Measurement of totally activated pyruvate dehydrogenase complex activity in human muscle: evaluation of a useful assay

A sensitive radiochemical method for the determination of the pyruvate dehydrogenase complex (PDHC) activity in skeletal muscle tissue, based on the decarboxylation of [1-14C]-pyruvate to 14CO2, is described. Measurements can be carried out either in muscle homogenate or in 600-g supernatant, both obtainable from a small muscle biopsy specimen (20 mg). In addition to NAD+, thiamine pyrophosphate and coenzyme A in the incubation mixture, a preparation of NADH:cytochrome c reductase (NADHCR) together with cytochrome c has a stimulating effect on the PDHC activity. NADHCR constitutes an oxidation system for NADH to prevent feedback inhibition. Addition of L-carnitine also results in stimulation of PDHC by trapping the produced acetyl-CoA as acetylcarnitine. Special care for radioactive pyruvate, with freeze drying and storage at -20 degrees C under nitrogen, and determination of the purity during every PDHC assay, is required. In the presented assay a Km value of 0.084 mmol/l was found for pyruvate. Nonsigmoidal kinetics was found with a Hill coefficient of 1.63. With the described method, a totally Mg2+,Ca(2+)-stimulated PDHC activity is measured. Addition of a purified specific pyruvate dehydrogenase phosphatase did not yield a higher PDHC activity. Finally, comparison of total PDHC activity with [1-14C]-pyruvate oxidation rates, both measured in the supernatant prepared from fresh muscle, shows an equimolar correlation, indicating that total PDHC activity is rate limiting in the assay for the pyruvate oxidation rate. Neonatal muscle exhibits five to ten times lower PDHC activities and pyruvate oxidation rates than controls (age > 3 years).