Purpose/Principle

Complex I is composed of 45 subunits of which 7 are encoded by the mitochondrial genome. This enzyme complex is assembled in several steps characterized by subcomplexes of increasing molecular weight (100, 230, 400, 460, 850, and 900 kDa) to the final holoenzyme complex of 1000 kDa. The detection of intermediates or a decrease in the amount of the holoenzyme can be observed in patient fibroblasts with complex I deficiencies that are due to defects that affect the assembly of complex I or in defects that affect the synthesis of the mitochondrial encoded subunits such as defects in mitochondrial translation, transcription or mitochondrial mRNA processing. The intermediates that are observed in patients but not in controls include 100, 400, 460, 850, and 900 kDa bands. A 230 kDa intermediate is observed in some control fibroblasts but an increase in the amount of this intermediate in patient samples is considered pathogenic. This assay can detect a disorder despite the presence of fully assembled holocomplex or normal complex I enzyme activity.