Regulation by Magnesium of Potato Tuber Mitochondrial Respiratory Activities

Abstract

Dehydrogenase activities of potato tuber mitochondria and corresponding phosphorylation rates were measured for the dependence on external and mitochondrial matrix Mg²⁺. Magnesium stimulated state 3 and state 4 respiration, with significantly different concentrations of matrix Mg²⁺ required for optimal activities of the several substrates. Maximal stimulation of respiration with all substrates was obtained at 2-mM external Mg²⁺. However, respiration of malate, citrate, and α-ketoglutarate requires at least 4-mM Mg²⁺ inside mitochondria for maximization of dehydrogenase activities. The phosphorylation system, requires a low level of internal Mg²⁺ (0.25 mM) to reach high activity, as judged by succinate-dependent respiration. However, mitochondria respiring on citrate or α-ketoglutarate only sustain high levels of phosphorylation with at least 4-mM matrix Mg²⁺. Respiration of succinate is active without external and matrix Mg²⁺, although stimulated by the cation. Respiration of α-ketoglutarate was strictly dependent on external Mg²⁺ required for substrate transport into mitochondria, and internal Mg²⁺ is required for dehydrogenase activity. Respiration of citrate and malate also depend on internal Mg²⁺ but, unlike α-ketoglutarate, some activity still remains without external Mg²⁺. All the substrates revealed insensitive to external and internal mitochondrial Ca²⁺, except the exogenous NADH dehydrogenase, which requires either external Ca²⁺ or Mg²⁺ for detectable activity. Calcium is more efficient than Mg²⁺, both having cumulative stimulation. Unlike Ca²⁺, Mn²⁺ could substitute for Mg²⁺, before and after addition of A23, showing its ability to regulate phosphorylation and succinate dehydrogenase activities, with almost the same efficiency as Mg²⁺.