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

4-Methylumbelliferyl-alpha -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-alpha -iduronate 2-sulphate requires the sequential action of IDS and alpha -iduronidase. A normal level of alpha -iduronidase activity was insufficient to complete the hydrolysis of the reaction intermediate 4-methylumbelliferyl-alpha -iduronide formed by IDS. A second incubation step in the presence of excess purified alpha -iduronidase is needed to avoid underestimation of the IDS activity.