Abstract
Myosin of the obliquely striated mantle muscle of squid, Ommastrephes sloani pacificus, was isolated by the authors' method and its ATPase profiles were studied under various reaction con-ditions. With Ca2+ (5mM) or Mg2+ (1mM) present and in 600mM KCI, the activity-pH curve showed 2 peaks, at pH 7.0 and near pH 8.5, where the activity with Ca2+ was higher than that with Mg2+. The activity-ionic strength curve gave a peak at 0.2M KCI with declining slope beyond that concentration. Ca2+ activated the ATPase with a maximum effect at 1-9mM, while Mg2+ caused a slight activation with a maximum effect at 0.1mM. The activity was depressed with EDTA for all concentrations between 0.1-1mM, but in a different manner from the ATPase of skeletal muscle myosins. PCMB did not affected either Ca2+-ATPase or EDTA-ATPase. These profiles of the squid myosin ATPase were compared with those of the myosin ATPases from other sources. The squid myosin ATPase was found to be very different from the skeletal muscle myosin ATPases and, in some detailed points, distinct from the myosin ATPases from invertebrate and nonmuscular sources.
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