The low abundance lysosomal enzyme N-acetylgalactosamine 4-sulphatase (4-sulphatase) has been quantified using a microimmunopurification step and a monoclonal-based ELISA detection system. The assay is similar in principle to a two-site ELISA but uses a single monoclonal antibody against one epitope to bind 4-sulphatase in two separate assay steps. The sensitivity of this assay is sufficient to allow the quantification of 4-sulphatase in human cultured skin fibroblasts derived from normal controls and patients deficient in 4-sulphatase activity (mucopolysaccharidosis type VI or Maroteaux-Lamy syndrome). The results obtained suggest a range of mucopolysaccharidosis type VI or 4-sulphatase deficient mutants, from those expressing little or no quantifiable 4-sulphatase protein to those examples with quantifiable levels of 4-sulphatase protein which is enzymically inactive. Phenotypic variability in patients with a 4-sulphatase deficiency may therefore be partially attributed to a range of protein expressions. The method should allow the determination of 4-sulphatase specific activity in mucopolysaccharidosis type VI patients.
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