Monomerization of cytochrome oxidase may be essential for the removal of subunit III.

1. Incubation of cytochrome oxidase, under conditions used as initial steps in treatment to remove subunit III, causes at least partial monomerization of the enzyme. 2. The extent of removal of subunit III by anion-exchange fast protein liquid chromatography (FPLC) is much increased if the enzyme is fully monomerized before it is applied to the column. 3. Subunit III is incompletely removed by chymotrypsin treatment. A digestion product of subunit III migrating in SDS-PAGE like subunit IV, is detected with specific antibodies. The amount of this product is reduced when monomerization is increased by raising the detergent/protein ratio. 4. The results suggest that monomerization facilitates removal of subunit III and exposes it to further chymotrypsin digestion. We propose that subunit III is at least in part located in the junction between the monomers in the cytochrome oxidase dimer.