Abnormal liver enzyme activity or concentration Reduced hepatic phosphorylase kinase activity 2024-02-12T08:07:37Z Activity of phosphorylase kinase in liver tissue below the lower limit of normal. Phosphorylase kinase (PhK) has a major regulatory role in the breakdown of glycogen. The enzyme PhK comprises four copies each of four subunits, encoded by PHKA1, PHKA2, PHKB, and PHKG. Phosphorylase kinase (PhK) deficiency causing glycogen storage disease type IX (GSD IX) results from deficiency of the enzyme phosphorylase b kinase, which has a major regulatory role in the breakdown of glycogen. While liver biopsy or muscle biopsy was done routinely in previous years to measure PhK activity, the availability of molecular genetic testing has reduced the need for such invasive procedures. Decreased hepatic phosphorylase kinase activity