Abstract Oxidative phosphorylation in mitochondria is the main source of ATP in most eukaryotic cells. Concentrations of ATP, ADP, and AMP affect numerous cellular processes, including macromolecule biosynthesis, cell division, motor protein activity, ion homeostasis, and metabolic regulation. Variations in ATP levels also influence concentration of free Mg 2+ , thereby extending the range of affected reactions. In the cytosol, adenine nucleotide concentrations are relatively constant and typically are around 5 mM ATP, 0.5 mM ADP, and 0.05 mM AMP. These concentrations are mutually constrained by adenylate kinases operating in the cytosol and intermembrane space and are further linked to mitochondrial ATP and ADP pools via the adenine nucleotide translocator. Quantitative data on absolute adenine nucleotide concentrations in the mitochondrial matrix are limited. Total adenine nucleotide concentration lies in the millimolar range, but the matrix ATP/ADP ratio is consistently lower than the cytosolic ratio. Estimates of nucleotide fractions show substantial variability (ATP 20-7
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