MODIFICATION OF CALCIUM REQUIREMENTS FOR ACTIVATION OF CARDIAC MYOFIBRILLAR ATPASE BY CYCLIC-AMP DEPENDENT PHOSPHORYLATION

Phosphorylation of dog cardiac myofibrils with cyclic AMP dependent protein kinase incorporated 0.8 nmol phosphate mg/protein. This phosphorylation was associated with a righward shift of the relationship between free calckum and myofibrillar ATPase with no change in basal or macimal ATPase. Half-maximal activating free calcium was 0.2 μM in control and 0.1 μM in phosphorylated myofibrils. Calcium bound to phosphorylated myofibrils was significantly depressed at all free calcium concentrations studied. Dephosphorylation of control myofibrils with alkaline phosphotase caused a relatively small leftward shift of the relation between free calcium and myofibrillar ATPase and a significant elevation in myofibrillar calcium binding. Our data suggest that phosphorylation of myofibrillar troponin I alters the calcium binding properties of troponin C. © 1979.