Tandem Mass Spectrometry for the Direct Assay of Lysosomal Enzymes in Dried Blood Spots: Application to Screening Newborns for

BACKGROUND: Treatments now available for mucopo-lysaccharidosis I require early detection for optimum therapy. Therefore, we have developed an assay appro-priate for newborn screening of the activity of the rel-evant enzyme, -L-iduronidase. METHODS: We synthesized a new -L-iduronidase sub-strate that can be used to assay the enzyme by use of tandem mass spectrometry together with an internal standard. The assay uses a dried blood spot on a new-born screening card as the enzyme source. The assay protocol uses a simple liquid-liquid extraction step be-fore mass spectrometry. We optimized enzyme reac-tion conditions and procedures for the assay, including the concentration of substrate, the reaction pH, the incubation time, andmass spectrometer operation.W