The muscle specificity and structure of two closely related fast-twitch white muscle myosin heavy chain isozymes.

The structures of the heavy chains of fast-twitch white muscle myosin isolated from a number of specific rabbit muscles were examined by the technique of tryptic peptide mapping of iodinated proteins from sodium dodecyl sulfate polyacrylamide gels. The results suggest that the distribution of the two closely related heavy chain isozymes postulated to exist by Starr and Offer ((1973) J. Mol. Biol. 81, 17-31) vary in a muscle-specific manner. One isozyme appears to be preferentially located in the extensor digitorum longus muscle, while the other appears to be preferentially located in the semitendinosus muscle. Other muscles, such as the gastrocnemius and the latissimus dorsi, appear to contain the two isozymes in a nearly 1:1 ratio. Fine structure mapping of myosin fragments revealed that the two muscle-specific isozymes differ in a number of locations throughout the head and tail portions of the myosin heavy chain.