Control of Respiration by Cytochrome c Oxidase in Intact Cells: ROLE OF THE MEMBRANE POTENTIAL*

Metabolic control analysis was applied to intact HepG2 cells. The effect on the control coefficient of cytochrome c oxidase (CcOX) over cell respiration of both the electrical (Δψ) and chemical (ΔpH) component of the mitochondrial transmembrane proton electrochemical gradient (Δμ_H⁺) was investigated. The overall O₂ consumption and specific CcOX activity of actively phosphorylating cells were titrated with cyanide under conditions in which Δψ and ΔpH were selectively modulated by addition of ionophores. In the absence of ionophores, CcOX displayed a high control coefficient (C_IV = 0.73), thus representing an important site of regulation of mitochondrial oxidative phosphorylation. A high control coefficient value (C_IV = 0.85) was also measured in the presence of nigericin, i.e. when Δψ is maximal, and in the presence of nigericin and valinomycin (C_IV = 0.77), when Δμ_H⁺ is abolished. In contrast, CcOX displayed a markedly lower control coefficient (C_IV = 0.30) upon addition of valinomycin, when Δψ is converted into ΔpH. These results show that Δψ is responsible for the tight control of CcOX over respiration in actively phosphorylating cells.