Myosin denaturation in heated myofibrils of scallop adductor muscle

The thermal inactivation of Ca²⁺ ATPase of scallop myofibrils (0.1 M KCl, pH 7.5) was found to be unaffected by the presence of Ca²⁺. Monomeric myosin content and salt solubility decreased much faster than Ca²⁺ ATPase inactivation in both Ca and EDTA media, which was well explained by faster denaturation of the rod portion than subfragment-1 of myosin. In contrast, when the myofibrils were heated at 0.5 M KCl, a slow decrease in salt solubility was observed, which was also explained by slow denaturation of the rod portion of myosin. Myofibrils from scallop smooth muscle showed the same denaturation pattern as those from adductor muscle. These results show that mollusk myosin is not always stabilized by Ca²⁺.