Multiplex lysosomal enzyme activity assay on dried blood spots using tandem mass spectrometry

Deficiencies in any of the 50 degradative enzymes found in lysosomes results in the accumulation of undegraded material and subsequently cellular dysfunction. Early identification of deficiencies before irreversible organ and tissue damages occur leads to better clinical outcomes. In the method which follows, lysosomal alpha-glucosidase, alpha-galactosidase, beta-glucocerebrosidase, acid sphingomyelinase, and galactocerebrosidase are extracted from dried blood spots and incubated individually with an enzyme-specific cocktail containing the corresponding substrate and internal standard. Each enzyme cocktail is prepared using commercially available mixture of substrate and internal standard at the predetermined optimized molar ratio. After incubation, the enzymatic reactions are quenched using an ethyl acetate/methanol solution and all five enzyme solutions are combined. The mixtures of the reaction products are prepared using liquid-liquid and solid-phase extractions and quantified simultaneously using selected ion monitoring on LC-MS-MS system.

Multiplex enzyme assay screening of dried blood spots for lysosomal storage disorders by using tandem mass spectrometry

BACKGROUND

Reports of the use of multiplex enzyme assay screening for Pompe disease, Fabry disease, Gaucher disease, Niemann-Pick disease types A and B, and Krabbe disease have engendered interest in the use of this assay in newborn screening. We modified the assay for high-throughput use in screening laboratories.

METHODS

We optimized enzyme reaction conditions and procedures for the assay, including the concentrations of substrate (S) and internal standard (IS), assay cocktail compositions, sample clean-up procedures, and mass spectrometer operation. The S and IS for each enzyme were premixed and bottled at an optimized molar ratio to simplify assay cocktail preparation. Using the new S:IS ratio, we validated the modified assay according to CLSI guidelines. Stability of the S, IS, and assay cocktails were investigated. Dried blood spots from 149 healthy adults, 100 newborns, and 60 patients with a lysosomal storage disorder (LSD) were tested using the modified assay.

RESULTS

In our study, the median enzyme activity measured in adults was generally increased 2-3-fold compared to the original method, results indicating higher precision. In the multiplex format, each of the 5 modified enzyme assays enabled unambiguous differentiation between samples from healthy individuals (adults and newborns) and the corresponding disease-specific samples.

CONCLUSIONS

The modified multiplex enzyme assay with premixed S and IS is appropriate for use in high-throughput screening laboratories.

Evaluation of silyl-blocked p-nitrophenylmaltoheptaoside as a substrate for alpha-amylase reagents

We describe a reagent for measuring alpha-amylase (EC 3.2.1.1) activity in serum with use of a thexyldimethylsilyl ether of p-nitrophenyl-alpha-D-maltoheptaoside (SB7) as substrate. This substrate differs from Genzyme's benzylidene-blocked p-nitrophenylmaltoheptaoside substrate (B-PNPG7). The reagent, optimized for the characteristics of the silyl-blocked substrate, contains 4-(2-hydroxyethyl)-1-piperazineethane sulfonate buffer at pH 7.3, alpha-glucosidase (maltase; EC 3.2.1.20), and glucoamylase (EC 3.2.1.3). Comparison with Ciba Corning Diagnostics Corp.'s, amylase reagent with B-PNPG7 as substrate (x) yielded a regression equation of y = 1.20x-2.7 (r = 0.9997). The linear range exceeded amylase concentrations > 2500 U/L and total precision (CV) was 2.3% at an amylase concentration of 112 U/L with the Ciba Corning 550 Express analyzer. Reconstituted reagent is stable for 30 days at 5 degrees C and 7 days at ambient (18-25 degrees C) temperatures.

Evaluation of silyl-blocked p-nitrophenylmaltoheptaoside as a substrate for alpha-amylase reagents

We describe a reagent for measuring alpha-amylase (EC 3.2.1.1) activity in serum with use of a thexyldimethylsilyl ether of p-nitrophenyl-alpha-D-maltoheptaoside (SB7) as substrate. This substrate differs from Genzyme's benzylidene-blocked p-nitrophenylmaltoheptaoside substrate (B-PNPG7). The reagent, optimized for the characteristics of the silyl-blocked substrate, contains 4-(2-hydroxyethyl)-1-piperazineethane sulfonate buffer at pH 7.3, alpha-glucosidase (maltase; EC 3.2.1.20), and glucoamylase (EC 3.2.1.3). Comparison with Ciba Corning Diagnostics Corp.'s, amylase reagent with B-PNPG7 as substrate (x) yielded a regression equation of y = 1.20x-2.7 (r = 0.9997). The linear range exceeded amylase concentrations > 2500 U/L and total precision (CV) was 2.3% at an amylase concentration of 112 U/L with the Ciba Corning 550 Express analyzer. Reconstituted reagent is stable for 30 days at 5 degrees C and 7 days at ambient (18-25 degrees C) temperatures.