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Phosphocreatine synthesis by isolated rat skeletal muscle mitochondria is not dependent upon external ADP: a 31P NMR study.

Phosphocreatine synthesis by mitochondria isolated from rat skeletal muscle was determined in presence of inorganic phosphate, creatine, and either ATP or ADP, using 31P NMR spectroscopy in a new protocol maintaining mitochondria for several hours in a well-coupled state. Maximal velocity of phosphocreatine synthesis was identical with 0.4 mM of ADP or 0.5 mM ATP at a rate of 0.063 mM/min. External ATP and ADP were always present in the spectra, demonstrating that in skeletal muscle cells as in heart muscle cells, mitochondrial creatine kinase coupled to translocase has a very strong amplifying effect on oxidative phosphorylation and converts external inorganic phosphate and creatine into phosphocreatine without net adenine nucleotide consumption. Therefore, adenine nucleotides can be considered as cofactors rather than regulators of mitochondria metabolism. This is in agreement with the "phosphocreatine-circuit" theory.