Principle of Method:
Hydrolysis of the synthetic substrate 4-methylumbelliferyl-a-L-iduronide 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 a-iduronidase is the primary defect in mucopolysaccharidosis (MPS) type I (Hurler disease and Scheie disease and intermediate phenotypes). These diseases are characterised by increased glycosaminoglycan (GAG) excretion in urine, predominantly of dermatan and heparan sulphates, and the enzyme is assayed to confirm the diagnosis following such abnormal GAG results. All types of MPS I show a marked enzyme deficiency and it is not possible to distinguish them by enzyme assays alone, although study of the Km in cultured fibroblasts may give some information. Prenatal diagnosis of MPS I is possible by assaying the enzyme in chorionic villi or cultured amniotic cells but particular care has to be exercised in the direct assay of chorionic villi. This is because activity of a-iduronidase is quite low in chorionic villi, but high in decidua, so even low levels of maternal contamination could give rise to a false-negative result. Very careful selection of the villi is therefore necessary. The method has application for carrier detection in affected families but is not suitable for screening the general population.
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.