnitine profiles usually suggest directly which step in the mitochondrial beta-oxidation system is defective (Fig. ).
Overview of the different mitochondrial fatty acid beta-oxidation deficiencies and the plasma acylcarnitine abnormalities identified in each of them
In order to rapidly identify the enzyme defect in patients after the finding of an abnormal acylcarnitine profile, we have set up enzyme activity measurements for CPT1, CACT, CPT2, VLCAD, MCAD, SCAD, LCHAD/MTP, and SCHAD in lymphocytes. This is especially relevant since existing neonatal screening programs have been extended in many countries around the world including the Netherlands and now include many of the mitochondrial beta-oxidation deficiencies. We will now discuss the carnitine cycle plus the actual beta-oxidation machinery in some detail including the enzymes involved with particular emphasis on the different enzyme assays currently in use for patient diagnostics. Special attention will be given to the enzyme assays developed in our own laboratory (see Table for details on each of these enzyme assays).
Enzymatic analysis of the different mitochondrial fatty acid beta-oxidation enzymes and the experimental conditions used in our laboratory for each of these enzymes