The aim of this study has been to explore the site specific phosphorylation pattern of rat heart glycogen synthase paying particular attention to phosphorylations that are important to the in vivo control of enzyme activity. This problem has been approached using techniques of immuneprecipitation of ³²P labeled synthase from hormonally responsive, freshly isolated adult rat cardiomyocytes.

Identification of the active subunit of rat heart glycogen synthase was accomplished by immuneprecipitating synthase from ³²P-labeled cardiomyocytes and performing Western blot analysis on DEAE-cellulose fractions containing synthase activity. Using these methods, glycogen synthase activity has been localized to a protein of 88,000 daltons.

Reverse phase HPLC analysis of synthase tryptic peptides from either hormone responsive cardiomyocytes or synthase treated in vitro with cAMP-dependent protein kinase and protein phosphatase-1 (PP-1) resulted in finding six reproducible peaks of phosphopeptides. The incorporation of radioactivity into peaks 1 and 2 was associated with both the treatment of cardiomyocytes with epinephrine and the in vitro phosphorylation of rat heart synthase with cAMP-dependent protein kinase. These same two peaks are selectively dephosphorylated when cAMP-dependent kinase treated synthase is incubated with protein phosphatase-1. This dephosphorylation of peaks 1 and 2 are coincident with the conversion of synthase from the D to the I form. Peak 3 is dephosphorylated upon treatment of cardiomyocytes with insulin and hyperphosphorylated in cardiomyocytes derived from alloxan diabetic animals.

Taken together these results demonstrates the direct relationship between the phosphopeptides in peaks 1 and 2 and the inhibition of synthase activity in response to epinephrine treatment in the cell. This inhibition can be explained by the activity of cAMP-dependent protein kinase which can duplicate the intracellular, epinephrine-stimulated synthase phosphopeptide pattern. This inhibition can be relieved in vitro by protein phosphatase-1 which dephosphorylates peaks 1 and 2. The effect of insulin and alloxan diabetes is localized to peak 3 whose phosphorylation is unaffected in vitro by either cAMP-dependent protein kinase or protein phosphatase-1.