Abstract
The effect of varying the Mg2+ concentration on the 2-oxoglutarate dehydrogenase (2-OGDH) activity and the rate of oxidative phosphorylation of rat heart mitochondria was studied. The ionophore A23187 was used to modify the mitochondrial free Mg2+ concentration. Half-maximal stimulation (K0.5) 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 K0.5 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 Pi concentration promoted a decrease in the K0.5 values of ATP synthesis, membrane potential formation and state 4 respiration for Mg2+ with 2-OG, but not 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 Pi 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 Pi (and ADP) in intact mitochondria which is not associated to variations in matrix pH.
Highlights
The notion that the cytosolic concentration of free Mg2ϩ ([Mg2ϩ]c)1 had a constant value around 1 mM under different conditions has changed in recent years
We show that variations of external Mg2ϩ, and in [Mg2ϩ]m, can modulate the activities of the 2-oxoglutarate dehydrogenase (2-OGDH) and the ATP synthase and, in consequence, Mg2ϩ may affect the rate of oxidative phosphorylation in isolated rat heart mitochondria
Similar results were previously reported for rat liver mitochondria [12]
Summary
The notion that the cytosolic concentration of free Mg2ϩ ([Mg2ϩ]c) had a constant value around 1 mM under different conditions has changed in recent years. The transition from basal (state 4) to active (state 3) respiration led to a small, but significant elevation in the mitochondrial matrix free Mg2ϩ concentration ([Mg2ϩ]m) from 0.5 mM to 0.6 – 0.7 mM. This increase in [Mg2ϩ]m persisted during ATP synthesis, until added ADP was exhausted; at this time [Mg2ϩ]m returned to basal levels. Modulation of mitochondrial glutaminase by 0 –2 mM Mg2ϩ has been observed [13] All of these reports describing an active movement of Mg2ϩ in cells and mitochondria of different tissues and in response to different agonists suggest that Mg2ϩ may play a role as a second messenger in the cell. We show that variations of external Mg2ϩ, and in [Mg2ϩ]m, can modulate the activities of the 2-OGDH and the ATP synthase and, in consequence, Mg2ϩ may affect the rate of oxidative phosphorylation in isolated rat heart mitochondria
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
