Mitochondrial Oxidative Phosphorylation Enzymes

In this chapter, an overview is presented of methods to investigate the activity of the enzymes of the mitochondrial oxidative phosphorylation system, as part of diagnostic investigations of patients suspected to have a mitochondrial disorder due to the defect in one or more OXPHOS enzymes. As there are also other cellular components that influence the OXPHOS activity, additional methods are mentioned that can measure the activity of intact mitochondria by measuring oxygen consumption, ATP production, and substrate oxidation. Most diagnostic OXPHOS enzyme tests were developed as screening methods to establish a biochemical diagnosis that directs investigations targeted investigations of candidate genes. However, in recent years, the development of holistic genetic screening methods (exome and genome sequencing) has resulted in changes in diagnostic protocols in which genetic tests are performed in the early stages of the diagnostic investigations. As a consequence, OXPHOS enzyme test and related mitochondrial parameters, like respiration, ATP production, and substrate oxidations, are increasingly being used to validate genetic findings. Nevertheless, as the diagnostic yield of exome/genome sequencing has been estimated to be around 50%, a large group of patients still depend on OXPHOS activity screening in order to obtain a biochemical diagnosis.