Chapter 154

Glycogen, the storage form of glucose in animal cells, is composed of glucose residues joined in straight chains by α–1,4 linkages and branched at intervals of 4 to 10 residues with α–1,6 linkages. The treelike molecule can have a molecular weight of many millions and may aggregate to form structures recognizable by electron microscopy. In muscle, glycogen forms β particles, which are spherical and contain up to 60,000 glucose residues. Each β particle contains a covalently linked protein called glycogenin. Liver contains β particles and rosettes of glycogen called α particles, which appear to be aggregated β particles.

The primary function of glycogen varies in different tissues. In skeletal muscle, stored glycogen is a source of fuel that is used for short-term, high-energy consumption during muscle activity; in the brain, the small amount of stored glycogen is used during brief periods of hypoglycemia or hypoxia as an emergency supply of energy. In contrast, the liver takes up glucose from the bloodstream after a meal and stores it as glycogen. When blood glucose levels start to fall, the liver converts glycogen back into glucose and releases it into the blood for use by tissues such as brain and erythrocytes that cannot store significant amounts of glycogen.

Glycogen storage diseases (GSDs) are inherited disorders that affect glycogen metabolism. Disorders in virtually every enzyme involved in the synthesis or degradation of glycogen and its regulation cause some type of glycogen storage disease. Excluded from this section are those conditions in which tissue glycogen accumulation is secondary, such as overtreatment of diabetes mellitus with insulin or administration of pharmacological amounts of glucocorticoids.

There are more than 12 forms of glycogenoses (see Fig. 154-1). The glycogen found in these disorders is abnormal in quantity or quality, or both. Historically, the glycogen storage diseases were categorized numerically in the order in which the enzymatic defects were identified. This section classifies the diseases by the organs involved and the clinical manifestations.

Liver and muscle have abundant glycogen and are the most commonly and seriously affected tissues. The glycogen storage diseases that principally affect the liver (see Fig. 154-1) include glucose-6-phosphatase deficiency (type I), debranching enzyme deficiency (type III), branching enzyme deficiency (type IV), liver phosphorylase deficiency (type VI), phosphorylase kinase deficiency (type IX), liver glycogen synthetase deficiency (type 0), and glucose transporter-2 defect (type XI). Because carbohydrate metabolism in the liver affects plasma glucose levels, the disorders of hepatic glycogen degradation and glucose release typically cause fasting hypoglycemia and hepatomegaly. Some glycogen storage diseases are associated with liver complications; for example, hepatic adenomas with risk for malignant transformation in GSD I and liver cirrhosis with risk for progression to end-stage liver failure or malignancy (types III, IV, and some forms of type IX). Other organs besides liver may also be involved; for example, renal dysfunction in glycogen storage disease type I; cardiac involvement in types II, III, and IX; myopathy ...

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