About Muscle Glycogen Phosphorylase Deficiency (GSD V)
Posted by biochemistryquestions on May 5, 2008
Answer to CM-02
Answer: (c)
An important energy source for exercising muscle is anaerobic glycolysis, using glucose from the blood, that is converted to glucose 6 (P) inside the muscle cells, and glucose 6 (P) formed from glycogen stored in the muscle. The final product of anaerobic glycolysis is lactate.
The lactate diffuses from the muscle to the blood and then is transported to the liver where it is converted in pyruvate.
This patient is not able to use the glycogen stored in his muscle to obtain energy, since he lacks muscle glycogen phosphorylase, which degrades glycogen to glucose 1 (P). Normally this glucose 1 (P) becomes glucose 6 (P) in a reaction catalyzed by phosphoglucomutase and then glucose 6 (P) enters glycolysis. Due to this patient inability to degrade glycogen, it accumulates in the muscle fiber (it is called Glycogen Storage Disease Type V or McArdle Disease), the patient present fatigue very early in the test and the concentration of lactate in blood drawn from the forearm is lower than in a normal person.
Glucose in blood is not affected since muscle glycogen does not buffer blood glucose.
For more information about McArdle Disease, click on these links
Cupler, E.J.: Glycogen Storage Disease Type V
Stojanov, L.:Glycogen Storage Diseases Type I-VII
Association for Glycogen Storage Disease
The association for Glycogen Storage Disease UK
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