The structure and metabolism of mammalian glycogens

Abstract

Mammalian liver and skeletal muscle glycogens were extracted using mild procedures and characterised according to their fine structure, molecular weight distribution and electron microscopic appearance. The role of protein in their structure was investigated. Rat, rabbit and mouse liver and muscle glycogens were polydisperse ranging in size up to more than one thousand million daltons. Whilst there are reports of liver glycogen covering such a size range, skeletal muscle glycogen of this size has not been previously reported. The glycogens contained a significant level of protein, some of which could be removed without altering the molecular weight distribution. The residual protein accounted for approximately 1% by weight of the purified glycogen, and it was concluded that this protein was covalently bound. Protease or concentrated alkali treatment of the glycogen digested the protein and resulted in a dramatic lowering of the molecular weight of glycogen, indicating that it was covalently bound protein which was responsible for the formation of high molecular weight material. Disulphide bond reduction also caused a lowering of molecular weight, indicating that high molecular weight glycogen arises by disulphide bridging between the protein backbones upon which the low molecular weight glycogen is synthesised. Thus liver and skeletal muscle glycogens are constructed in a similar manner. The fine structure of the glycogens was typical of that previously reported. When observed by electron microscopy the material appeared as spherical β-particles and large aggregates of these, the α-particles, thus confirming the presence of a high molecular weight component of skeletal muscle glycogen. The sizes of the products of TCA or KOH extraction of tissue glycogen were explained by the effects of these agents upon the glycogen molecule. Alkali digests the protein backbone and splits the glycogen β-particles, while TCA causes insolubility of the high protein content, high molecular weight glycogen. The protein backbone of rat liver glycogen was isolated. It had a molecular weight of 60,000 daltons and was rich in serine, glutamate, and the hydrophobic amino acids. Lysosome-enriched fractions were isolated from rat liver and skeletal muscle. Both contained glycogen; approximately 10% of tissue glycogen for the liver fraction and approximately 5% for the muscle fraction. The lysosomal glycogen of both tissues was enriched in the high molecular weight component. Thus in both tissues glycogen metabolism is compartmentalised, and degradation is via both phosphorolytic and hydrolytic pathways. The lysosomal acid α-glucosidases showed a preference for low rather than high molecular weight glycogen as substrate. A model for the regulation of lysosomal breakdown of glycogen was proposed. High and low molecular weight liver and muscle glycogens showed inhomogeneous responses upon starvation and rapid post mortem glycogenolysis. Both phosphorolysis and hydrolysis were involved. The increase in glycogen level upon refeeding following starvation was inhomogeneous with respect to glycogen size, in both tissues. The metabolic inhomogeneity of glycogen was related to its structural inhomogeneity and the association of the high molecular weight component with the lysosome.