Analysis of muscle mitochondrial function with techniques applicable to needle biopsy samples

Abstract

Summary. Methods are described for the assessment of mitochondrial function in small (less than 50 mg) samples of human skeletal muscle using spectrophotometric assays to follow the reduction or oxidation of added cytochrome c. Activities of pyruvate plus malate cytochrome c reductase [(P + M)CR], α‐oxo‐glutarate cytochrome c reductase (OgCR) glutamate cytochrome c reductase (GCR), succinate cytochrome c reductase (SCR) and cytochrome c oxidase (COx) were measured in various rat tissues and samples of human skeletal muscle. In all tissues the activities were in the order (P + M) CR<SCR<COx, although there were wide variations in the absolute levels of activity. Rates of (P + M)CR and SCR activity measured spectrophotometrically in hypotonic medium were similar to the equivalent rates of oxygen uptake measured in isotonic medium with an oxygen electrode. Because the spectrophotometric assays have to be carried out in hypotonic medium, mitochondria are uncoupled and the method cannot be used to examine this aspect of mitochondrial function. Measurements can be reliably made on a single needle biopsy specimen of human muscle and by measuring a number of different activities the function of the whole electron transport chain and a number of tricarboxylic acid cycle enzymes can be determined. The clinical use of the methods is illustrated with results from selected patients with specific defects of mitochondrial metabolism. The methods offer a convenient way both for screening and the detailed and repeated study of patients.