Principle Of Method:
Hydrolysis of the synthetic substrate 4-methylumbelliferyl-a-D-galactopyranoside at acid pH is followed by measuring the fluorescence of the liberated 4-methylumbelliferone after stopping the reaction with alkaline buffer.
Uses And Limitations Of The Method:
Deficiency of lysosomal b-galactosidase activity is the primary defect in GM1-gangliosidosis. The substrate used in this method is an artificial alternative for ganglioside GM1. Variant forms of the disease referred to as type 1 (or generalised gangliosidosis), type 2 (juvenile) and type 3 (adult) exist. All types display low b-galactosidase activity and the deficiency tends to be greater the earlier the onset. The enzyme is also deficient in Morquio disease type B (mucopolysaccharidosis IVB), which is allelic with GM1-gangliosidosis, and in galactosialidosis, which is non-allelic. The latter disease should always be considered when low enzyme activity is found. It can be distinguished by the presence of low neuraminidase in fibroblasts in addition to low a-galactosidase but normal activities of other lysosomal enzymes. Prenatal diagnosis is possible using chorionic villi or cultured amniotic cells for GM1-gangliosidosis and Morquio B disease. The laboratory has no experience in prenatal diagnosis of galactosialidosis. The method has application in carrier detection for GM1-gangliosidosis in affected families but is not suitable for screening the general population.
Patients with all these diseases have abnormal oligosacchariduria and also have keratan sulphate or keratan sulphate-like material in the urine, visible by two-dimensional electrophoresis. These tests may help in the differential diagnosis.
b-galactosidase is a convenient “reference enzyme” to assay in investigation of other storage disorders to confirm adequacy of the sample.
The assay is included in all our lysosomal enzyme screening procedures.
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.