Abstract
1. 1. Horseshoe crab ( Limulus polyphemus) myofibrillar ATPase has good Ca 2+ activation at pH 9 with activity ranging over 1 μmole P (mg protein) −1 (5 min) −1 at 25°C. 2. 2. Magnesium also activates this enzyme system at neutral pH with activities only one-third or one-quarter as high but nevertheless a true activation when compared with the activity in the absence of divalent cations. 3. 3. Manganese activates but only slightly at pH 7, whereas the activity at pH 9 is almost as high as the activity of isolated myosin B with Mn 2+. 4. 4. There is a double pH optimum, at pH 7 and 9, and the system hydrolyzes ATP, ITP, UTP and CTP. 5. 5. EDTA and EGTA appear to inhibit the Ca 2+ system by chelating an equimolar amount of Ca 2+. 6. 6. Inhibition of the Mg 2+ system, on the other hand, cannot be explained by chelation of the activating Mg 2+.
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