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Kinetics of the inhibition of mitochondrial respiration by NO
A Koivisto1, A Matthias, G Bronnikov
1The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, Sweden.
Abstract:
The kinetics of the inhibition of mitochondrial respiration by NO was examined in isolated mitochondria (here obtained from rat brown adipose tissue). The Ki of NO for the inhibition was approximately 27 nM; the IC50 of NO increased in proportion to the square of an increase in O2 tension. The Km of O2 for respiration was approximately 16 microM; in the presence of NO, the dependence of respiration on O2 tension had a Hill coefficient of approximately 2. The unusual kinetics is probably related to the ability of cytochrome c oxidase to use 2 NO or 1 O2 as electron acceptor. The interaction between NO and O2 in the control of respiration could be described by the formula VO2(O2, NO) = VO2max x ([O2]2/((16 microM x (1 + [NO]/27 nM))2 + [O2]2)). Thus, the kinetics is such that respiration in the presence of physiological levels of NO is very sensitive to decreasing O2 tension.
Insights
Mitochondrial respiration is inhibited by nitric oxide (NO). This NO inhibition is highly sensitive to oxygen levels, impacting cellular energy production.
Area of Science:
- Biochemistry
- Cellular Respiration
- Mitochondrial Function
Background:
- Mitochondria are crucial for cellular energy production.
- Nitric oxide (NO) is a signaling molecule with complex roles in physiology.
- Understanding NO's interaction with mitochondrial respiration is key to cellular metabolism.
Purpose of the Study:
- To investigate the kinetics of mitochondrial respiration inhibition by nitric oxide (NO).
- To elucidate the relationship between oxygen tension and NO's inhibitory effect.
- To characterize the interaction of NO and O2 at cytochrome c oxidase.
Main Methods:
- Isolated mitochondria from rat brown adipose tissue were used.
- Mitochondrial respiration rates were measured under varying O2 and NO concentrations.
- Enzyme kinetics, including Ki, IC50, Km, and Hill coefficient, were determined.
Main Results:
- The inhibition constant (Ki) for NO was approximately 27 nM.
- NO's IC50 increased with the square of O2 tension.
- Respiration's O2 dependence showed a Hill coefficient of ~2 in the presence of NO, suggesting a complex interaction at cytochrome c oxidase.
Conclusions:
- Cytochrome c oxidase's ability to accept either 2 NO or 1 O2 explains the observed kinetics.
- A formula describing VO2(O2, NO) was derived, highlighting the interplay between NO and O2.
- Physiological NO levels render respiration highly sensitive to reductions in O2 tension.