Biochemical characteristics of ATPase and Ca2+ [calcium ions]-sensitivity of myosin from Akazara smooth adductor and surf-clam foot muscles

Myosins were successfully isolated from the smooth muscles of Akazara adductor and surf-clam foot by the method of EBASHI, which has been used for preparation of chichen gizzard myosin. The properties of ATPase activity of these myosins were similar to those of Akazara striated adductor myosin and of rabbit skeletal myosin, except for the specific activity level and its KCl concentration dependence. Judging from the SDS-and urea-gel electrophoretic patterns, Akazara smooth muscle myosin was shown to contain two different kinds of regulatory light chains (so-called EDTA-LC) besides heavy chain and essential light chain (so-called SH-LC), while surf-clam foot muscle myosin was found to contain only one kind of regulatory light chain, as other muscle myosins do. The regulatory light chains can be reversibly and completely removed from both smooth muscle myosins by washing with 10mM EDTA at 0-4°C, resulting in complete loss of Ca2+ -sensitivity of Mg-ATPase activity and of superprecipitaion ability. From these results, both smooth muscle myosins appeared to be grouped into the same type of Akazara and scallop striated adductor myosins, whose regulatory light chains are readily removable.