Modulation of oxidative phosphorylation by Mg2+ in rat heart mitochondria

The effect of varying the Mg2+ concentration on the 2-oxoglutarate dehydrogenase (2-OGDH) activity and the rake of oxidative phosphorylation of rat heart mitochondria was studied, The ionophore A23187 was used to modify the mitochondrial free Mg2+ concentration. Half-maximal stimulation (K-0.5) of ATP synthesis by Mg2+ was obtained with 0.13 +/- 0.02 mM (n = 7) with succinate (+rotenone) and 0.48 +/- 0.13 mM (n = 6) with 2-oxoglutarate (2-OG) as substrates. Similar K-0.5 values were found for NAD(P)H formation, generation of membrane potential, and state 4 respiration with 2-OG. in the presence of ADP, an increase in P-i concentration promoted a decrease in the K-0.5 values of ATP synthesis, membrane potential formation and state 4 respiration for Mg2+ with 2-OG,, but blot with succinate, These results indicate that 2-OGDH is the main step of oxidative phosphorylation modulated by Mg2+ when 2-OG is the oxidizable substrate; with succinate, the ATP synthase is the Mg2+-sensitive step. Replacement of P-i by acetate, which promotes changes on intramitochondrial pH abolished Mg2+ activation of 2-OGDH. Thus, the modulation of the 2-OGDH activity by Mg2+ has an essential requirement for P-i (and ADP) in intact mitochondria which is mot associated to variations in matrix pH.