Fluorescence Quenching‐based Assay for Measuring Golgi endo ‐α‐Mannosidase

: Golgi endo-α-mannosidase (G-EM) catalyzes an alternative deglucosylation process for N-glycans and plays important roles in the post-endoplasmic reticulum (ER) quality control pathway. To understand the post-ER quality control mechanism, we synthesized a tetrasaccharide probe for the detection of the hydrolytic activity of G-EM based on a fluorescence quenching assay. The probe was labeled with an N-methylanthraniloyl group as a reporter dye at the non-reducing end and a 2,4-dinitrophenyl group as a quencher at the reducing end. This probe is hydrolyzed to disaccharide derivatives by G-EM, resulting in increased fluorescence intensity. Thus, the fluorescence signal is directly proportional to the amount of disaccharide derivative present, allowing the G-EM activity to be evaluated easily and quantitatively.