Detergent-solubilized bovine cytochrome c oxidase: Dimerization depends onthe amphiphilic environment

The extent to which bovine cytochrome c oxidase (COX) dimerizes in nondenat uring detergent environments was assessed by sedimentation velocity and equ ilibrium. In contrast to generally accepted opinion, the COX dimer is diffi cult to maintain and is the major oligomeric form only when COX is solubili zed with a low concentration of dodecylmaltoside, i.e., similar to1 mg/mg p rotein. The dimer form is intrinsically unstable and dissociates into monom ers with increased detergent concentration, i.e., >5 mg/ mg protein. The st ructure of the solubilizing detergent, however, greatly alters detergent ef fectiveness by inducing either monomerization or aggregation. Triton X-100 is most effective at solubilizing COX, but it destabilizes COX dimers, even at low concentration. Undecylmaltoside, decylmaltoside, and octaethylenegl ycolmonododecyl ether (C12E8) are less effective at solubilizing COX. Each prevents COX aggregation at high detergent concentration, but also destabil izes the COX dimer. Other detergents, e.g., Tween 20, sodium cholate, sodiu m deoxycholate, CHAPS, or CHAPSO, are completely ineffective COX solubilize rs and do not prevent aggregation even at 10-40 mg/mL. The transition from dimers to monomers depends on many factors other than detergent structure a nd concentration, e.g., protein concentration, phospholipid content and pH. We conclude that the intrinsic dimeric structure of COX can be maintained only after solubilization with low concentrations of dodecylmaltoside at ne ar neutral pH, and even then precautions must be taken to prevent its disso ciation into monomers.