Kinetic Modeling of Energy Metabolism and Superoxide Generation in Hepatocyte Mitochondria

Abstract

Direct nonenzymatic oxidation of semiquinone by oxygen is one of the main sources of superoxide radicals $\left( {{O}_{2}^{\overline \cdot } } \right)$ in mitochondria. Using all the known data on hepatocyte mitochondria, we have revealed the correlation between the rate of superoxide generation by the bc ₁complex and the transmembrane potential (ΔΨ). Assuming that the main electrogenic stage of the Qcycle is the electron transfer between the cytochrome bhemes, then the rate of superoxide generation sharply increases when ΔΨ grows from 150 to 180 mV. However, this interrelation is ambiguous. Indeed, the increase of the generation rate with the growth of the potential can occur faster when succinate dehydrogenase is inhibited by malonate than when external ADP is exhausted. When the potential is changed by adding phosphate or potassium (K⁺), the rate of $\left( {{O}_{2}^{\overline \cdot } } \right)$ production remains constant, although the comparison of the rates at the same ΔΨ reveals the effect of phosphate or potassium. It turned out that the rate of $\left( {{O}_{2}^{\overline \cdot } } \right)$ generation is a function of $\Delta \overline {\mu } _{H}$ rather than any of its components. Phosphate and K⁺have practically no influence on $\Delta \overline {\mu } _{H}$, since the change in ΔΨ is compensated by ΔpH. The rate of superoxide generation by the bc ₁complex is a multiple function of the electron-transfer activity of enzymes, the processes determining the membrane potential (e.g., loading), and the oxygen concentration. The kinetic model proposed in this work may serve to understand how the superoxide production is regulated.