Tandem mass spectrometry for the direct assay of lysosomal enzymes in dried blood spots: application to screening newborns for mucopolysaccharidosis VI (Maroteaux-Lamy syndrome)

Abstract

We report a new assay of N-acetylgalactosamine-4-sulfatase (aryl sulfatase B) activity in dried blood spots (DBS) for the early detection of mucopolysaccharidosis VI (Maroteaux-Lamy syndrome) in newborn screening. The assay uses a synthetic substrate consisting of N-acetylgalactosamine-4-sulfate moiety glycosidically linked to a hydrophobic residue and furnished with a tert-butyloxycarbamido group as a marker for specific mass spectrometric fragmentation. Incubation with aryl sulfatase B present in DBS converts the substrate to a desulfated product which is detected by electrospray tandem mass spectrometry and quantified using a homologous internal standard. Assay and workup procedures were optimized to be compatible with the work flow in newborn screening laboratories. Analysis of DBS from human newborns showed clear distinction of aryl sulfatase B activity from 89 healthy individuals where it ranged between 1.4 and 16.9 μmol/(h L of blood), with an average activity of 7.4 μmol/(h L of blood), and an MPS-VI patient that had an activity of 0.12 μmol/(h L of blood). Results are also reported for the aryl sulfatase B assay in DBS from groups of normal felines and felines affected with MPS-VI.

Figures

Figure 1

Distribution of ASB activities in DBS from humans. Black bars show data for 89 unaffected newborns obtained using the liquid-liquid extraction method. Blue bars show data for 10 unaffected newborns obtained using the solid phase extraction method. The red bar isfor 1 MPS VI patient. ASB activity values for each sample are given inTable s 1 and 2.

Figure 1

Distribution of ASB activities in DBS from humans. Black bars show data for 89 unaffected newborns obtained using the liquid-liquid extraction method. Blue bars show data for 10 unaffected newborns obtained using the solid phase extraction method. The red bar isfor 1 MPS VI patient. ASB activity values for each sample are given inTable s 1 and 2.

Scheme 1

Scheme 2