THE SUBUNIT STRUCTURE OF CYTOCHROME-C-OXIDASE FROM TUNA HEART AND LIVER

Cytochrome-c oxidase was isolated from tuna Liver and heart, and the s ubunit 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-c oded subunits were determined after blotting onto poly(vinylidene difl uoride) membranes or by tryptic hydrolysis of gel bands and HPLC separ ation of peptides, respectively. 13 subunits were identified with isof orms 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 su bunits 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 t he heart-type subunit VIa [Frank, V. & Kadenbach, B. (1996) FEBS Lett. 382, 121-124], appears not to occur in tuna, because-no isoforms of s ubunit VIa were found.