Interplay of Mg2+, ADP, and ATP in the cytosol and mitochondria: Unravelling the role of Mg2+ in cell respiration

The respiration of heterotrophic cells, where most of the ATP demand is met by mitochondrial oxidative phosphorylation, is generally thought to be regulated either by the ATP/ADP ratio and/or energy charge or by nucleotide concentration. The way in which ADP and ATP may directly mediate respiration remains unclear, however. Furthermore, because only free nucleotides are exchanged by the mitochondrial ADP/ATP carrier, whereas MgADP is the substrate of ATP-synthase, Mg²⁺ compartmentation must be known. For this purpose, we performed simultaneous measurements of free and Mg-complexed nucleotides and Mg²⁺ in the cytosol and mitochondrial matrix using NMR-based techniques. Physiological alterations induced by Mg starvation helped unravel the key role of cytosolic and mitochondrial Mg²⁺ and free ADP in the regulation of cell respiration.