The subunit structure of cytochrome-c oxidase from tuna heart and liver

Cytochrome-c oxidase was isolated from tuna liver and heart, and the subunit composition was analysed by SDS/PAGE by two separation systems. Two additional subunits of the enzyme complex were immunoprecipitated from solubilized mitochondria with an antibody against bovine subunit IV. The N-terminal and internal amino acid sequences of all nuclear-coded subunits were determined after blotting onto poly(vinylidene difluoride) membranes or by tryptic hydrolysis of gel bands and HPLC separation of peptides, respectively. 13 subunits were identified with isoforms for subunits Va, VIc, VIIb and VIII. The isoforms for subunits Va and VIIb are found in liver and heart, isoforms for subunit VIc only in heart, and isoforms for subunit VIII only in liver. Isoforms for subunits Va, VIc and VIIb have not been described in other species. The postulated mechanism of thermogenesis in mammals, based on decreased H+/e- stoichiometry at high ATP/ADP ratios due to binding of ATP to the heart-type subunit VIa [Frank, V. & Kadenbach, B. (1996) FEBS Lett. 382, 121-124], appears not to occur in tuna, because no isoforms of subunit VIa were found.