Implications of adenylate kinase-governed equilibrium of adenylates on contents of free magnesium in plant cells and compartments

On the basis of the equilibrium of adenylate kinase (AK; EC 2.7.4.3). which interconverts MgATP and free AMP with MgADP and free ADP, an approach has been worked out to calculate concentrations of free magnesium (Mg2+), based on concentrations of total ATP, ADP and AMP in plant tissues and in indivi dual subcellular compartments. Based on reported total adenylate contents, [Mg2+] in plant tissues and organelles varies significantly depending on li ght and dark regimes, plant age and developmental stage. In steady-state co nditions, [Mg2+] in chloroplasts is similar in light and darkness (in the m illimolar range), whereas in the cytosol it is very low in the light and in creases to about 0.4 mM in darkness. During the dark-to-light transition (p hotosynthetic induction), the [Mg2+] in chloroplasts falls to low values (0 .2 mM or less), corresponding to a delay in photosynthetic oxygen evolution . This delay is considered to result from lower activities of Mg-dependent enzymes in the Calvin cycle. In mitochondria, the changes in [Mg2+] are sim ilar but smoother. On the other hand, when the transition from light to dar kness is considered, an initial increase in [Mg2+] occurs in both chloropla sts and mitochondria, which may be of importance for the control of key reg ulatory enzymes (e.g. mitochondrial malic enzyme and pyruvate dehydrogenase complex) and for processes connected with light-enhanced dark respiration. A rationale is presented for a possible role of [MgATP]/[MgADP] ratio (rat her than [ATP(total)]/[ADP(total)]) as an important component of metabolic cellular control. It is postulated that assays of total adenylates may prov ide an accurate measure of [Mg2+] in plant tissues/cells and subcellular co mpartments, given that the adenylates are equilibrated by AK.