The role of free Mg2 and its chelated forms as a fine mechanism to control succinate dehydrogenase (SDH) and
the phosphorylative activities, in particular that of H-PPiase, of pea stem mitochondria was examined. Free
pyrophosphate (PPi) was found to be a competitive inhibitor of SDH in sub-mitochondrial particles (SMP), but this
inhibition was relieved when PPi was chelated by Mg2. Neither Mg2 alone in SMP, nor Mg2 depletion in intact
mitochondria affected SDH activity. Chelation of Mg2 by ATP, ADP, EDTA or citrate restored PPi-induced
inhibition of SDH activity. Being (Mg)PPi or (Mg)2PPi the substrate of the H-PPiase, the subtraction of Mg2 from
this complex by stronger chelators, such as ATP or ADP, liberates free PPi causing an inhibition of the H-PPiase
activity. In particular, the formation of the (Mg)ADP chelate could favor the H-ATPase, being its substrate.
Therefore, the activities of SDH, H-PPiase and H-ATPase appear to be interconnected and regulated by the
equilibrium between free Mg2 and its chelates with PPi, ADP and ATP.
