Assays / Genetic
Enzymes / b-Galactosidase
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
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
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.
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.
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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