Characterization of cyclic 3′:5′-AMP-dependent protein kinase in sarcoplasmic reticulum and cytosol of canine myocardium

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Canine cardiac sarcoplasmic reticulum vesicles contain intrinsic cAMP-dependent and Ca 2+-calmodulin-dependent protein kinase (EC 2.7.1.37) activities and a common substrate, phospholamban, for these enzymes. Cyclic AMP-dependent protein kinase associated with sarcoplasmic reticulum membranes was solubilized with Triton X-100. Solubilization of the sarcoplasmic reticulum protein kinase did not affect its dependency on cAMP or its substrate specificity. The solubilized cAMP-dependent protein kinase was purified by DEAE-cellulose chromatography and was characterized as a type II enzyme on the basis of its elution at high ionic strength. DEAE-purified cAMP-dependent protein kinase exhibited no Ca 2+-calmodulin-dependent protein kinase activity. Cytosol from canine cardiac muscle cells, chromatographed on DEAE-cellulose under conditions identical to those used with sarcoplasmic reticulum, exhibited the presence of both type I and type II cAMP-dependent protein kinase isozymes. The properties of the DEAE-cellulose purified type II protein kinases from sarcoplasmic reticulum and cytosol were similar. We conclude that cardiac sarcoplasmic reticulum contains primarily type II cAMP-dependent protein kinase and this is probably the enzyme which phosphorylates sarcoplasmic reticulum in vivo and regulates Ca 2+ transport.