Debranching Enzyme from Rabbit Skeletal Muscle

Purification, Properties and Physiological Role

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Abstract

Debranching enzyme was purified 150-fold from rabbit skeletal muscle by a three-step procedure which utilised ammonium sulphate precipitation, ion-exchange chromatography on DEAE-cellulose and “hydrophobic” chromatography on Sepharose-NH(CH2)4 NH2. The preparation was completed within three days, and 200 mg enzyme was isolated from 1000 g muscle, which represented an overall yield of 60 %. The preparation was homogeneous by the criteria of poly-acrylamide gel electrophoresis and ultracentrifugal analysis. The sedimentation coefficient, s20,w, was 8.1 S. The amino acid composition was determined, and the absorption coefficient, A1%280, measured refractometrically was 17.5.

The subunit molecular weight determined by gel electrophoresis in the presence of sodium dodecyl sulphate was 166000 and this value was supported by sedimentation equilibrium in the presence of 6 M guanidinium chloride (155000). The molecular weight of the native enzyme measured by high-speed sedimentation equilibrium was 164000, showing that the debranching enzyme is a monomeric protein at the concentrations which exist in muscle (0.7 mg/ml). The results indicate that the two different enzyme activities which are associated with debranching enzyme, 1,4-glucan-4-glycosyltransferase and amylo-1,6-glucosidase, reside on the same polypeptide chain.

Protein-glycogen particles isolated from skeletal muscle showed seven major protein-staining components by polyacrylamide gel electrophoresis, one of which was identified as debranching enzyme. Four of the other components were the α and β subunits of phosphorylase kinase, glycogen phosphorylase and glycogen synthetase.

A new titrimetric assay for debranching enzyme was developed; it was used to demonstrate that the maximum potential activity of debranching enzyme is only 5–10% that of phosphorylase at the concentrations of the two enzymes in skeletal muscle. Since the activity of debranching enzyme is unaffected by every mechanism which leads to the activation of glycogen phosphorylase and phosphorylase kinase, the evidence suggests that the hormonal control of muscle glycogenolysis by adrenalin might be confined to a stimulation of rate of degradation of the outermost branches of glycogen.

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