Assays / Genetic Enzymes / Glycogen Brancher Enzyme

PRINCIPLE OF METHOD: The assay is an indirect one and depends on measuring the stimulating effect of the branching enzyme on the activity of phosphorylase in polymerising glucose from glucose 1-phosphate in the presence of AMP. The liberated inorganic phosphate is determined with a kit from Boehringer.

USES AND LIMITATIONS OF THE METHOD: The normal function of the branching enzyme is to transfer six or more a1,4-linked glucose units from the outer region of a glycogen chain into an a1,6- position on the same or neighbouring chain. In the absence of the enzyme, as occurs in glycogen storage disease type IV (Andersen disease), glycogen of abnormal structure accumulates in the liver leading to cirrhosis, ascites and portal hypertension. Prenatal diagnosis is possible by assaying the enzyme in chorionic villi or cultured amniotic cells but is not offered by the SAS laboratory.

SPECIMEN REQUIREMENTS: Fibroblasts cultured from a skin biopsy are needed. Biopsy material should be collected aseptically into a sterile bottle containing tissue culture medium (available from the laboratory), and sent at room temperature to arrive at the laboratory within 24 hours. Biopsies for tissue culture should not be frozen. Fibroblast cultures established in other laboratories should be sent in plastic 25 cm2 flasks filled with medium.

THE LABORATORY RECOMMENDS USE OF A COURIER SERVICE OR ROYAL MAIL SPECIAL DELIVERY FOR SENDING ALL SPECIMENS TO THE LABORATORY.

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