High Molecular Weight Phosphorylase Phosphatase from Muscle Glycogen Particles
Type-1 Protein Phosphatase (Phosphorylase Phosphatase) (1) is a key enzyme of glycogen metabolism: it activates Glycogen Synthase and inactivates Phosphorylase, thus stimulating glycogen synthesis. One would expect Phosphatase to be the target of both insulin and adrenaline, the major hormonal regulators of this metabolism. And in fact it is known that adrenaline inhibits phosphatase through the activation of phosphatase inhibitor-1 (2) and possibly also by removing the enzyme from its substrates (3). On the other hand, stimulation of Phosphatase by insulin has never been proven. In the recent years type-1 Phosphatase has been purified to homogeneity and there were great advances in the understanding of the Phosphatase system and of its activation by the kinase FA (1, 4). It has also been realized recently that the same Phosphatase is present at different cell sites (cytosol, glycogen particles and microsomal membranes) where the catalytic subunit is complexed to different regulatory subunits (5). Since the studies on the purified enzyme did not clarify yet some aspects of its regulation, and specifically those related to insulin stimulation, it may be useful to investigate Phosphatase in more crude fractions. It is known in fact that Phosphatase in a crude system, such as muscle extract, has different properties from the purified enzyme and displays a much higher molecular weight (250,000 by gel filtration). High molecular weight Phosphatase complexes are present not only in the cytosol (6), but also in the Phosphatase obtained from glycogen particles (7) and from microsomal membranes (E. Villa-Moruzzi and L.M.G. Heilmeyer Jr., in preparation). It is possible that such complexes are the primary target of the hormone-related signals directed toward Phosphatase. Here it is reported the isolation of high molecular weight Phosphatase from skeletal muscle glycogen particles and its disruption in smaller complexes by affinity chromatography on poly-lysine.
KeywordsCatalytic Subunit Microsomal Membrane Glycogen Metabolism Rabbit Skeletal Muscle Small Complex
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