Separation and properties of human brain hexosaminidase C

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Nutrient regulation of signaling and transcription

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

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.

Structural analyses of enzymes involved in the O-GlcNAc modification

In order to study the O-GlcNAc modification in vivo, it is evident that a range of specific small molecule inhibitors would be a valuable asset. One strategy for the design of such compounds would be to utilise 3-D structural information in tandem with knowledge of catalytic mechanism. The last few years has seen major breakthroughs in our understanding of the 3-D structure of the enzymes involved in the O-GlcNAc modification notably from the study of the tetratricopeptide repeat (TPR) domain of the human O-GlcNAc transferase, of the bacterial homologs of the O-GlcNAc hydrolase and more latterly bacterial homologs of the O-GlcNAc transferase itself. Of particular note are the bacterial O-GlcNAc hydrolase homologs that provide near identical active centres to the human enzyme. These have informed the design and/or subsequent analysis of inhibitors of this enzyme which have found great use in the chemical dissection of the O-GlcNAc in vivo, as described by Macauley and Vocadlo elsewhere in this issue.

Mammalian cells contain a second nucleocytoplasmic hexosaminidase

Some thirty years ago, work on mammalian tissues suggested the presence of two cytosolic hexosaminidases in mammalian cells; one of these has been more recently characterized in a recombinant form and has an important role in cellular function due to its ability to cleave beta-N-acetylglucosamine residues from a variety of nuclear and cytoplasmic proteins. However, the molecular nature of the second cytosolic hexosaminidase, named hexosaminidase D, has remained obscure. In the present study, we molecularly characterize for the first time the human and murine recombinant forms of enzymes, encoded by HEXDC genes, which appear to correspond to hexosaminidase D in terms of substrate specificity, pH dependency and temperature stability. Furthermore, a Myc-tagged form of this novel hexosaminidase displays a nucleocytoplasmic localization. Transcripts of the corresponding gene are expressed in a number of murine tissues. On the basis of its sequence, this enzyme represents, along with the lysosomal hexosaminidase subunits encoded by the HEXA and HEXB genes, the third class 20 glycosidase to be identified from mammalian sources.

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.

Purification and characterization of blood group A-degrading isoforms of alpha-N-acetylgalactosaminidase from Ruminococcus torques strain IX-70

To cleave blood group A immunodeterminants from erythrocytes (Hoskins, L. C., Larson, G., and Naff, G. B. (1995) Transfusion 35, 813-821), we purified and characterized alpha-N-acetylgalactosaminidase (EC 3.2.1.49) activity from culture supernatants of the human fecal bacterium Ruminococcus torques strain IX-70. Three isoforms separated during hydrophobic interaction chromatography. Hydroxyapatite chromatography further resolved the most hydrophilic, isoform I, into isoforms IA and IB. The most hydrophobic, isoform III, differed from IA and IB by a more acidic pH optimum, greater heat resistance, greater sensitivity to alkylating agents, and anomalous retardation during gel filtration chromatography. Isoform IB differed from IA and III in N-terminal amino acid sequence and in sensitivity to EDTA inhibition. Each cleaved nonreducing alpha(1-->3)-N-acetylgalactosamine residues from human blood group A and AB mucin glycoproteins, Forssman hapten, and blood group A lacto series glycolipids. The apparent molecular mass of denatured isoform subunits of IA, IB, and III-PII (158, 173, and 201 kDa, respectively) bore no integer relationship to the apparent molecular mass of the native isoforms (265, 417, and 530 kDa), but the latter bore a ratio of 1.96:3.09:3.93 to the weight-average apparent molecular mass of native IA (135 kDa), suggesting that the isoforms are multimers of a 135-kDa sequence. Isoforms IA and III-PII had an identical N-terminal amino acid sequence which showed homologies to the N-terminal sequence of sialidases produced by Bacteroides fragilis SBT3182, another commensal enteric bacterium.

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.

An evaluation of the mechanisms developmentally involved on cellular and extracellular glycosaminoglycans accumulation in chick embryo skin fibroblasts

1. Ammonium chloride, a lysosomotropic amine known to inhibit lysosomal function, was administered to 7-day cultured and 14-day chick embryo skin fibroblasts to evaluate the relationship between synthesis, degradation and uptake of glycosaminoglycans (GAG). 2. Following amine treatment, the amount of 3H-glucosamine and 35SO4 labelled cellular GAG increased, was more at 14 days than at 7 days. Hyaluronic acid (HA) incorporation was mainly interested at 7 days and that of sulphated GAG at 14 days. 3. The extracellular accumulation declined proportionally to the cellular increase of undegraded GAG. HA was mainly affected at 7 days and sulphated GAG at 14 days. 4. The amine did not change 3H-HA uptake and it was unable to inhibit its degradation. 5. The products of degradation of uptaken 3H-HA were retained inside the cell. Those released by degradation of newly synthesized GAG flowed out of the cell.

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

A neutral beta-N-acetylglucosaminidase has been purified to homogeneity from carp blood by a seven-step procedure. It was localized in the cytosol of red blood cells. The purified enzyme was specific to beta-N-acetylglucosaminide and inactive to beta-N-acetylgalactosaminide. It was competitively inhibited by free N-acetylglucosamine, but not by free N-acetylgalactosamine. The optimum pH of the enzyme was 6.5, with a stable pH range of 7.0 to 11.0. The enzyme showed greater heat-lability and anodal electrophoretic mobility than acidic beta-N-acetylglucosaminidases. The Mr value, estimated by sucrose density gradient centrifugation, was 122,000, and the enzyme dissociated into two nonidentical subunits with Mr values of 66,000 and 53,000, based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. With respect to the major characteristics, the neutral enzyme in carp blood was supposed to be a counterpart of hexosaminidase C in human and other mammalian tissues.

beta-Hexosaminidase expression in chick embryo fibroblasts in vitro

1. Two forms of beta-hexosaminidase, similar to hexosaminidase A and hexosaminidase C, were separated by DEAE-cellulose chromatography in chick embryo skin fibroblasts in vitro. 2. beta-Hexosaminidase specific activity increases during development in cultured chick embryo skin fibroblasts in vitro. 3. Concanavalin-A treatment determines the increase of the neutral form, hexosaminidase C, during development. 4. Concanavalin-A reduces the specific activity of beta-hexosaminidase during development.

beta-N-acetylhexosaminidase isoenzymes during chick embryo development

1. Two forms (I and II) with acidic pH optima and a neutral form of beta-hexosaminidase has been separated by DEAE-cellulose chromatography and characterized in skin and lung of 7, 9, 11, 14 day chick embryos and 1 day old chicken. 2. Forms I and II are similar to hexosaminidase A and B for their behaviour on DEAE-cellulose chromatography, Concanavalin A-Sepharose column and thermal stability. 3. Neutral form has a neutral pH optimum and higher molecular weight and a more acidic I. P. than forms I and II, a low beta-N-acetylgalactosaminidase activity and it is not bound by a Concanavalin A-Sepharose column and in that resemble hexosaminidase C and/or other neutral hexosaminidases. 4. We have found differences in the percentage of neutral form and in the specific activities of the extracts in the skin in different stages of development. 5. No significant differences were observed in the lung.

N-acetyl-beta-D-hexosaminidase A from rat urine: partial purification and characterization

N-Acetyl-beta-D-hexosaminidase A was purified from rat urine by ion-exchange chromatography on DEAE-cellulose, followed by concanavalin A chromatography, and finally by chromatography on 2-acetamido-N-(epsilon-aminocaproyl)-2-deoxy-beta-glucosylamine-Se pharose 4B. The enzyme was purified 482-fold with a yield of about 7%. The optimal pH was 4.5 for N-acetyl-glucosaminidase activity and 4.0-4.5 for N-acetylgalactosaminidase activity. The enzyme was heat-labile and stable from pH 4.5 to pH 7.0 but it was very unstable at lower pH values. Km values were 0.55 mM and 0.059 mM, respectively. The glycoprotein nature of the enzyme was deduced from its behavior on concanavalin A. The effect of some carbohydrates and ionic compounds on the activities of the enzyme was studied. When N-acetyl-D-glucosaminolactone and N-acetyl-D-galactosaminolactone were used as inhibitors, Ki values were also calculated.

Molecular forms of beta-N-acetylhexosaminidase in Epstein-Barr virus-transformed lymphoid cell lines from normal subjects and patients with Tay-Sachs disease

In whole leukocytes and in lymphocytes from normal subjects, the percentage activity of heat-stable beta-N-acetylhexosaminidase (30 +/- 5% and 45 +/- 5%, respectively) was higher than in the transformed lymphoid cell line (19 +/- 3%). In Tay-Sachs transformed cells as well as non-transformed beta-N-acetylhexosaminidase was almost completely heat-stable (95 - 98%). In the transformed cells from normal subjects, the beta-N-acetylhexosaminidase B (Hex B) activity (5% of total) was significantly lower than in blood lymphocytes (average 25 - 30% of total activity), whereas Hex A and Hex I were similar in the either cell type. Blood lymphocytes and lymphoid cell lines established from a Tay-Sachs patient lacked heat-labile Hex A and expressed high heat-stable Hex I and Hex B activities (3-6-fold). After neuraminidase treatment, Hex A peak sharpened while Hex I peaks switched to higher pI than normal Hex I, in the region of Hex B. PreHex A/S pI was not affected. Hydrolytic properties using the both substrates (4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside and 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-galactopyranoside) of each molecular form were similar in transformed and non-transformed cells. Data derived from the use of a mixture of substrates were consistent with the model which proposes a common active site for either substrate in the case of preHex A, Hex B and Hex I, but not for Hex A. Thus Epstein-Barr virus-transformed lymphoid cell lines represent an accurate model system for studies on Tay-Sachs disease.

Separation and properties of a "neutral" hexosaminidase from embryonic chicken brain

1. A "neutral" hexosaminidase has been separated from other hexosaminidase forms (I and II) by DEAE-cellulose chromatography and characterized in embryonic (16-days old) and 1-day old chicken brains. 2. Its properties differ from those of the forms I and II. It has optimum activity at about pH 6.0 and can be eluted from DEAE-cellulose with 0.25 M KCl only. 3. It has no N-acetylgalactosaminidase activity and cannot be successfully detected after isoelectric focusing since it is very acidic and completely unstable below pH 5.0. 4. "Neutral" hexosaminidase is heat-stable at pH 6.0 and is inhibited by chloride. 5. These properties, very different from those of forms I and II, suggest that this "neutral" form of hexosaminidase would be very similar to known hexosaminidase C separated from other materials. 6. We have found no significant differences for the above-mentioned three forms in chick embryos (16-days old) in comparison with those from 1-day old chicken.

Cleavage of the (1 goes to 3)-2-acetamido-2-deoxy-beta-D-glucopyranosyl linkage present in keratan sulfate. The A and B isoenzymes of human liver hexosaminidase (EC 3.2.1.30)

The disaccharide 2-acetamido-2-deoxy-beta-D-glucopyranosyl-(1 goes to 3)-D-[1-3H]-galactitol, prepared from keratan sulfate, was rapidly hydrolyzed by the A and B isoenzymes of normal human liver hexosaminidase (EC 3.2.1.30), and by the B isoenzyme prepared from the liver of a patient who had died of Tay-Sachs disease. The disaccharide substrate was also hydrolyzed by extracts of normal, cultured-skin fibroblasts, and fibroblasts of patients with Tay-Sachs disease, whereas it was not hydrolyzed by fibroblast extracts of patients with Sandhoff disease. Thus, effective degradation of keratan sulfate, secondary to a defect of the beta subunits present in the A and B isoenzymes of hexosaminidase, may contribute to the appearance of skeletal lesions in patients affected by Sandhoff disease.

Isolation and further characterization of bovine brain hexosaminidase C

Hexosaminidase C (2-acetamido-2-deoxy-beta-D-glucoside acetamidodeoxyglucohydrolase, EC 3.2.1.30) was partially purified from bovine brain tissue. The resulting preparation, free of its lysosomal counterparts, was used for the characterization of the enzyme and for further purification (lectin affinity chromatography, hydrophobic interaction chromatography, substrate-ligand affinity chromatography, ion-exchange chromatography, chromatography on activated thiol-Sepharose 4B). Only ion-exchange chromatography on DEAE-Sephacel appeared to improve the purity. The Michaelis constant was 0.46 mM for the substrate 4-methyl-umbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside. The enzyme was not inhibited by acetate or N-acetylgalactosamine. Inhibition by N-acetylglucosamine was competitive, with a Ki value of 8.0 mM. Inhibition by divalent metal ions increased in the order Fe less than Zn less than Cu. Dithiothreitol and beta-mercaptoethanol, at an optimum concentration of about 10 mM, stimulated the activity. The enzyme is apparently not a glycoprotein since it did not bind to various lectins, nor did sialidase change its isoelectric point.

Basic findings and current developments in sphingolipidoses

Sphingolipidoses are caused by recessively inherited deficiencies of lysosomal hydrolases. The clinical backgrounds of and current biochemical and genetic approaches to the different forms and variants of gangliosidoses, trihexosylceramidosis (Fabry's disease), galactosylceramidosis (Krabbe's disease), sulfatidoses (metachromatic leukodystrophies), glucosylceramidosis (Gaucher's disease), sphingomyelinoses (Niemann-Pick disease) and ceramidosis (Farber's disease) are presented.

The multiple forms and kinetic properties of the N-acetyl-beta-D-hexosaminidases from colonic tumours and mucosa of rats treated with 1,2-dimethylhydrazine

The separation and purification of the N-acetyl-beta-D-hexosaminidase activities from tumours induced by 1,2-dimethylhydrazine in the rat colon and from colonic mucosa of tumour-bearing animals are reported. Mucosa contained N-acetylhexosaminidases A and B, as well as a third form whose properties with regard to electrophoretic mobility and thermostability lay between those of A and B. Tumours contained only N-acetylhexosaminidase A and B activities. Each form possessed both N-acetylglucosaminidase (EC 3.2.1.30) and N-acetylgalactosaminidase (EC 3.2.1.53) activities, which could not be separated by a variety of techniques. The alteration of the ratio of the two specific activities in each form during purification, together with differences in the kinetic inhibition constants and behaviour during inactivation by various reagents or a temperature of 50 degrees C, supported the belief that each form contains the two enzyme activities, glucosaminidase and galactosaminidase, at separate active sites. This model is in contrast with that reported for these activities from a number of other sources. A variety of treatments reported to cause the conversion of form A into a form resembling B failed to produce such an effect on the rat colonic hexosaminidases.

Binding of human liver hydrolases by immobilized lectins

The binding of 22 human liver hydrolase activities by immobilized lectins of six different carbohydrate specificities, namely alpha-D-mannose (glucose), D-N-acetylglucosamine, D-N-acetylgalactosamine, L-fucose, alpha-D-galactose and beta-D-galactose, were examined. Differences in binding among these enzymes and within specific enzymes were observed. For example, the neutral forms of alpha-mannosidase and beta-xylosidase were bound by the Ulex europaeus lectin I (specific for L-fucose), whereas the acidic forms were not. Bandierea simplicifolia lectin (specific for alpha-galactose) bound 65% of beta-glucuronidase activity; recycling experiments demonstrated complete binding of the enzyme that had been eluted with the competitor D-galactose and no binding of the fraction that was not initially bound. These results suggested the presence of two forms of this enzyme. Similar data were obtained for acidic beta-galactosidase activity. These experiments may provide the basis for the expanded use of immobilized lectins for purification and characterization of hydrolases and other glycoproteins.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

N-acetyl-beta-d-glucosaminidase in marmoset kidney, serum and urine

N-Acetyl-beta-D-glucosaminidase activities were determined in homogenates of marmoset kidney, in serum and in urine by using the 4-methylumbelliferyl substrate. The enzyme activity was separated into several components by DEAE-cellulose ion-exchange chromatography, starch-gel electrophoresis and isoelectric focusing. The kidney contained two major forms of the enzyme, A and B, which had similar pH optima and Km values. The A-form bound to DEAE-cellulose at pH 6.8, migrated towards the anode on starch-gel electrophoresis and had a pI of 5.0. The B-form did not bind to DEAE-cellulose at pH 6.8, remained near the origin on starch-gel electrophoresis and had a pI of 7.64. The isoenzymes also differed in heat stability, the B-form being the more stable. Serum contained B-form activity and, in addition, two intermediate forms (I1 and I2) were loosely bound to DEAE-cellulose. The serum A-form activity was less firmly bound to DEAE-cellulose than was the tissue A-form and was designated As. Serum from a pregnant marmoset contained a form which may be analogous to the human P-isoenzyme. Urine contained only a small amount of B-form activity, the majority being present in the A-form. The kidney A- and B-forms both had mol.wts. of 96000--100000 and the activity was predominantly lysosomal. Partial purification of the kidney A isoenzyme was undertaken. Immunoprecipitation studies indicated a relationship between marmoset kidney A-form and human liver A-form activity.

A difference in the specificities of human liver N-acetyl-beta-hexosaminidases A and B detected by their activities towards glycosaminoglycan oligosaccharides

N-Acetyl-beta-hexosaminidases A and B differ in their activities towards oligosaccharides prepared from glycosaminoglycans. Trisaccharides from hyaluronic acid and desulphated chondroitin 4-sulphate were hydrolysed by N-acetyl-beta-hexosaminidase A, but not by N-acetyl-beta-hexosaminidase B.

Characterization of proteins structurally related to human N-acetyl-beta-D-glucosaminidase

Those proteins of human liver that cross-reacted with antibodies raised to apparently homogenous hexosamindases A and B were detected by immunodiffusion. Cross-reacting proteins with high molecular weights (greater than 2000000) and intermediate molecular weights (70000--200000) were present both in the unadsorbed fraction and in the 0.05--0.2M-NaCl eluate obtained by DEAE-cellulose chromatography at pH7.0. The unadsorbed fraction also contained a cross-reacting protein of low molecular weight (10000--70000). The possible structural and functional relationships between hexosaminidase and the cross-reacting proteins are discussed. An apparently cross-reacting protein present in the 0.05M-NaCl eluate from the DEAE-cellulose column was serologically unrelated to hexosaminidase, but it gave a reaction of immunological identify with one of the apparently cross-reacting proteins having the charge and size characteristics of hexosaminidase A. It is suggested that immunochemical methods may provide criteria for the homogeneity of enzyme preparations superior to those of conventional methods.

Glycosphingolipid hydrolases: properties and molecular genetics

This is a review of the properties and molecular genetics of six lysosomal hydrolases: beta-galactosidase, hexosaminidases A and B, alpha-galactosidase, beta-glucosidase and alpha-fucosidase. Each enzyme is discussed with regards to isoenzymes and substrate specificity, subunit structure, genetic relationship of isoenzymes and genetic variants. The molecular genetics of human diseases caused by deficiencies of each enzyme are discussed.

The tissue distribution of hexosaminidase S and hexosaminidase C

The proportion of hex S to hex C in normal and Sandhoff's fibroblasts was determined to be between 1:1 and 1:2 by differential staining of hex S at pH 4.4 with 4-methylumbelliferyl-beta-N-acetylgalactosaminide and of hex C at pH 7.0 with 4-methylumbelliferyl-beta-N-acetylglucosaminide. Hex S and hex C were also semi-quantitated in various normal tissues--brain, liver, spleen, heart, kidney, intestine, placenta, skeletal muscle and fibroblasts. Hex C was most prominent in brain and, somewhat less so, in liver, skeletal muscle and fibroblasts. The greatest amount of hex S activity was found in fibroblast, but it was also observed in lesser amounts in liver, kidney, intestine and placenta.

Purification of two hexosaminidases from human kidney

Hexosaminidase forms A and B were isolated from human kidney in a homogeneous state as demonstrated by electrophoretic and enzymic criteria. The enzymes were stable for at least 18 months when stored at -20 degrees C in 0.025 M-phosphate buffer, pH 6.5. The molecular weights of forms A and B were estimated by gel filtration to be 111 000 +/- 1500 and 114 000 +/- 1600 respectively. The molecular weights of hexosamidase A and B subunits were determined by using polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate. Hexosaminidase A dissociated into one subunit with mol.wt. 68 000. Hexosaminidase B dissociated into three subunits with mol. wts. 100 000, 68 000 and 37000 respectively, and one protein band of mol.wt. 140 000. After treatment of hexosaminidases A and B with iodoacetic acid, the molecular weights of the carboxymethylated polypeptide subunits were also estimated. Carboxymethylated hexosaminidase A dissociated into one major subunit of mol.wt. 18 000 and two other protein bands of mol.wts. 65 000 and 100 000. Carboxymethylated hexosaminidase B dissociated into one major subunit for mol.wt. 19 000 and an additional band of mol.wt. 37 000. The Km of the enzymes for the synthetic substrate p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside was 0.8 mM. Both enzymes were inhibited or activated by various metal ions. Double pH optima for the enzymes were found at pH 4.5 and 4.8.

Lysosomal hydrolase secretion by Tetrahymena: a comparison of several intralysosomal enzymes with the isoenzymes released into the medium

Tetrahymena pyriformis were grown in proteose-peptone medium and then washed and incubated in a dilute salt solution for one hour. The cells were then discarded and the lysosomal hydrolases that had been secreted were subjected to DEAE cellulose column chromatography. At least three isoenzymes of acid phosphatase, three of acid protease, and two of beta-N-acetylhexoseaminidase were found, as well as single peaks of alpha-mannosidase, beta-galactosidase, and beta-fucosidase. The latter two activities were not resolved by the DEAE column and could not be separated in a second chromatographic step on CM-cellulose. Cells were also grown under identical conditions and homogenized in 0.25 M sucrose in order to allow comparison of some of the intracellular lysosomal hydrolases with their secreted counterparts. Two lysosomal populations were resolved by sucrose density gradient sedimentation, a heavy lysosomal fraction, contered at a density of about 1.25 gm/cm3, and a light lysosomal fraction, centered at a density of about 1.16 gm/cm3. These two populations differed in that the light lysosomes did not appear to contain significant amounts of beta-fucosidase, beta-galactosidase, or acid protease, whereas all six of the hydrolase activities studied were present in the heavy lysosomes. The light lysosomal peak occurred in cells grown to transition phase, but was markedly reduced in cells from cultures grown to stationary phase. In addition to these two fractions a third very light particle, containing only alpha-mannosidase activity, was detected just inside the gradient. Measurements were made of the effect of heat (10 minutes at 66 degrees) and of a change in pH from 4.5 (standard assay condition) to 6.0 on the three acid phosphatases and two beta-N-acetylhexoseaminidase isoenzymes resolved by DEAE column chromatography of the secreted hydrolases and on these hydrolyases in the heavy and light lysosomal fractions on the sucrose gradient. Use of the thermostability and pH criteria permitted computation of the expected properties of the intralysosomal acid phosphatase and hexoseaminidase activities if these consisted of the respective isoenzymes in the proportions secreted. It was found that neither the intralysosomal acid phosphatase nor the intralysosomal hexoseaminidase had the properties expected if they consisted of the secreted mixture of the respective isoenzymes, indicating that modification of some of these isoenzymes may have occurred during the 1-hour starvation period or after secretion.

Characterization and tissue distribution of N-acetyl hexosaminidase C: suggestive evidence for a separate hexosaminidase locus

1. An electrophoretic system in which N-acetyl hexosaminidase C (HEX(C)) MIGRATES LESS ANODALLY THAN N-acetyl hexosaminidase A (HEX(A)) is described. 2. HEX(C) is shown to differ from HEX(A) and HEX(B) in substrate specificity, molecular size and affinity for Concanavalin-A. 3. HEX(C) is present in a wide range of adult and foetal tissues and in tissues from patients with Tay-Sachs and Sandhoff's diseases. It is particularly prominent in brain, testis, thymus and lymphoblastoid cell extracts and in several foetal tissues. 4. It is suggested that HEX(C) is coded at a separate gene locus from HEX(A) and HEX(B).

Studies on human N-acetyl-Beta-d-hexosaminidase C separated from neonatal brain

Human brain hexosaminidase C was separated from isoenzymes A and B by Sephadex G-200 gel filtration. Properties of the enzyme were studied, particularly its isoelectric-focusing profile, pI4.80. These findings indicate that hexosaminidase C is identical with the major residual component of Sandhoff fibroblasts with respect to substrate specificity, pI and activity pH optimum.

The separation of bovine brain beta-N-acetyl-D-hexosaminidases. Abnormal gel-filtration behaviour of beta-N-acetyl-D-glucosaminidase C

Bovine brain tissue was extracted and the 50 000g supernatant was separated by electrophoresis, DEAE-Sephadex chromatography and gel filtration on Sephadex G-200 and Bio-Gel P-200. The electrophoretic separation showed that the beta-N-acetyl-D-hexosaminidases (hexosaminidases) of bovine brain tissue were composed of four different fractions. Two fractions (A and B) exerted both glucosaminidase and galactosaminidase activity, a third fraction (C) showed only glucosaminidase activity, whereas a fourth form (D) with specificity towards the galactosaminide moiety was found to be present. DEAE-Sephadex chromatography at pH 7.0 showed that the B form was eluted with the void volume, whereas the A and D forms could be eluted in one peak by raising that salt concentration. The C form could not be detected in the eluate. Gel filtration on Sephadex G-200 showed that the B, A and D forms had almost equal molecular weights. In this case also the C form could not be detected in the column eluates. Gel filtration on Bio-Gel P-200 revealed that the C form was eluted with the void volume.

Characterization of Hex S, the major residual beta hexosaminidase activity in type O Gm2 gangliosidosis (Sandhoff-Jatzkewitz disease)

Hex S, the major residual beta hexosaminidase activity present in tissues, fluids, and cultured skin fibroblasts of patients with type 0 GM2 gangliosidosis, was isolated and characterized biochemically and immunologically. when appropriate tissue homogenates were tested by electrophoresis on cellulose acetate gels, hex S as well as hex C, the corresponding minor beta hexosaminidase component found in normal visceral tissues, migrated with greater anodic mobilities than hex A. However, a small but reproducible electrophoretic difference was observed between partially purified hex S and hex C components. Hex S and hex C had slightly higher apparent molecular weights than those of hex A or hex G; no major differences were found between hex S and hex A in thermostability, pH optimum, or kinetic properties. Hex S, like hex C from placenta, reacted with an antiserum directed towards the unique antigenic determinants alpha of hex A, indicating that hex S, hex C, and hex A share a common antigenic determinant. No reactivity of hex S was detected with an antiserum directed toward the common antigenic determinant beta of hex A and hex B. These results suggest that further biochemical and immunologic characterization of hex S and elucidation of its relationships with hex A, hex B, and hex C may significantly contribute to the understanding of the molecular defects in the GM2 gangliosidoses.

Studies on beta-D-N-acetylhexosaminidase. Various isozymes in tissues of normal subjects and Sandhoff's disease patients

Hexosaminidase (EC 3.2.1.51) activity from human liver and kidney extract was completely precipitated by anti-hexosaminidase A antiserum and 80 to 90% by anti-hexosaminidase B antiserum. Immunologically distinct hexosaminidase "C" could not be detected in these tissues. The final fractions of hexosaminidase A eluted from DE-52 chromatography were resolved into several enzymatically active components by rechromatography. Compared to hexosaminidase A and B, these minor components are more anodal in polyacrylamide disc electrophoresis. The residual activity of hexosaminidase from liver and fibroblasts of patients with Sandhoff's disease has also been resolved into similar components. The enzyme activity of these more anodal hexosaminidase components was precipitated completely by anti-hexosaminidase A anti-serum and partially by anti-hexosaminidase B antiserum. The minor, more anodal components probably represent hexosaminidase molecules having an altered ratio of subunits or the degradation products of hexosaminidase A.