Morquio B syndrome: A primary defect in β‐galactosidase

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Abstract
Fibroblasts from patients with Morquio B syndrome contain normal numbers of β‐galactosidase molecules with normal turnover but strongly reduced activity per enzyme molecule. Various substrate affinities are abnormal: the KM for methylum belliferyl (MU)‐β‐galactoside is 4–10‐fold elevated and affinity for keratan sulphate and oligosaccharides, isolated from Morquio B urine, was not detectable. In contrast, these substrate affinities are normal for β‐galactosidase in adult type GM1‐gangliosidosis fibroblasts. Cell hybridization studies demonstrate that Morquio B syndrome and infantile and adult type GM1‐gangliosidosis belong to the same complementation group. From these results we conclude that Morquio B syndrome is caused by a mutation in the structural gene for β‐galactosidase, which is allelic to the mutations in infantile and adult type GM1‐gangliosidosis. Urinary excretion of keratan sulphate and oligosaccharides is abnormal in Morquio B syndrome but normal in adult type GM1‐gangliosidosis. The catalytic properties of β‐galactosidase in Morquio B syndrome and GM1‐gangliosidosis provide a possible explanation for the distinct clinical manifestations in these disorders.