A Specific Activity‐Based Probe to Monitor Family GH59 Galactosylceramidase, the Enzyme Deficient in Krabbe Disease

Galactosylceramidase (GALC) is the lysosomal β‐galactosidase responsible for the hydrolysis of galactosylceramide. Inherited deficiency in GALC causes Krabbe disease, a devastating neurological disorder characterized by accumulation of galactosylceramide and its deacylated counterpart, the toxic sphingoid base galactosylsphingosine (psychosine). We report the design and application of a fluorescently tagged activity‐based probe (ABP) for the sensitive and specific labeling of active GALC molecules from various species. The probe consists of a β‐galactopyranose‐configured cyclophellitol‐epoxide core, conferring specificity for GALC, equipped with a BODIPY fluorophore at C6 that allows visualization of active enzyme in cells and tissues. Detection of residual GALC in patient fibroblasts holds great promise for laboratory diagnosis of Krabbe disease. We further describe a procedure for in situ imaging of active GALC in murine brain by intra‐cerebroventricular infusion of the ABP. In conclusion, this GALC‐specific ABP should find broad applications in diagnosis, drug development, and evaluation of therapy for Krabbe disease. Developments in diagnostics: We report the design and application of a fluorescently labeled β‐galactopyranose‐configured cyclophellitol‐epoxide probe. This activity‐based probe allows sensitive detection of active molecules of the lysosomal β‐galactosidase galactosylceramidase (GALC) in vitro, in situ, and in vivo, showing great promise for laboratory diagnosis of Krabbe disease.