Muscle pyruvate kinase deficiency: glycogen storage disease or mitochondrial myopathy?

Defects of muscle glycogen metabolism are well documented causes of metabolic myopathy, presenting with a spectrum of symptoms which show some relationship to the position of defective enzyme within the glycolytic pathway. We present three women with metabolic myopathic conditions which show some features associated with a glycogen storage disease and some features of a mitochondrial defect. Muscle histochemistry and electron microscopy showed only minor and non-specific changes. However biochemical analysis of muscle biopsies in these three cases revealed a defect in glycolysis at the level of pyruvate kinase (PK), a defect as yet undescribed. Further investigation of the enzyme's properties, revealed that the residual muscle PK activity was due to the muscle (M1) isoform.