Biochemical analyses of deficiencies in the oxidative phosphorylation system in human muscle
MitochondriaI disorders are caused by biochemical abnormalities of the mitochondrial respiratory chain (RC), a key component of oxidative phosphorilation (OXPHOS). The diagnosis of a RC deficiency can be invasive, expensive, time consuming and labour intensive. The corner stone for diagnosis of mitochondrial disorders and the only two definitive tests are DNA analysis and respiratory chain enzyme analysis (Naviaux, 2004). The aim of this study was to use biochemical analysis (polarographic analysis and enzyme assay) to identify patients with an OXPHOS defect by setting up reference values from healthy control data. The most commonly used approach in many centres is the interpretation of enzyme activity data based on retrospectively compiled reference values obtained from the data from diseased children, because of the unavailability of muscle tissue from healthy children (Thorburn, 2004). Ethics approval (protocol 91/98) was given to obtain muscle biopsies from selected healthy children. Biochemical tests were done on the muscle biopsies of the healthy children (control group) and those of the selected possible patients, based on their clinical phenotypes. The methods used in the literature differ significantly, so the first aim was to evaluate and partly standardise the method. Reference values were determined from the control group, based on four different approaches used in the literature. The most accurate approach was chosen, the patient data were compared to the reference range and the patients with OXPHOS deficiencies were diagnosed. During this study 18 controls were collected and 26 possible patients identified. In 69% of the patients an OXPHOS defect could be diagnosed based on their enzymatic assay data. Only 11% of the enzymatic assay data were comparable with the respiratory analysis data. It is thus recommended that in future respiratory analysis is no longer utilised.