Identification of tissue‐specific isoforms for subunits Vb and VIIa of cytochrome c oxidase isolated from rainbow trout

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Abstract
Cytochrome c oxidase was isolated from heart and liver of rainbow trout (Salmo gairdnerii). SDS/PAGE analysis showed the presence of 11 different polypeptide subunits in the fish enzyme. The nuclear‐coded subunits IV, Va, Vb, VIc, VIIa, VIIc and VIII could be identified by their N‐terminal amino acid sequences. The mammalian subunits VIa and VIlb appear to be absent (or blocked at the N‐terminal) in cytochrome c oxidase from trout. For subunit Vb, two polypeptides of different electrophoretic mobilities were found which differed in their N‐terminal sequences, and represent a new pair of cytochrome‐c‐oxidase subunit isoforms, not found in mammalia. Both isoforms of subunit Vb were found in cytochrome c oxidase from heart and liver, but at different ratios. Subunit VIIa also seemed to occur in different isoforms, whereas subunit VIII had the same N‐terminal amino acid sequence in cytochrome c oxidase of liver and heart, similar to the human‐type subunit but different from rat, bovine and chicken.