Analysis of a tissue sample obtained on biopsy of the liver in a patient with GSD shows normal activities of glycogen phosphorylase, glycogen debranching enzyme, and glucose 6-phosphatase. A defect in the signal transduction pathway for glycogen metabolism is suspected. Which of the following enzymes is most likely to be defective?

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Phosphorylase kinase is involved in the signal transduction pathway for glycogen metabolism, activating glycogen phosphorylase to break down glycogen. A defect here would affect glycogen metabolism despite normal enzyme activities.  A deficiency in the activity of phosphorylase kinase results in a glycogen storage disease (GSD9) whose symptoms include hepatomegaly, mild hypoglycemia, hyperlipidemia, and ketosis. These symptoms are similar to those exhibited by GSDVI, also known as Hers disease.

Analysis of a Tissue Sample Obtained on Biopsy of the Liver