Deficiency of muscle phosphorylase
(myophosphorylase or alpha-1,4-glucan orthophosphate glycosyl transferase),
which initiates glycogenolysis by removing 1,4-glucosyl groups with release
of glucose-1-phosphate. Myophosphorylase is the only isoform present in
skeletal muscle (but also in the heart and the brain). Patients are unable
to release glucose from glycogen in muscle. The production of adenosine
triphosphate via the Krebs cycle is compromised and the exercising muscle
derives its energy from blood glucose and free fatty acids, which might
account for the second-wind phenomenon experienced by these patients, (i.e., progressive fatigue
and muscle weakness develops after 10 to 15 min of exercise. Following a rapid recovery,
exercise can be resumed without difficulties for a prolonged time period.)