A fluorimetric enzyme assay for the diagnosis of MPS II (hunter disease)

4-Methylumbelliferyl-α-iduronate 2-sulphate was synthesized and shown to be a specific substrate for the lysosomal iduronate-2-sulphate sulphatase (IDS). Fibroblasts (n = 17), leukocytes (n = 3) and plasmas (n = 9) from different MPS II patients showed < 5% of mean normal IDS activity. The enzymatic liberation of the fluorochrome from 4-methylumbelliferyl-α-iduronate 2-sulphate requires the sequential action of IDS and α-iduronidase. A normal level of α-iduronidase activity was insufficient to complete the hydrolysis of the reaction intermediate 4-methylumbelliferyl-α-iduronide formed by IDS. A second incubation step in the presence of excess purified α-iduronidase is needed to avoid underestimation of the IDS activity.

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