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

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Abstract
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.