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Matrix free Mg2+ changes with metabolic state in isolated heart mitochondria
D W Jung1, L Apel, G P Brierley
1Department of Physiological Chemistry, Ohio State University Medical Center, Columbus 43210.
Abstract:
The concentration of free Mg2+ in the matrix of isolated heart mitochondria has been monitored by using the fluorescent probe furaptra (mag-fura-2). Beef heart mitochondria respiring in a KCl medium in the absence of external Mg2+ maintain free matrix Mg2+ near 0.50 mM. Addition of Pi under these conditions decreases free Mg2+ by 0.12-0.17 mM depending on the substrate. This decrease in free Mg2+ appears to reflect changing ligand availability in the matrix. The decrease is prevented when the Pi transporter is blocked by mersalyl. Addition of ADP to initiate state 3 respiration causes a marked increase in free matrix Mg2+ (0.1-0.2 mM) that persists as long as ATP formation is taking place; free Mg2+ then returns to the base level. This cyclic change is blocked by oligomycin and carboxyatractyloside and appears to reflect to a large extent the decrease in matrix Pi that accompanies oxidative phosphorylation. Exchange of external ADP for matrix ATP may also contribute to the increase in free matrix Mg2+. Addition of an uncoupler promotes anion efflux and increases free matrix Mg2+. Similar changes in free Mg2+ on addition of Pi, ADP, or uncoupler are seen when extramitochondrial Mg2+ is buffered from 0.5 to 2 mM, but the basal free matrix Mg2+ increases as external Mg2+ concentration increases in this range. Free matrix Mg2+ also increases when total mitochondrial Mg2+ is increased by respiration-dependent uptake in the presence of Pi. It is concluded that matrix free Mg2+ changes significantly with changing ligand availability and that such changes may contribute to the regulation of Mg2(+)-sensitive matrix enzymes and membrane transporters.
Insights
Mitochondrial matrix free Mg2+ concentration fluctuates with metabolic activity, influenced by phosphate and ADP levels. These dynamic Mg2+ shifts may regulate key mitochondrial enzymes and transporters.
Area of Science:
- Mitochondrial Physiology
- Bioenergetics
- Cellular Metabolism
Background:
- Mitochondrial magnesium (Mg2+) homeostasis is crucial for cellular energy production.
- The precise regulation of free Mg2+ within the mitochondrial matrix remains incompletely understood.
- Understanding matrix free Mg2+ dynamics is key to elucidating mitochondrial function and dysfunction.
Purpose of the Study:
- To monitor and characterize the concentration of free Mg2+ in the matrix of isolated heart mitochondria.
- To investigate the influence of metabolic substrates and inhibitors on matrix free Mg2+ levels.
- To explore the regulatory role of matrix free Mg2+ in mitochondrial processes.
Main Methods:
- Utilized the fluorescent probe furaptra (mag-fura-2) for real-time measurement of free Mg2+.
- Studied isolated beef heart mitochondria under various respiratory states (state 3, uncoupling).
- Manipulated matrix substrate availability (Pi, ADP) and employed specific inhibitors (mersalyl, oligomycin, carboxyatractyloside).
Main Results:
- Basal free matrix Mg2+ was maintained around 0.50 mM in respiring mitochondria.
- Phosphate (Pi) addition decreased free matrix Mg2+, reflecting ligand availability changes.
- ADP addition during state 3 respiration increased free matrix Mg2+, linked to ATP synthesis and Pi consumption.
- Uncouplers promoted anion efflux, leading to increased free matrix Mg2+.
- Matrix free Mg2+ increased with external Mg2+ concentration and total mitochondrial Mg2+ uptake.
Conclusions:
- Matrix free Mg2+ concentration is highly dynamic and sensitive to metabolic conditions and ligand availability.
- Changes in matrix free Mg2+ likely play a significant role in regulating Mg2+-sensitive mitochondrial enzymes.
- These fluctuations may also influence the activity of mitochondrial membrane transporters, impacting overall mitochondrial function.