Human liver glycogen phosphorylase. Kinetic properties and assay in biopsy specimens

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Abstract
The 2 forms of glycogen phosphorylase were purified from human liver, and some kinetic properties were examined in the direction of glycogen synthesis. The b form has a limited catalytic capacity, resembling that of the rabbit liver enzyme. It is characterized by a low affinity for glucose 1-phosphate, which is unaffected by AMP, and a low V, which becomes equal to that of the a form in the presence of the nucleotide. Lyotropic anions stimulate phosphorylase b and inhibit phosphorylase a by modifying the affinity for glucose 1-phosphate. Both enzyme forms are easily saturated with glycogen. These kinetic properties allowed the design of a simple assay method for total (a + b) phosphorylase in human liver. It requires only 0.5 mg of tissue, and its average efficiency is 90% when the enzyme is predominantly in the b form. The assay of total phosphorylase allows the unequivocal diagnosis of hepatic glycogen-storage disease caused by phosphorylase deficiency. One patient with a complete deficiency is reported. The assay of human liver phosphorylase a is based on the preferential inhibition of the b form by caffeine. The a form displays the same activity when measured by either of the 2 assays.