Phosphorylation of glycogen synthase in a homogenate of human polymorphonuclear leukocytes

Summary

Glycogen synthase I in a homogenate of human polymorphonuclear leukocytes was phosphorylated under imitated physiological conditions utilizing the endogenous protein kinases. At subsequent steps of phosphorylation the³²P-labelled synthase was purified and characterized. Limited tryptic hydrolysis of the³²P-labelled synthase released four phosphopeptides (t-A, t-B, t-C, t-D) and subsequent chymotrypsinization of the trypsin resistant core released three phosphopeptides (c-A, c-B, c-C).

One P_i/subunit was incorporated within 8–10 min and 2.2 P_i/subunit within 60 min increasing the K_c for Gle-6-P to 4–6 mM. The initial phosphorylation up to 0.8 P_i/subunit occurred mainly in peptide c-A and a linear relation between ratio of independence (RI) of glycogen synthase in the interval RI 0.85 to RI 0.05 and phosphorylation of this peptide to 0.5 P_i was observed. Phosphorylation of this peptide is responsible for the decrease in ratio of independence.

From experiments with inhibitors and activators, the initial phosphorylation was found predominantly catalysed by the endogenous cAMP independent synthase kinase, however, the endogenous cAMP dependent protein kinase and phosphorylase kinase also phosphorylate endogenous glycogen synthase I to a minor degree. Circumstantial evidence for a Ca-dependent synthase kinase different from phosphorylase kinase is presented.

The endogenous Gle-6-P dependent glycogen synthase occurring in a homogenate of leukocytes disrupted in the presence of NaF incorporated 1.07 P_i/subunit and K_c for Glc-6-P was increased from 6–8 mM to 20 mM.

From the present and previous experiments [7] a total of 8 major phosphorylatable sites have been defined, one on each of the peptides t-A, t-B, t-C, c-B and c-C and two on peptide c-A, which in addition may contain a third site for phosphorylase kinase. Assuming identical subunits, only 13 out of 32 sites are thus covalently modified at maximum phosphorylation. The operational defined synthase R (K_c for Glc-6-P 0.5 mM) and D (K_c for Glc-6-P 2–8 mM) activities correspond to synthase with about 0.8 P_i and 1.8–2.3 P_i/subunit, respectively.