Enzymatic characterization of O-GlcNAcase isoforms using a fluorogenic GlcNAc substrate

A highly sensitive fluorogenic hexosaminidase substrate, fluorescein di(N-acetyl-beta-D-glucosaminide) (FDGlcNAc) was prepared essentially as described previously [Chem. Pharm. Bull. 1993, 41, 314] with some modifications. The fluorescent analog is a substrate for a number of hexosaminidases but here we have focused on the cytoplasmic O-GlcNAcase isoforms. Kinetic analysis using purified O-GlcNAcase and its splice variant (v-O-GlcNAcase) expressed in Escherichia coli suggests that FDGlcNAc is a much more efficient substrate (K-m = 84.9 mu M) than the conventional substrate, para-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside (pNP-beta-GlcNAc, K-m = 1.1 nM) and a previously developed fluorogenic substrate, 4-methylumbelliferyl 2-acetamido-2-deoxy-beta -D-glucopyranoside [MUGlcNAc, K-m = 0.43 mM; J. Biol. Chem. 2005, 280, 25313] for O-GlcNAcase. The variant O-GlcNAcase. a protein lacking the C-terminal third of the full-length O-GlcNAcase, exhibited a K-m of 2.1 mM with respect to FDGlcNAc. This shorter isoforni was not previously thought to exhibit O-GlcNAcase activity based on in vitro Studies with pNP-beta-GlcNAc. However, both O-GlcNAcase isoforms reduced O-GlcNAc protein levels extracted from HeLa and HT-29 cells in vitro. indicating that the splice variant is a bona fide O-GlcNAcase. Fluorescein di-N-acetyl-beta-D-galactosaminide (FDGalNAc) is not cleaved by these enzymes, consistent with previous findings that the O-GlcNAcase has substrate specificity toward O-GlcNAc but not O-GalNAc. The enzymatic activity of the shorter isoforni of O-GlcNAcase was first detected by using highly sensitive fluorogenic FDGlcNAc substrate. The finding that O-GlcNAcase exists as two distinct isoforms has a number of important implications for the role of O-GlcNAcase in hexosamine signaling. Published by Elsevier Ltd.