Protocol to measure endogenous GlcNAc-1-phosphotransferase activity in SK-MEL-30 cells

GlcNAc-1-phosphotransferase (GNPT) is the key enzyme for tagging lysosomal hydrolases with the mannose 6-phosphate moiety for delivery to the lysosome. Here, we present the assay for measuring endogenous GNPT activity in SK-MEL-30 cells. We provide details for preparing the [³H]UDP-GlcNAc donor substrate and conditions for the enzymatic transfer of [³H]GlcNAc-1-P to the methyl α-D-mannopyranoside acceptor (α-MM). We then describe the chromatography steps to specifically separate the [³H]GlcNAc-1-P-αMM GNPT reaction product from unreacted [³H]UDP-GlcNAc that is bound to the quaternary aminoethyl (QAE) Sephadex A-25 matrix. For complete details on the use and execution of this protocol, please refer to Doray et al.¹ and Yang et al.².