Spectrophotometric evaluation of mitochondrial respiratory chain (MRC) enzymatic complexes is the main approach to the biochemical investigation and diagnosis in oxidative phosphorylation disorders (also known as mitochondrial cytopathies). Regular dual beam spectrophotometers may be used, but we describe the protocols for double wavelength devices, allowing the analysis of complex activities from a small amount of tissue, with high sensitivity. An important concern is which tissue should be selected for analysis. Accordingly, we present the results obtained with different tissues and control values to be used. There are no standards available for the determinations and no interlaboratory quality control schemes are implemented. Additionally, different laboratories may use different protocols and comparison of results may be difficult. Currently, there is no consensus in literature for defining a criterion of an MRC deficiency to be used in biochemical diagnosis. There is statistical evidence that the most adequate criterion to define an MRC deficiency is below 40% of the mean control value normalized to citrate synthase activity.
