CYTOSOLIC AND SARCOPLASMIC RETICULUM-ASSOCIATED LOW KM, CGMP-INHIBITED CAMP PHOSPHODIESTERASE IN MAMMALIAN MYOCARDIUM

The distribution and phosphoprotein band patterns of low Km, cGMP-inhibited cAMPphosphodiesterase (cGI PDE) activity were examined in cytosolic and microsomal fractions of human, canine, rabbit and guinea pig left ventricular myocardium following phosphorylation by cAMP-dependent protein kinase, immunoprecipitation with anti-cGI PDE antibodies and SDS-PAGE. The recovery of cGI PDE activity in cytosolic and microsomal fractions was comparable in all four species. Microsomal cGI PDE was comprised chiefly of a ∼135 kDa phosphoprotein. Cytosolic cGI PDE was comprised solely of ∼116 kDa and lower molecular weight phosphoproteins. The ∼135 kDa phosphoprotein probably corresponds to the holoenzyme encoded by the recently cloned cDNA for human myocardial cGI PDE, whose predicted molecular weight is 126 kDa. The ∼116 kDa phosphoprotein may result from deletion or removal of putative membrane-association domains from the N-terminal region of the holoenzyme. These results suggest that the cytosolic and sarcoplasmic reticulum-associated forms of mammalian myocardial cGI PDE are separate molecular species. © 1993 Academic Press, Inc.