A fluorimetric enzyme assay for the diagnosis of Sanfilippo disease type D (MPS IIID)

4-Methylumbelliferyl-α-N-acetylglucosamine 6-sulphate was synthesized and shown to be a substrate for the lysosomal N-acetylglucosamine-6-sulphate sulphatase (GlcNAc-6S sulphatase). Fibroblasts and leukocytes from 3 different Sanfilippo D patients showed <1% of mean normal GlcNAc-6S sulphatase activity. The enzymatic liberation of the fluorochrome from 4-methyl-umbelliferyl-α-N-acetylglucosamine 6-sulphate requires the sequential action of the GlcNAc-6S sulphatase and α-N-acetylglucosaminidase. A normal level of α-N-acetylglucosaminidase activity was insufficient to complete the hydrolysis of the reaction intermediate 4-methylumbelliferyl-α-N-acetylgluco-saminide formed by the GlcNAc-6S sulphatase. A second incubation in the presence of excess α-N-acetyglucosaminidase is needed to avoid underestimation of the GlcNAc-6S sulphatase activity