INTRINSIC REGULATION OF GLYCOGEN PARTICLES: LIVER v. SKELETAL MUSCLE

Abstract

Regulation of glycemia requires the precise control of glycogen metabolism. Regulation intrinsic to the protein-glycogen complex can be studied by “flash activation” of glycogenolysis. In this procedure, the addition of Ca++ and MgATP to purified rabbit skeletal muscle glycogen particles resulted in conversion of 80% of phosphorylase (P) to the active form in 1 min, followed by return to baseline (5%) by 2 min. Using 32P-ATP, phosphorylation of P, β-subunit of phosphorylase kinase (PK), glycogen synthase (GS) and an unidentified 32kDa protein was detected by PAGE-SDS. Activation of P and 32p labeling were Ca++-dependent and temporally parallel, as were inactivation and dephosphorylation. Unidentified proteins at 69kDa and 19kDa were labeled independently of Ca++ and were not dephosphorylated. In contrast, hepatic glycogen particles could not be “flash activated”, even with attempts to further optimize conditions. Labeling showed no Ca++-dependent phosphorylation, but addition of cAMP-dependent protein kinase phosphorylated α- and β-subunits of PK (as well as GS and the 32kDa protein) in liver, as it had in skeletal muscle preparations. Nonetheless, “flash activation” of hepatic glycogen particles did not occur even upon addition of cAMP or the protein kinase catalytic subunit. We conclude: 1) Only Ca++ and MgATP are needed to trigger glycogenolysis in muscle glycogen particles; 2) Additional unknown factors are required in liver glycogen particles, even following cAMP-dependent phosphorylation; 3) There are fundamental differences in the regulation of liver and muscle glycogen metabolism.