Histiocytoid cardiomyopathy of infancy: Deficiency of reducible cytochrome b in heart mitochondria

A 3–week–old girl with failure to thrive and cardiomegaly died of cardiac arrest at age 4 weeks. Morphologic studies of the heart showed enlarged muscle fibers with large accumulations of mitochondria, characteristic of histiocytoid cardiomyopathy. Biochemical studies showed markedly decreased succinate–cyto–chrome c reductase and rotenone–sensitive NADH–cytochrome c reductase activities, while other mitochondrial enzymes were normal. In isolated mitochondria, cytochrome spectra showed a severe defect of reducible cytochrome b and a less marked defect of cytochrome cc₁, while the content of cytochrome aa₃ (cyto–chrome c oxidase) was normal. Histiocytoid cardiomyopathy appears to be due to a defect of complex III (reduced coenzyme Q–cytochrome c reductase) in the respiratory chain of heart mitochondria.