Enzyme activity measured in single muscle fibers in partial cytochrome c oxidase deficiency.

Single-fiber analyses using a kinetic microphotometric method were performed on three patients with chronic progressive external ophthalmoplegia and proximal myopathy accompanied by a partial deficiency of cytochrome c oxidase. Two patients had subsarcolemmal accumulation of mitochondria (ragged-red fibers). Qualitative histochemical demonstration of cytochrome c oxidase showed a mosaic of fibers without detectable cytochrome c oxidase activity. Quantitative single fiber measurements in the patients' biopsies showed that the majority of the muscle fibers had decreased cytochrome c oxidase activity without selective involvement of a specific fiber type. Succinate dehydrogenase was measured and the ratio of the activities (succinate dehydrogenase/cytochrome c oxidase) was calculated. Normal muscle showed a ratio of about 2, whereas diseased muscle showed values between 10 and 20, due to a decrease in cytochrome c oxidase activity. Ragged-red fibers showed very low or undetectable cytochrome c oxidase activity.