A fluorimetric enzyme assay for the diagnosis of Morquio disease type A (MPS IV A)

4-Methylumbelliferyl-β-D-galactopyranoside-6-sulphate was synthesized and used for the determination of galactose-6-sulphate sulphatase activity. Fibroblasts and leucocytes from 12 different Morquio A patients, showed 0.0–2.7% of mean normal galactose-6-sulphate sulphatase activity. Heterozygotes showed intermediate activities. The enzymatic liberation of the fluorochrome from 4-methylumbelliferyl-β-D-galactopyranoside-6-sulphate requires the sequential action of galactose-6-sulphate sulphatase and β-galactosidase. Normal β-galactosidase activity caused nearly complete hydrolysis of non-fluorescing 4-methylumbelliferyl-galactoside, formed during incubation. In cell extracts with a β-galactosidase deficiency however, a second incubation in the presence of excess β-galactosidase is needed to avoid underestimation of galactose-6-sulphate sulphatase activity.