Khal, S. K.; Linhart, N. A.; Jain, S.; Rosario Acevedo, G.; Boyce, M.

Glycosylation depends on tightly regulated pools of nucleotide-sugars (NS), yet the mechanisms controlling mammalian NS homeostasis and their downstream effects on glycoprotein biosynthesis remain poorly understood. UDP-galactose 4'-epimerase (GALE) catalyzes the reversible interconversion of UDP-galactose/UDP-glucose and UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine, making it a central regulator of glycan precursor pools and an excellent model enzyme for studying NS metabolism. Here, we report the discovery of a cell-active small molecule inhibitor of human GALE through a high-throughput chemical screening strategy. Using a coupled luminescence-based assay, we identified the FDA-approved drug disulfiram as a GALE inhibitor. Biochemical analyses demonstrated that disulfiram directly inhibits GALE through covalent modification of cysteine residues, including C153, likely via its reactive metabolite diethyldithiocarbamate. In cultured human cells, disulfiram treatment phenocopied genetic GALE deletion, reducing terminally sialylated glycans, mucin-type O-glycans, and properly glycosylated mucin-domain glycoproteins. These effects were rescued by galactose supplementation, consistent with a mechanism of on-target GALE inhibition. Similar phenotypes were observed in human lung adenocarcinoma cells, supporting a broader role for GALE in regulating glycosylation and mucin biosynthesis across tissue types. Together, these studies establish a platform for the discovery of pharmacological GALE inhibitors as new research tools, identify disulfiram as a cell-active chemical probe for studying NS regulation, and suggest that targeting GALE might modulate mucin hypersecretion in muco-obstructive diseases and mucinous cancers.