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Chapter 14: Carbohydrates: Glycogen Metabolism

You are examining the biochemical characteristics of the liver dysfunction in your patient who is exhibiting signs of a GSD. You have isolated the microsomal fraction (contains the endoplasmic reticulum) of a liver biopsy homogenate from your patient and a control individual for your studies. Incubation with radioactive phosphate–labeled glucose 6-phosphate results in an increase in isotope associated with the microsomes from your control sample but no increase in association with the microsomes from your patient. These results are best explained by a defect in which of the following?

A. glucose-6-phosphatase activity in the microsomes

B. microsomal glucose-6-phosphate transporter

C. cytosolic glucose 6-phosphatase

D. microsomal glucose transporter

E. microsomal phosphate transporter

Answer B: The patient is suffering from a form of type I glycogen storage disease (GSDI). The most common form of the disorder is referred to as von Gierke disease (also called type Ia) and results from a defect in the gene encoding glucose 6-phosphatase. However, some type I patients, although exhibiting identical symptoms turn out not to be deficient in glucose 6-phosphatase. These latter patients are identified as having type Ib GSD. The mechanism by which free glucose is released from glucose 6-phosphate involves several different steps. Glucose 6-phosphate must first be transported from the cytosol into the lumen of the endoplasmic reticulum, ER. Inside the ER, the phosphate is removed through the action of ER-localized glucose 6-phosphatase. Type Ib disease results from defects in the glucose-6-phosphate transporter 1. Thus, liver cells from type Ib patients will not accumulate glucose inside the membranes of the ER.

You are examining a 5-month-old infant who was brought to your office by distressed parents. Their infant has demonstrated progressive weakness and difficulty eating and appears to have a bulging abdomen. Upon examination, you find striking hypotonia and demonstrable palpable liver. Chest x-ray indicates prominent cardiomegaly. Before a correct diagnosis could be made, the infant lapses into a coma and dies. At autopsy, the heart was sectioned and stained with the result shown. Which of the following diseases is most likely given the symptoms and outcomes?

Image not available.

A. Andersen disease

B. Cori (Forbes) disease

C. McArdle disease

D. Pompe disease

E. von Gierke disease

Answer D: Glycogen storage disease type II (GSDII) is also known as Pompe disease and results from defects in the gene encoding lysosomal acid maltase. Thus, the disease is also referred to as ...

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