Reversible Changes in the ATPase Activity and in the Regulatory Light Chain Content upon Heat (30°C)-Treatment of “Akazara” Striated Adductor Myosin

Myosin from striated adductor muscle of “Akazara” scallop was incubated at 30°C for 5 min in a medium containing 2 mM MgCl2 and various concentrations of Ca2+ ions. It was observed that the 30°C-treatment resulted in a decrease in the Ca2+-sensitivity of myosin-ATPase as well as in the release of the regulatory light chain (EDTA-LC) of myosin. The 30°C-treated myosin was then subjected to a cooling treatment, being kept for 18 h at 0°C. It was found that EDTA-LC recombined with myosin and that Ca2+-sensitivity of myosin-ATPase was restored. It was also found that Ca2+ alone was about 70 times more effective than Mg2+ alone in preventing the heat-induced release of EDTA-LC from occurring and also in recombination of EDTA-LC with the heat-treated myosin.