Detergent-Solubilized Monomeric and Dimeric Cytochrome bc₁ Isolated from Bovine Hearth

Mitochondrial cytochrome bc₁ complex, isolated from frozen bovine heart, was solubilized with five different “non-denaturing” detergents: dodecyl maltoside, octaethylene glycol monododecyl ether (C₁₂E₈), Triton X-100, Tween 20, and sodium cholate. The hydrodynamic properties of the solubilized complex III's were then investigated by sedimentation analysis. Complex III exists as a stable and monodisperse dimer when it is solubilized in low ionic strength buffer with a low concentration of any of the five detergents. At pH 7.8, 20 °C, the protein sediments as a homogeneous species with an s_obs of about 14 S. The protein molecular weight of the 14S particle, after correction for bound detergent, is 465 000 ± 30 000 as measured by sedimentation equilibrium analysis. The aggregation state and/or homogeneity of cytochrome bc₁ is strongly dependent upon the concentration of the solubilizing detergent and ionic strength. The enzyme becomes a homogeneous, monomeric complex with a protein molecular weight of 235 000 ± 20 000 and an s_obs of 10-10.5 S after it is solubilized in high concentrations of Tween 20 (more than 5 mg/mg of protein) and sodium chloride (more than 0.5 M). However, a heterogeneous mixture of subcomplexes is produced upon solubilization of the complex with high concentrations of the other detergents and 0.5 M NaCl. Monomerization of cytochrome bc₁ by Tween 20 and 0.5 M NaCl has no effect on either the spectral properties, the subunit composition, or the enzymatic activity and is reversible since the dimeric 14S particle is regenerated upon removal of the high concentration of salt. Pretreatment of complex III with antimycin A also stabilizes the enzyme and permits the generation of the homogeneous monomeric complex by the other detergents with high ionic strength; e.g., a homogeneous 10S particle with a protein molecular weight of 230 000 ± 20 000 is also generated with 10 mg of dodecyl maltoside/mg of protein and 1 M sodium chloride if the complex is first inhibited by antimycin A.