Background and Purpose: The relevance of mitochondria in the generation of oxidative stress is well established. In this respect the mitochondrial enzymes monoamine oxidases (MAO) are a relevant source of reactive oxygen species (ROS) and their inhibition has been shown to prevent both reperfusion injury and maladaptive remodeling. However, the mechanisms responsible for the increase in MAO activity in injured cardiomyocytes have not been elucidated. This study aims at characterizing fluidity changes of mitochondrial membranes as a factor modulating MAO activity.
Methods: Membrane fluidity in mitochondria isolated from mouse hearts was evaluated as changes in fluorescence anisotropy of added hematoporphyrin that probes phospholipid heads interacting with protein polar domains. Previous studies have shown that an increase in mitochondrial membrane fluidity (MMF) is associated with abnormalities, such as a decreased respiration and an increased propensity to opening of the permeability transition pore. However, its relationship with mitochondrial ROS generation was never investigated. Mitochondrial integrity and function were assessed by measuring membrane potential with Rhodamine123. The activity of MAO using tyramine as substrate was measured by assessing H2O2 formation.
Results: The addition of the omega3 fatty acid docohexanoic acid (DHA) caused a 60±4% increase in membrane rigidity that was paralleled by a 40±7.8% decrease in MAO activity. The effect of DHA was abrogated by doxorubicin. This cardiotoxic agent promotes oxidative stress by interacting with mitochondria. The anisotropy measurement of its intrinsic fluorescence showed a dose-dependent increase in MMF associated with an increase in MAO activity.
The relationship between MMF and MAO activity was supported by measurements in mitochondria isolated from hearts of mice subjected to pathophysiological conditions, such as high fat diet and aging. Eight weeks of high fat diet resulted in a significant increase (25±8.3%) in both MMF and MAO activity. Similar results were obtained by comparing young mice (4 months old (mo.)) with adult (14 mo.) and senescent (24 mo.) littermates. In a time-dependent fashion aging increased MMF and MAO activity.
Conclusions: For the first time evidence is provided of a tight relationship between membrane fluidity of heart mitochondria and the ROS producing activity of MAO. Mitochondrial membrane fluidity is likely to contribute to cardiac oxidative stress by means of MAO modulation.