beta-N-acetylhexosaminidases in the spleen of a patient with hairy-cell leukaemia

Dynamic O-glycosylation of nuclear and cytosolic proteins: cloning and characterization of a neutral, cytosolic beta-N-acetylglucosaminidase from human brain

Dynamic modification of cytoplasmic and nuclear proteins by O-linked N-acetylglucosamine (O-GlcNAc) on Ser/Thr residues is ubiquitous in higher eukaryotes and is analogous to protein phosphorylation. The enzyme for the addition of this modification, O-GlcNAc transferase, has been cloned from several species. Here, we have cloned a human brain O-GlcNAcase that cleaves O-GlcNAc off proteins. The cloned cDNA encodes a polypeptide of 916 amino acids with a predicted molecular mass of 103 kDa and a pI value of 4.63, but the protein migrates as a 130-kDa band on SDS-polyacrylamide gel electrophoresis. The cloned O-GlcNAcase has a pH optimum of 5.5-7.0 and is inhibited by GlcNAc but not by GalNAc. p-Nitrophenyl (pNP)-beta-GlcNAc, but not pNP-beta-GalNAc or pNP-alpha-GlcNAc, is a substrate. The cloned enzyme cleaves GlcNAc, but not GalNAc, from glycopeptides. Cell fractionation suggests that the overexpressed protein is mostly localized in the cytoplasm. It therefore has all the expected characteristics of O-GlcNAcase and is distinct from lysosomal hexosaminidases. Northern blots show that the transcript is expressed in every human tissue examined but is the highest in the brain, placenta, and pancreas. An understanding of O-GlcNAc dynamics and O-GlcNAcase may be key to elucidating the relationships between O-phosphate and O-GlcNAc and to the understanding of the molecular mechanisms of diseases such as diabetes, cancer, and neurodegeneration.

Distribution of active alpha- and beta-subunits of beta-N-acetylhexosaminidase as a function of leukaemic cell types

beta-N-Acetylhexosaminidase isoenzymes, and the distribution of the alpha- and beta-subunits forming the enzyme in a representative series of fresh leukaemic cells and in established leukaemic cell lines, were obtained by using a combination of DEAE-cellulose chromatography and assay with the fluorogenic substrates 4-methylumbelliferyl-beta-N-acetylglucosaminide hydrolyzed by both alpha and beta subunits, and 4-methylumbelliferyl-beta-N-acetylglucosaminide-6-SO4 hydrolyzed only by hexosaminidase isoenzymes containing alpha-subunits. The presence of hexosaminidase S (the alpha alpha dimer), was found in all the leukaemic cell populations we surveyed, but not in normal human cells. The presence of this isoenzyme can therefore be considered as an additional marker of leukaemic cells. A prevalence of hexosaminidase A and A-like intermediate forms (alpha beta structure), characterize leukaemic cells of myeloid origin, whereas greater amounts of hexosaminidase B and B-like intermediate forms (beta beta structure), were consistent attributes of leukaemic cells of lymphoid origin. An over-expression of beta-subunits in blasts might be related to their undifferentiated status. These changes in the isoenzymes of hexosaminidase may prove informative about a variety of changes in the biology of leukaemic cells that could range from chromosomal alterations to changes in the proteolytic processing and glycosylation.

Purification and properties of human urinary beta-D-mannosidase

Two forms of the lysosomal enzyme beta-mannosidase were identified and purified from human urine. The purification strategy employed allowed sufficient quantities of both forms to be obtained for subunit analysis and for further characterizations. The two beta-mannosidases were identified as beta-mannosidase B and A, in order of their elution from an ion-exchange column. In all samples examined, the A form was predominant, and the B/A ratio was consistently 0.14. The two forms displayed the same optimum pH (i.e., 4.3) and both were retained by a Concanavalin-A Sepharose column, but showed different isoelectric points, molecular masses and subunit compositions. Native- and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of pure beta-mannosidases B and A suggest that active protein B (160 kDa) consists of three subunits, one 75 kDa and two 49 kDa subunits. Protein A is smaller and appears to be composed of three subunits of 75 kDa, 49 kDa and 37 kDa. Two forms of beta-mannosidase, exhibiting a chromatographic behaviour comparable to the urinary forms, were also detected in human kidney. Nevertheless, in this tissue their relative distribution was different, the B/A ratio being 19.

N-acetyl-beta-D-glucosaminidase (NAG) isoenzymes in primary cultures of rabbit kidney proximal tubule cells: a cellular model for studies on nephrotoxicity?

N-Acetyl-beta-D-glucosaminidase (NAG) isoenzyme profile in primary cultures of rabbit kidney proximal tubule cells was studied. Confluent cells had high levels of NAG activity, but ion exchange chromatography showed that the NAG isoenzyme profile in cultured cells was different from that of rabbit renal cortex homogenates and freshly isolated cells. Confluent cultured cells contained an atypical acidic isoform, absent in homogenates and freshly isolated cells in which the predominant isoform is NAG-A (a heterodimer alpha beta). The fact that this atypical isoform was able to hydrolyse the synthetic substrate 4-methylumbelliferyl-beta-N-acetylglucosaminide-6-sulphate indicated that it probably was an alpha-subunit homodimer. These results suggest subunit rearrangement within NAG polypeptide chains linked to down-regulation of beta-subunit production in cultured rabbit proximal cells. The change in isoenzyme profile in cultured cells may make it difficult to use primary cultures of rabbit proximal tubule cells to establish correlations between in vitro and in vivo studies using NAG isoenzymes as a nephrotoxicity index, as illustrated by the effects of gentamicin.

Platelet glycohydrolase activities: characterization and release

Granules containing acid hydrolases have been detected in human platelets but have not been thoroughly characterized. We have studied the activity and characteristics of glycohydrolases present in normal human platelets, evaluated their release upon stimulation with thrombin, and assessed the contribution of platelet - released lysosomal contents to the glycohydrolase activity present in normal serum. Platelets contained a remarkable glycohydrolase activity with a prevalence of beta - N-acetylhexosaminidase. All glycohydrolases were released to some extent upon stimulation with thrombin and contributed to the glycohydrolase activity found in human serum. alpha-Mannosidase and alpha-galactosidase were partially inactivated after release by a mechanism as yet undefined. In addition, thrombin stimulation affects the intraplatelet isoenzyme pattern of beta-N-acetylhexosaminidase by producing the appearance of a new form.

Increase of intermediate forms of beta-N-acetylhexosaminidase during rat liver development and regeneration

1. Rat liver beta-N-acetylhexosaminidase was separated into several different molecular forms by DEAE-cellulose chromatography. 2. The subunit composition of the isoenzymes, as well as the similarities to human hexosaminidases, were determined by using the specific active alpha subunit substrate 4-methylumbelliferyl-beta-N-acetylglucosamine-6-sulphate. 3. As in human tissues, the intermediate form lacked the active alpha subunit and resembled hexosaminidase B rather than A. 4. The intermediate form was markedly increased in foetal liver and in regenerating liver after partial hepatectomy. 5. The variations in isoenzyme expression were accompanied by variations in specific activity of hexosaminidase. 6. Sulphated substrate analysis and thermal stability experiments indicated that the rapid cell proliferation had a greater effect on the formation of beta-subunit of hexosaminidase than on that of alpha-subunit.

Treatment of HL-60 cells with dimethyl sulphoxide inhibits the formation of beta-N-acetylhexosaminidase S

beta-N-Acetylhexosaminidase of HL-60 cells was separated into two main forms, A and S, by chromatography on DEAE-cellulose. Analysis of developmental changes in the isoenzyme pattern was complicated by the fact that the specific activity of beta-N-acetylhexosaminidase underwent a 6-fold change during the normal growth cycle. Two other lysosomal enzymes, beta-galactosidase and alpha-mannosidase, behaved similarly. Induction of differentiation of HL-60 cells with dimethyl sulphoxide at a low cell density (3 x 10(5) cells/ml) had a greater effect on the abundance of alpha-subunits of beta-N-acetylhexosaminidase, measured with 4-methylumbelliferyl-beta-N-acetylglucosaminide 6-sulphate, than of beta-subunits, measured with 4-methylumbelliferyl-beta-N-acetylglucosamine, and resulted in an isoenzyme profile in which A and B were the major forms, with the levels of form S greatly decreased.