Mitochondrial respiration and ROS emission during β-oxidation in the heart: An experimental-computational study

Lipids are main fuels for cellular energy and mitochondria their major oxidation site. Yet unknown is to what extent the fuel role of lipids is influenced by their uncoupling effects, and how this affects mitochondrial energetics, redox balance and the emission of reactive oxygen species (ROS). Employing a combined experimental-computational approach, we comparatively analyze β-oxidation of palmitoyl CoA (PCoA) in isolated heart mitochondria from Sham and streptozotocin (STZ)-induced type 1 diabetic (T1DM) guinea pigs (GPs). Parallel high throughput measurements of the rates of oxygen consumption (VO2) and hydrogen peroxide (H2O2) emission as a function of PCoA concentration, in the presence of L-carnitine and malate, were performed. We found that PCoA concentration 600 nmol/mg mito prot, in both control and diabetic animals. Also, for the first time, we show that an integrated two compartment mitochondrial model of β-oxidation of long-chain fatty acids and main energy-redox processes is able to simulate the relationship between VO2 and H2O2 emission as a function of lipid concentration. Model and experimental results indicate that PCoA oxidation and its concentration-dependent uncoupling effect, together with a partial lipid-dependent decrease in the rate of superoxide generation, modulate H2O2 emission as a function of VO2. Results indicate that keeping low levels of intracellular lipid is crucial for mitochondria and cells to maintain ROS within physiological levels compatible with signaling and reliable energy supply.Author summary: Lipids are main sources of energy for liver and cardiac and skeletal muscle. Mitochondria are the main site of lipid oxidation which, in the heart, supplies most of the energy required for its blood pumping function. Paradoxically, however, lipids over supply impair mitochondrial function leading to metabolic syndrome, insulin resistance and diabetes. In this context, scientific debate centers on the impact of lipids and mitochondrial function on diverse aspects of human health, nutrition and disease. To elucidate the underlying mechanisms of this issue, while accounting for both the fundamental role of lipids as energy source as well as their potential detrimental effects, we utilized a combined experimental and computational approach. Our mitochondrial computational model includes β-oxidation, the main route of lipid degradation, among other pathways that include oxygen radical generation and consumption. Studies were performed in heart mitochondria from type 1 diabetic and control guinea pigs. Model and experimental results show that, below a concentration threshold, lipids fueling proceeds without disrupting mitochondrial function; above threshold, lipids uncouple mitochondrial respiration triggering excess emission of oxidants while impairing antioxidant systems and the mitochondrial energy supply-demand response. These contributions are of direct use for interpreting and predicting functional impairments in metabolic disorders associated with increased circulating levels of lipids and metabolic alterations in their utilization, storage and intracellular signaling.