Arylsulphatase A

Principle of Method:
Hydrolysis of the synthetic substrate p-nitrocatechol sulphate (2-hydroxy-5-nitrophenyl sulphate) is followed by measuring the 5-nitrocatechol formed as determined from its absorbance in alkaline solution at 515 nm. Since other arylsulphatases (ASB and ASC) will also hydrolyse this substrate, incubation conditions are chosen to maximise ASA activity but minimise activity of ASB and ASC. The most convenient method is to carry out the incubation for a relatively long time at 0° C, under which condition ASA displays activity but ASB and ASC are virtually inactive. However, for assaying ASA in plasma or serum for diagnosis of I-cell disease a short incubation at 37° C is carried out.

Uses and Limitations of the Method:
Deficiency of ASA is the primary defect in
metachromyatic leucodystrophy (MLD), and this disease can be diagnosed by demonstrating a severe deficiency of ASA in white cells or fibroblasts. Prenatal diagnosis can be achieved by assaying chorionic villi or cultured amniotic cells. ASA is also deficient in multiple sulphatase deficiency (MSD), and the possibility of this diagnosis should always be considered when low ASA is found. MSD can be confirmed or excluded by assaying one or more further sulphatases. Low ASA activity also occurs in some healthy individuals due to a relatively common allele (the “pseudodeficiency” allele) that is present in 7-15% of the population. This allele can be tested for by a DNA-based method. All patients with low ASA activity should have a histological test for urinary metachromatic material. This requires a fresh random urine which is centrifuged at low speed. The sediment is sent for analysis. A positive result coupled with low ASA activity confirms diagnosis of MLD. Very high ASA activity is observed in plasma and serum from patients with I-cell disease and pseudo-Hurler polydystrophy.

The assay is included in our lysosomal enzyme screening procedure for patients with a neurodegenerative disorder.

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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