Ultracentrifugation analysis of detergent induced monomerization and dimerization of bovine cytochrome c oxidase

The self-association of cytochrome c oxidase, solubilized in a variety of detergents and phospholipid/detergent mixtures, was investigated by sedimentation velocity and sedimentation equilibrium. Homogeneous dimeric enzyme was only obtained in a 1:1 (w/w) cholate-phospholipid mixture, the condition most commonly used to reconstitute the enzyme into small unilamellar vesicles. In all cases in which oxidase was solubilized only by detergent, the enzyme was either homogeneous and monomeric, a mixture of monomers and dimers, or contained a significant amount of higher aggregates. With low concentrations of dodecyl maltoside, undecyl maltoside or Triton X 100 (1 mg/mg pr) in low ionic strength neutral pH buffer, the enzyme was a mixture of dimers and monomers; at higher detergent concentrations (10 mg/mg pr) it was homogeneous and monomeric. Monomer formation was also favored at low detergent concentrations if samples were incubated at pH > 8.50. Decyl maltoside or C12E8 were less effective since the enzyme was always either aggregated, or a mixture of monomers and dimers even at the high detergent/protein ratios. Solubilization of oxidase by bile salts (sodium cholate, CHAPS, or CHAPSO). or Tween 20 was quite different. At very high concentrations of these detergents (40 mg/mg pr), the enzyme was a heterogeneous mixture of high molecular weight aggregates.