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The cardiac sarcolemmal protein kinase reaction. Kinetic characteristics and sensitivity to cyclic AMP.

Isolated and carefully purified preparations of the sarcolemmal membrane from rat cardiac muscle were used to study the membrane-bound protein kinase (PrK) reaction. Contamination of the membrane preparation by mitochondria and sarcoplasmic reticulum was practically absent, and the sarcolemmal membrane vesicles were mostly oriented inside out. These membrane vesicles contained protein kinase tightly bound to the membrane and able to phosphorylate both the endogenous membrane protein with molecular mass of 11,500 daltons and the exogenous added protein (histone, type II). Endogenous protein phosphorylation was completely independent of cAMP which only stimulated the phosphorylation of histone in the membrane-bound PrK reaction. The kinetics of the membrane-bound PrK reaction with MgATP and histone as substrates was studied, and the kinetic mechanism was found to be of sequential Bi-Bi type. Kinetic characteristics of the enzyme were very close to those found earlier for the soluble muscle enzyme. On the basis of data obtained, it is suggested that membrane-bound PrK may be of the same origin as the soluble enzyme and that cAMP may influence the binding of PrK to the surface membrane of cardiac cells.