b-Glucosidase

Principle of Method: Hydrolysis of the synthetic substrate 4-methylumbelliferyl-a-D-glucopyranoside at acid pH and in the presence of sodium taurocholate (which inhibits non-specific enzyme activity) is followed by measuring the fluorescence of the liberated 4-methylumbelliferone after stopping the reaction with alkaline buffer.

Uses and Limitations of Method: Deficiency of lysosomal a-glucosidase (glucocerebrosidase) activity is the primary defect in Gaucher disease. Glucocerebroside builds up in the tissue, the spleen usually being the first organ to be enlarged. The substrate used in this method is an artificial alternative to glucocerebroside. Diagnosis of all three main clinical types of Gaucher disease is possible by showing low activity of the enzyme in leucocytes or fibroblasts, but it is not possible to distinguish between types. Prenatal diagnosis is possible by assaying a-glucosidase in chorionic villi or cultured amniotic cells. Heterozygote detection is unreliable because of considerable overlap between heterozygotes and normals.

Metabolite assays are not routinely available to demonstrate excess accumulation of glucocerebroside in Gaucher tissues and so assay of the enzyme is an important diagnostic technique. Serum acid phosphatase is often elevated, and this test may have been done before the patient is referred to the laboratory, and the enzyme chitotriosidase may be elevated in plasma.

The assay is included in our lysosomal enzyme screening procedure for patients with hepato(spleno)megaly.

Specimen Requirements: Blood. 5 ml lithium heparin (orange capped tube) unseparated and unfrozen. Send at room temperature to arrive at the laboratory within 24h of venepuncture. THE LABORATORY RECOMMENDS USE OF A COURIER SERVICE OR ROYAL MAIL SPECIAL DELIVERY FOR SENDING ALL SPECIMENS TO THE LABORATORY.

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