Defects in the cytochrome bc1 complex in mitochondrial diseases.

The clinical and biochemical findings of 14 patients with an isolated defect of the bc1 complex have been summarized. The heterogeneity of this group of disorders reflects the severity and tissue specific expression of the defect and the complexity of this multisubunit protein with components that are coded on both nuclear and mitochondrial DNA. The data on several patients with a combined defect of cytochrome oxidase and the bc1 complex or with multiple respiratory chain defects have also been presented and discussed in relation to our knowledge of the biosynthesis and assembly of the respiratory chain complexes. The severity of the defect in vivo is illustrated in one patient with isolated complex III deficiency by measurement of O2 consumption and CO2 production following exercise, or by 31P-NMR. The latter also provides a means by which response to therapy can be followed.