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Transient state kinetic studies of calcium dependent atpase and calcium transport by cardiac sarcoplasmic reticulum effect of cyclic amp dependent protein kinase ec catalyzed phosphorylation of phospholamban

, : Transient state kinetic studies of calcium dependent atpase and calcium transport by cardiac sarcoplasmic reticulum effect of cyclic amp dependent protein kinase ec catalyzed phosphorylation of phospholamban. Journal of Biological Chemistry 255(5): 1985-1992

Phosphorylation of the 22,000-dalton protein phospholamban of cardiac microsomes (sarcoplasmic reticulum) by cyclic[c]AMP-dependent protein kinase altered profoundly the intermediary steps of microsomal Ca2+-dependent ATPase in dogs. To define the control of ATPase by its putative regulator phospholamban, transient kinetics of the formation of the phosphorylated intermediate EP [the phosphorylated intermediate of Ca2+-dependent ATPase of cardiac sarcoplasmic reticulum] of ATPase and Ca binding by the membrane through a rapid quenching device, after microsomes were incubated with and without cAMP-dependent protein kinase were examined. When microsomes were suspended with ethylene glycol bis(.beta. aminoethyl ether) N,N'-tetraacetic acid (EGTA) prior to the assay (Ca2+-free microsomes), the initial rates of EP formation and Ca binding were markedly enhanced in phosphorylated microsomes, whereas those incubated with Ca/EGTA buffer (Ca2+-bound microsomes) exhibited less pronounced enhancement. In Ca2+-free microsome, these effects were evident within a wide range of ionized Ca2+ from 0.1-20 .mu.M. Under these conditions, the initial rates and maximal amounts of EP formation were enhanced significantly by microsomal phosphorylation above 10 .mu.M ATP, while those were reduced at lower ATP (1-5 .mu.M). Latter effects were attributed to the predominance of a protein kinase-induced increase in EP decomposition at lower ATP ranges, where the EP formation is rate-determining. cAMP-dependent phosphorylation of phospholamban evidently produces marked increases in the rate of a step associated with Ca binding to the ATPase enzyme and the rate at which EP is subsequently formed, resulting in an increased rate at which Ca is translocated across the microsomal membrane. Together with the report that phospholamban phosphorylation induces a marked increase in the rate of EP decomposition, phospholamban, through modulation of elementary steps of ATPase, evidently could function as a regulator controlling the active Ca transport by cardiac sarcoplasmic reticulum.


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