Specific Determination of β-Galactocerebrosidase Activity via Competitive Inhibition of β-Galactosidase

Abstract

Abstract Background: The determination of cellular β-galactocerebrosidase activity is an established procedure to diagnose Krabbe disease and monitor the efficacy of gene/stem cell-based therapeutic approaches aimed at restoring defective enzymatic activity in patients or disease models. Current biochemical assays for β-galactocerebrosidase show high specificity but generally require large protein amounts from scanty sources such as hematopoietic or neural stem cells. We developed a novel assay based on the hypothesis that specific measurements of β-galactocerebrosidase activity can be performed following complete inhibition of β-galactosidase activity. Methods: We performed the assay using 2–7.5 μg of sample proteins with the artificial fluorogenic substrate 4-methylumbelliferone-β-galactopyranoside (1.5 mmol/L) resuspended in 0.1/0.2 mol/L citrate/phosphate buffer, pH 4.0, and AgNO3. Reactions were incubated for 30 min at 37 °C. Fluorescence of liberated 4-methylumbelliferone was measured on a spectrofluorometer (λex 360 nm, λem 446 nm). Results: AgNO3 was a competitive inhibitor of β-galactosidase [inhibition constant (Ki) = 0.12 μmol/L] and completely inhibited β-galactosidase activity when used at a concentration of 11 μmol/L. Under this condition, the β-galactocerebrosidase activity was preserved and could be specifically and accurately measured. The assay can detect β-galactocerebrosidase activity in as little as 2 μg cell protein extract or 7.5 μg tissue. Assay validation was performed using (a) brain tissues from wild-type and twitcher mice and (b) murine GALC−/− hematopoietic stem cells and neural precursor cells transduced by GALC-lentiviral vectors. Conclusions: The procedure is straightforward, rapid, and reproducible. Within a clinical context, our method unequivocally discriminated cells from healthy subjects and Krabbe patients and is therefore suitable for diagnostic applications.