| Abstract
| - For both cardiac and skeletal myosin, the Ca-ATPase activity of myosin at acidic pH was shown to be different from that at alkaline pH, in the susceptibility to heat-inactivation, the effects of organic solvents, and the effect of trinitrophenylation of the myosin. It is therefore suggested that there are two different types of Ca-ATPase of both cardiac and skeletal myosin. Differences in the Ca-ATPase activity were also found between cardiac and skeletal myosins. (a) The Ca-ATPase activity of cardiac myosin was more susceptible to heat-inactivation at alkaline pH than at acidic pH. In contrast, the activity of skeletal myosin was more susceptible to heat-inactivation at acidic pH than at alkaline pH. (b) Dioxane weakly stimulated the activity of cardiac myosin at acidic pH, but strongly activated that of skeletal myosin at acidic pH. Acetone very strongly inhibited the activity of cardiac myosin at alkaline pH, but not so strongly that of skeletal myosin at alkaline pH. (c) Trinitrophenylation of the myosin resulted in loss of the activity optimum at acidic pH with skeletal myosin but not with cardiac myosin. As reported by Srivastava et al. (J. Biochem. 86, 725-731, 1979), 1 mol of lysine residue per mol of cardiac myosin quickly reacted with 2, 4, 6-trinitrobenzene sul-fonate (TNBS) either in the absence or presence of inorganic pyrophosphate (PP1). However, trinitrophenyl (TNP) groups bound to cardiac myosin in the presence of PP1 were significantly different, in the pH dependence of the absorption spectrum, from those bound (to cardiac myosin) in the absence of PP1. Moreover, the difference was found to be very similar to that observed between TNP groups bound to skeletal myosin in the presence of PP1 and those bound (to skeletal myosin) in the absence of PP1. It is therefore suggested that the sites on cardiac myosin for tri-nitrophenylation in the presence of PP1 are different from the sites for trinitrophenyl-ation in the absence of PP1, as much as the sites on skeletal myosin are.
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