Abstract
Myosin B was extracted at low ionic strength in the presence of ATP from the intestinal smooth muscle of pig. It exhibited a typical Ca++-sensitivity like that of skeletal myosin B. The results obtained were as follows:
1) By lowering the temperature from 25°to 15°C, the Mg++-activated ATPase activity and the rate of superprecipitation were more greatly decreased than those in the case of skeletal myosin B. On the other hand, for the change from 37° to 25°C, the amounts were the same as those of skeletal myosin B.
2) Temperature changes between 10°and 42°C had no significant effect on the Ca++-sensitivity. Viz., it was more stable than that of skeletal myosin B.
3) Without Mg++, the ATPase activity of intestinal myosin B was activated tenfold with the increase of KCl concentration from 0.2 to 0.5 M. With 4 mm Mg++, the ATPase activity at 0.5M KCl without Mg++ was much higher than that at 0.08 M KCl.
4) The ATPase activity at 0.5M KCl without Mg++ was activated maximally with an increase of Ca++ concentration from pCa 5.5 to pCa 4.0. Its Ca++ dependence was very different from that of skeletal myosin B.
The above results may indicate that the differences in the properties of smooth and skeletal myosin B may be due mainly to differences in the properties of myosin.
