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Two different isoforms of subunit VIIa have been found in cytochrome coxidase isolated from human skeletal muscle. The first 22 residues of the N‐terminal amino acid sequences showed 5 differences. Our results provide the first conclusive evidence for the existence of cytochrome coxida isoenzymes in man. Since the two cytochrome coxidase isoforms were both present in skeletal muscle tissue, though not necessarily in the same cell type, this suggests that human cytochrome coxidase isoforms are not strictly tissue‐specific. These findings may have important implications for the elucidation of genetic diseases in man in which a deficiency of cytochrome coxidase is restricted to certain tissues.

Subunit VIII was purified from a preparation of the human heart cytochrome coxidase and its complete amino acid sequence was determined. The sequence proved to be much more related to that of the bovine liver oxidase subunit VIII than to that found in bovine heart. Our finding of a ‘liver-type’ subunit VIII in the human heart enzyme suggests that either there are no isoforms of human subunit VIII or the human oxidase does not show the type of tissue specificity that has been reported for the oxidase in other mammals.