Thermal transitions were studied by means of differential scanning calorimetry (DSC) and a spectrophotometric method. Three endothermic peaks (40,43,5o"C: ordinary muscle; 46,54, 62°C: dark muscle) were observed in DSC thermograms of both myosins. Thermograms of S-l fragments showed one peak (41°C: ordinary muscle, 43°C: dark muscle). But ordinary and dark muscle rod fragments gave two peaks (41, 62°C) and one peak (58"C), respectively. The spectrophotometric results also showed two thermal transitions for both myosins and one transition for their S-l fragments. However, the rod from ordinary muscle myosin had two transitions, whereas that from dark muscle myosin had one transition.
Changes in activities of actomyosin, acto-heavy meromyosin (acto-HMM), and acto-subfragment-(acto-S-I) ATPases from tuna and sardine due to heat treatment (20", 25", 30", 35", 40°C) were compared for ordinary muscle and dark muscle. Activation of ordinary muscle actomyosin Mg-ATPases was more than doubled for tuna and tripled for sardine by heating at 35"C, while activation of dark muscle actomyosin was not observed at any temperature. The occurrence of thermal activation corresponded to a rapid loss of the EDTA-ATPase activity. Activation of hybrid actomyosins from dark and ordinary muscles was dependent upon myosin. For acto-HMM and acto-S-I thermal activation was not observed. The role of myosin tail fragments in thermal activation is discussed.
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