Background:The enzymatic analysis of mitochondrial respiratory chain (MRC) complexes of skeletal muscle is an important step in the diagnosis of mitochondrial disorders. Because of its lesser turbidity and increased sensitivity, mitochondrial fractionation has been increasingly considered the diagnostic method of choice compared with the more classical analysis of muscle homogenate. In circumstances in which mitochondria become abnormal in number, size or shape, the process of mitochondrial enrichment made by sequential centrifugation and washing may favor the selection of the most normal mitochondria, eliminating the most abnormal ones. In this situation, the study of muscle homogenate, paradoxically, may better reflect what happens in vivo.Case Report: To exemplify this situation we present a 60-year-old woman with a complete mitochondrial phenotype and a 70% heteroplasmic presence of the mtDNA A3243G mutation in muscle tissue. The respiratory and enzymatic activities from mitochondria-enriched muscle suspension were within normal control limits. In contrast, when muscle homogenate was studied, enzyme activities of complexes I, III, and V were found to be decreased.Conclusions: Although mitochondria-enriched muscle suspensions are usually more informative than muscle homogenates for studies of MRC, in some situations it may be necessary to study both to uncover the biochemical defect.

Keywords: Electron Transport Chain Complex Proteins - genetics, Biopsy, Brain - radiography, Cell Fractionation, DNA, Mitochondrial - analysis, Electron Transport, Electron Transport Chain Complex Proteins - metabolism, Mitochondria - ultrastructure, Mitochondrial Myopathies - physiopathology, Muscle, Skeletal - physiopathology, Oxidative Phosphorylation, Tissue Extracts - metabolism