Purification and properties of neutral beta-N-acetylglucosaminidase from carp blood

Enzymatic characterizations and activity regulations of N-acetyl-β-D-glucosaminidase from the spermary of Nile tilapia (Oreochromis niloticus)

N-Acetyl-β-D-glucosaminidase (NAGase) is proved to be correlated with reproduction of male animals. In this study, enzymatic characterizations of NAGase from spermary of Nile tilapia (Oreochromis niloticus) were investigated in order to further study its reproductive function in fish. Tilapia NAGase was purified to be PAGE homogeneous by the following techniques: (NH4)2SO4 fractionation (40-55%), DEAE-cellulose (DE-32) ion exchange chromatography, Sephadex G-200 gel filtration and DEAE-Sephadex (A-50). The specific activity of the purified enzyme was 4100 U/mg. The enzyme molecular weight was estimated as 118.0 kD. Kinetic studies showed that the hydrolysis of p-nitrophenyl-N-acetyl-β-D-glucosaminide (pNP-NAG) by the enzyme followed Michaelis-Menten kinetics. The Michaelis-Menten constant (Km) and maximum velocity (Vm) were determined to be 0.67 mM and 23.26 μM/min, respectively. The optimum pH and optimum temperature of the enzyme for hydrolysis of pNP-NAG was to be at pH 5.7 and 55°C, respectively. The enzyme was stable in a pH range from 3.3 to 8.1 at 37°C, and inactive at temperature above 45°C. The enzyme activity was regulated by the following ions in decreasing order: Hg(2+) > Zn(2+) > Cu(2+) > Pb(2+) > Mn(2+). The IC50 of Cu(2+), Zn(2+) and Hg(2+) was 1.23, 0.28, and 0.0027 mM, respectively. However, the ions Li(+), Na(+), K(+), Mg(2+) and Ca(2+) had almost no influence on enzyme activity. In conclusion, the enzymatic characterizations of NAGase from tilapia were special to the other animals, which were correlated with its living habit; besides, CuSO4 and ZnSO4 should used very carefully as insecticides in tilapia cultivation since they both had strong regulations on the enzyme.

Increasing O-GlcNAc levels: An overview of small-molecule inhibitors of O-GlcNAcase

The O-GlcNAc modification is found on many nucleocytoplasmic proteins. The dynamic nature of O-GlcNAc, which in some ways is reminiscent of phosphorylation, has enabled investigators to modulate the stoichiometry of O-GlcNAc on proteins in order to study its function. Although several genetic and pharmacological methods for manipulating O-GlcNAc levels have been described, one of the most direct approaches of increasing global O-GlcNAc levels is by using small-molecule inhibitors of O-GlcNAcase (OGA). As the interest in increasing O-GlcNAc levels has grown, so too has the number of OGA inhibitors. This review provides an overview of the available methods of increasing O-GlcNAc levels, with a special emphasis on inhibition of OGA by small molecules. Known inhibitors of OGA are discussed with particular attention on those most suitable for cell-based biological studies. Several examples in which OGA inhibitors have been used to study the functional role of the O-GlcNAc modification in biological systems are discussed, highlighting the pros and cons of different inhibitors.

Isoenzyme pattern and partial characterization of hexosaminidases in the membrane and cytosol of human erythrocytes

OBJECTIVES

Hexosaminidase activity is present in lysosomes, plasma membrane and cytosol of many human cells. Plasma membrane and cytosolic hexosaminidase is not well characterized, particularly as regards their isoenzyme forms and their relationship with the lysosomal ones.

DESIGN AND METHODS

Erythrocyte hexosaminidase isoforms were chromatographically separated, characterized and compared to those in the plasma of healthy individuals and in the erythrocytes of a Tay-Sachs patient.

RESULTS

Hexosaminidase isoenzymes were found in plasma membrane and cytosol and were composed of the same alpha- and beta-subunits as the lysosomal and plasma hexosaminidase A and B isoenzymes, though with some structural and kinetic differences. In addition, the cytosol contained a hexosaminidase that is a specific N-acetyl-beta-D-glucosaminidase, the one involved in the removal of N-acetylglucosamine residues O-linked to proteins, named O-GlcNAcase.

CONCLUSIONS

This work provides an additional step in the characterization of hexosaminidases helping better understand their role in non-lysosomal compartments and their involvement in physiological or pathological situations.

O-GlcNAcase uses substrate-assisted catalysis: kinetic analysis and development of highly selective mechanism-inspired inhibitors

The post-translational modification of serine and threonine residues of nucleocytoplasmic proteins with 2-acetamido-2-deoxy-d-glucopyranose (GlcNAc) is a reversible process implicated in multiple cellular processes. The enzyme O-GlcNAcase catalyzes the cleavage of beta-O-linked GlcNAc (O-GlcNAc) from modified proteins and is a member of the family 84 glycoside hydrolases. The family 20 beta-hexosaminidases bear no apparent sequence similarity yet are functionally related to O-GlcNAcase because both enzymes cleave terminal GlcNAc residues from glycoconjugates. Lysosomal beta-hexosaminidase is known to use substrate-assisted catalysis involving the 2-acetamido group of the substrate; however, the catalytic mechanism of human O-GlcNAcase is unknown. By using a series of 4-methylumbelliferyl 2-deoxy-2-N-fluoroacetyl-beta-D-glucopyranoside substrates, Taft-like linear free energy analyses of these enzymes indicates that O-GlcNAcase uses a catalytic mechanism involving anchimeric assistance. Consistent with this proposal, 1,2-dideoxy-2'-methyl-alpha-D-glucopyranoso-[2,1-d]-Delta2'-thiazoline, an inhibitor that mimics the oxazoline intermediate proposed in the catalytic mechanism of family 20 glycoside hydrolases, is shown to act as a potent competitive inhibitor of both O-GlcNAcase (K(I) = 0.070 microm) and beta-hexosaminidase (K = 0.070 microm). A series of 1,2-dideoxy-2'-methyl-alpha-D-glucopyranoso-[2,1-d]-Delta2'-thiazoline analogues were prepared, and one inhibitor demonstrated a remarkable 1500-fold selectivity for O-GlcNAcase (K(I) = 0.230 microm) over beta-hexosaminidase (K(I) = 340 microm). These inhibitors are cell permeable and modulate the activity of O-GlcNAcase in tissue culture. Because both enzymes have vital roles in organismal health, these potent and selective inhibitors of O-GlcNAcase should prove useful in studying the role of this enzyme at the organismal level without generating a complex chemical phenotype stemming from concomitant inhibition of beta-hexosaminidase.

Characterization of the isoenzyme profile of beta-N-acetylhexosaminidase in the urine of newborns

The urinary isoenzymes of beta-N-acetylhexosaminidase (Hex) in newborn infants were characterised by chromatography, electrophoresis, thermodynamic analysis and through substrate specificity. No qualitative difference was found for the major Hex A and Hex B isoenzymes between full-term or premature newborns and adults, although in the latter group the relative proportion of Hex B is much lower (18.5 +/- 2.7% vs. 36.3 +/- 1.0%). An additional minor enzyme form was found in some premature newborns, which eluted from the DEAE-cellulose column at a higher concentration of NaCl than Hex A and, like this isoenzyme, is able to hydrolyse 4-methylumbellipheryl-2-acetamido-2-deoxy-beta-D-glucopyranoside-6 -sulphate, which would suggest that it has alpha subunits in its molecule. These results do not confirm the hypothesis of other authors about the existence of a unique fetal Hex isoenzyme in neonatal urine which eluted before the application of the NaCl gradient, similarly to the Hex B.

Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Modification of Ser and Thr residues by attachment of O-linked N-acetylglucos-amine [Ser(Thr)-O-GlcNAcylation] to eukaryotic nuclear and cytosolic proteins is as dynamic and possibly as abundant as Ser(Thr) phosphorylation. Known O-GlcNAcylated proteins include cytoskeletal proteins and their regulatory proteins; viral proteins; nuclear-pore, heat-shock, tumor-suppressor, and nuclearoncogene proteins; RNA polymerase II catalytic subunit; and a multitude of transcription factors. Although functionally diverse, all of these proteins are also phosphoproteins. Most O-GlcNAcylated proteins form highly regulated multimeric associations that are dependent upon their posttranslational modifications. Evidence is mounting that O-GlcNAcylation is an important regulatory modification that may have a reciprocal relationship with O-phosphorylation and may modulate many biological processes in eukaryotes.

Demonstration and some properties of N-acetyl-beta,D-hexosaminidase (HEX) C isoenzyme in human renal tissues: relative increase of HEX C activity in renal cell carcinoma

The present study has demonstrated the presence of hexosaminidase (HEX) C activity in human renal tissues by an electrophoretic method. At the same time, its enzymatic properties, obtained as unbound fraction of renal tissue extracts passed through a concanavalin A Sepharose column, have been compared with those of HEX A and B. HEX C had the fastest electrophoretic mobility among HEX isoenzymes. The optimal pH of HEX C was 6.5, while that of HEX A and B was 4.9. The Km value of HEX C for synthetic glucosaminide substrate was 1.16 mmol/l, while that of HEX A and B was 0.18 mmol/l and 0.22 mmol/l, respectively. HEX C was inactive for a synthetic galactosaminide substrate, while HEX A and B were active. The percentage of HEX C activity to the total was 7.9 +/- 2.9% and 13.8 +/- 3.0% in the normal and the neoplastic tissues, respectively. A significant difference was observed between them (P < 0.01). These results indicate that the enzymatic properties of HEX C is quite different from those of A and B and also suggest that the determination of HEX C may become one of the useful clinical markers of the human renal cell carcinoma.