Phosphorylated cardiac myofibrils and their effect on ATPase activity.

Control guinea pig cardiac myofibrils were isolated in the presence of Triton X-100. Experimental myofibrils, prepared in the presence of Triton X-100, NaF, cyclic AMP and ATP, possessed a reduced myofibrillar ATPase activity. When myofibrils isolated under control conditions were incubated for two hours at 25°C with NaF, ATP and cyclic AMP, the ATPase activity was also decreased; however, the ATPase activity was not reduced as much as that of myofibrils isolated under experimental conditions. Incubation of myofibrils with $\text{E. coli}$ aklaline phosphatase and guinea pig heart phosphoprotein phosphatase resulted in an increase in ATPase activity and a decrease in phosphoprotein phosphate. Thus there appeared to be an inverse relationship between myofibrillar ATPase activity and phosphoprotein phosphate content. The results indicated that a protein kinase is associated with the Triton X-100 purified myofibrils and supports the notion that intact myofibrils can exist in at least two catalytic forms.