Rabbit cardiac myosin contains fewer cysteine residues than the skeletal myosin (7 and 8.8 moles/105 gm. of myosin, respectively). A similar difference is found between the cysteine content of rabbit cardiac and skeletal heavy meromyosins; the cardiac heavy meromyosin contains 8.9 moles/105 gm. of protein as compared to 11 moles in the skeletal heavy meromyosin. In skeletal myosin, actomyosin, and myofibrils the Ca++-ATPase, Ca++-ITPase, and EDTA-ATPase activities are about three times higher than those of cardiac myosin, actomyosin, and myofibrils; whereas the skeletal to cardiac actomyosin-ATPase activity ratio is higher. The ATPase activities of both cardiac and skeletal myosins, actomyosins, and myofibrils, however, are close to each other when determined in the presence of Mg++ at high ionic strength. The abilities of cardiac and skeletal myosins to combine with actin at either high or low ionic strength are also essentially the same. The Ca++-ATPase, Ca++-ITPase, EDTA-ATPase, and actomyosin-ATPase activities of cardiac myosin, heavy meromyosin, and myofibrils, unlike those of skeletal myosin, heavy meromyosin, and myofibrils, do not increase over pH 8.0. The ATPase activities of cardiac and skeletal myosins in the presence of Mg++ at high ionic strength, on the other hand, are affected similarly by changes of pH. In cardiac myosin, heavy meromyosin, and myofibrils, the Ca++activated ATPase is less sensitive to high KC1 concentrations than is that of skeletal myosin, heavy meromyosin, and myofibrils.
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