Electrophoretic study of hexosaminidases. Hexosaminidase C

O-GlcNAc and neurodegeneration: biochemical mechanisms and potential roles in Alzheimer's disease and beyond

Alzheimer disease (AD) is a growing problem for aging populations worldwide. Despite significant efforts, no therapeutics are available that stop or slow progression of AD, which has driven interest in the basic causes of AD and the search for new therapeutic strategies. Longitudinal studies have clarified that defects in glucose metabolism occur in patients exhibiting Mild Cognitive Impairment (MCI) and glucose hypometabolism is an early pathological change within AD brain. Further, type 2 diabetes mellitus (T2DM) is a strong risk factor for the development of AD. These findings have stimulated interest in the possibility that disrupted glucose regulated signaling within the brain could contribute to the progression of AD. One such process of interest is the addition of O-linked N-acetylglucosamine (O-GlcNAc) residues onto nuclear and cytoplasmic proteins within mammals. O-GlcNAc is notably abundant within brain and is present on hundreds of proteins including several, such as tau and the amyloid precursor protein, which are involved in the pathophysiology AD. The cellular levels of O-GlcNAc are coupled to nutrient availability through the action of just two enzymes. O-GlcNAc transferase (OGT) is the glycosyltransferase that acts to install O-GlcNAc onto proteins and O-GlcNAcase (OGA) is the glycoside hydrolase that acts to remove O-GlcNAc from proteins. Uridine 5'-diphosphate-N-acetylglucosamine (UDP-GlcNAc) is the donor sugar substrate for OGT and its levels vary with cellular glucose availability because it is generated from glucose through the hexosamine biosynthetic pathway (HBSP). Within the brains of AD patients O-GlcNAc levels have been found to be decreased and aggregates of tau appear to lack O-GlcNAc entirely. Accordingly, glucose hypometabolism within the brain may result in disruption of the normal functions of O-GlcNAc within the brain and thereby contribute to downstream neurodegeneration. While this hypothesis remains largely speculative, recent studies using different mouse models of AD have demonstrated the protective benefit of pharmacologically increased brain O-GlcNAc levels. In this review we summarize the state of knowledge in the area of O-GlcNAc as it pertains to AD while also addressing some of the basic biochemical roles of O-GlcNAc and how these might contribute to protecting against AD and other neurodegenerative diseases.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

Accumulation of free complex-type N-glycans in MKN7 and MKN45 stomach cancer cells

During the N-glycosylation reaction, it has been shown that ‘free’ N-glycans are generated either from lipid-linked oligosaccharides or from misfolded glycoproteins. In both cases, occurrence of high mannose-type free glycans is well-documented, and the molecular mechanism for their catabolism in the cytosol has been studied. On the other hand, little, if anything, is known with regard to the accumulation of more processed, complex-type free oligosaccharides in the cytosol of mammalian cells. During the course of comprehensive analysis of N-glycans in cancer cell membrane fractions [Naka et al. (2006) J. Proteome Res. 5, 88–97], we found that a significant amount of unusual, complex-type free N-glycans were accumulated in the stomach cancer-derived cell lines, MKN7 and MKN45. The most abundant and characteristic glycan found in these cells was determined to be NeuAcα2-6Galβ1-4GlcNAcβ1-2Manα1-3Manβ1-4GlcNAc. Biochemical analyses indicated that those glycans found were cytosolic glycans derived from lysosomes due to low integrity of the lysosomal membrane. Since the accumulation of these free N-glycans was specific to only two cell lines among the various cancer cell lines examined, these cytosolic N-glycans may serve as a specific biomarker for diagnosis of specific tumours. A cytosolic sialidase, Neu2, was shown to be involved in the degradation of these sialoglycans, indicating that the cytosol of mammalian cells might be equipped for metabolism of complex-type glycans.

Purification and characterization of beta-N-acetylhexosaminidase from bovine tick Boophilus microplus (Ixodide) larvae

beta-N-Acetylhexosaminidase (HEX, E.C. 3.2.1.52) from larvae of the ixodid tick Boophilus microplus was purified to capillary zone electrophoresis homogeneity, and characterized. Enzyme purification was carried out by sequential liquid chromatography on Sephadex G-200, p-aminobenzyl-N-acetyl-beta-D-thioglucosamine affinity, and Mono-Q FPLC columns. Purification was about 1600-fold, with a yield of 10%, as determined with p-nitrophenyl-N-acetylglucosaminide as substrate. The enzyme presented optimum pH 4.7, and optimum temperature 65 degrees C. The molecular weight of non-denatured enzyme was estimated as 127,000 by gel filtration chromatography, and 60,000 in SDS-PAGE. The tick hexosaminidase presented glycosyl residues, as evidenced by binding to Concanavalin-A. Among several p-nitrophenyl glycosides tested as substrate, HEX was active only on p-nitrophenyl-N-acetylglucosaminide and p-nitrophenyl-N-acetylgalactosaminide. The purified enzyme presented immunogenicity in rabbit, and the correspondent antibodies inhibited about 90% of its original, in vitro activity.

Adenoviral gene therapy of the Tay-Sachs disease in hexosaminidase A-deficient knock-out mice

The severe neurodegenerative disorder, Tays-Sachs disease, is caused by a beta-hexosaminidase alpha-subunit deficiency which prevents the formation of lysosomal heterodimeric alpha-beta enzyme, hexosaminidase A (HexA). No treatment is available for this fatal disease; however, gene therapy could represent a therapeutic approach. We previously have constructed and characterized, in vitro, adenoviral and retroviral vectors coding for alpha- and beta-subunits of the human beta-hexosaminidases. Here, we have determined the in vivo strategy which leads to the highest HexA activity in the maximum number of tissues in hexA -deficient knock-out mice. We demonstrated that intravenous co-administration of adenoviral vectors coding for both alpha- and beta-subunits, resulting in preferential liver transduction, was essential to obtain the most successful results. Only the supply of both subunits allowed for HexA overexpression leading to massive secretion of the enzyme in serum, and full or partial enzymatic activity restoration in all peripheral tissues tested. The enzymatic correction was likely to be due to direct cellular transduction by adenoviral vectors and/or uptake of secreted HexA by different organs. These results confirmed that the liver was the preferential target organ to deliver a large amount of secreted proteins. In addition, the need to overexpress both subunits of heterodimeric proteins in order to obtain a high level of secretion in animals defective in only one subunit is emphasized. The endogenous non-defective subunit is otherwise limiting.

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.

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.

Biochemical characterization of beta-hexosaminidase in different biological specimens from eleven patients with GM2-gangliosidosis B1 variant

GM2-gangliosidosis B1 variant is thought to be a rare disorder with a wide geographical and ethnic distribution. We report the biochemical findings obtained in different specimens from a group of 11 B1 variant patients originating from the north of Portugal. The biochemical data obtained seem to indicate that only one of these patients is a genetic compound presenting a clinical and biochemical pattern similar to the majority of B1 variant patients described in the literature, but somewhat different from the profile presented by the other patients reported here, who are homozygous for the 'DN-allele'.

Purification and properties of beta-N-acetylhexosaminidase A from pig brain

1. Adult pig brain beta-N-acetylhexosaminidase was separated into four different forms by ion exchange chromatography on diethylaminoethyl-cellulose. 2. Form A was purified 1300-1500 fold by an unusual procedure, the technique of ampholyte displacement, followed by chromatography on concanavalin A Sepharose. 3. The enzyme catalyses the hydrolysis of both beta-N-acetylglucosaminides and beta-N-acetylgalactosaminides. 4. The kinetic studies support the evidence of the association of both activities to a single protein, and at the same active site. 5. A natural substrate, N,N'-diacetylchitobiose, is also hydrolyzed, but not ovalbumin. 6. This enzyme may be considered as an exoglycosidase.

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.

A variant of mucolipidosis. II. Clinical, biochemical and pathological investigations

We present in this paper a patient with a clinically intermediate form of mucolipidosis (ML). Lysosomal hydrolase activity in fibroblasts was normal and levels of these enzymes in culture media were not elevated. There was a striking elevation of several hydrolases in serum and a deficiency (15% of normal) of N-acetyl-glucosamine phosphotransferase in fibroblasts. Atypical electron microscopic findings were also observed. There was no evidence of increased synthesis, slower turnover, unbalanced distribution or further changes in lysosomal enzymes. Phosphotransferase deficiency against endogenous beta-glucosaminidase and the fact that the electrophoretic mobility of lysosomal enzymes was identical to that of MLII suggest that these enzymes are not phosphorylated. Hypotheses that could explain this atypical pathology are discussed.

Irreversible inhibition of hexosaminidase C by medium-chain monocarboxylic acids and Triton X-100

The neutral beta-N-acetylhexosaminidase (hexosaminidase C) from human brain was partially purified (separated from lysosomal beta-N-acetylhexosaminidases by chromatography on a Con A-Sepharose column). Hexosaminidase C was inhibited by medium-chain fatty acids (monocarboxylic acids with chain-length between C6 and C9), whereas shorter-chain monocarboxylic acids showed no inhibitory effect. Studies on the inhibition mechanism showed an irreversible and pH-dependent inhibition which progresses with time and which is not reversed by the removal of fatty acids (by Bio-Beads SM-2). Similar inhibitory effects were also obtained using Triton X-100 (but not with homologous alkylamines). These results suggest that the hexosaminidase C inactivation is related to the hydrophobic properties of the inhibitor which acts as a denaturing agent mainly at acidic pH. The possibility has been discussed that this inactivation effect of monocarboxylic acid on hexosaminidase C could constitute a molecular model of the toxicity of medium-chain-length fatty acids.

Separation and characterization of four forms of beta-N-acetylhexosaminidase from chicken brain

Chicken brain beta-N-acetylhexosaminidases from embryos (16 and 21 days old), newborns (1 and 4 days old), and adults (3 1/2 months and 2 years old) were separated into four different forms by ion exchange chromatography on diethylaminoethyl-cellulose. Three of these forms were "acid" hexosaminidases (I, IIA, and IIB), and the fourth was a "neutral" form. Throughout development of the chicken, forms IIA and III maintained the same activity ratio, whereas that for form I decreased and that for form IIB showed an increase.

Biochemical basis of type AB GM2 gangliosidosis in a Japanese spaniel

The biochemical basis of a case of GM2 gangliosidosis in a Japanese Spaniel was studied. This dog had a massive accumulation of GM2 ganglioside in the brain. The beta-hexosaminidase activity in this affected dog brain was approximately 12 times higher than that of normal brain. However, the activity toward p-nitrophenyl-6-sulfo-2-acetamido-2-deoxyglucopyranoside was only four times higher in the affected brain than in normal brain. The GM2 activator preparation obtained from the normal dog brain could stimulate the hydrolysis of GM2 ganglioside by beta-hexosaminidase isolated from the affected dog. However, the corresponding activator fraction from the affected dog could not stimulate such a reaction. It was concluded that the biochemical basis of the GM2 gangliosidosis in this Japanese Spaniel was due to the attenuation in the stimulatory activity of GM2 activator. This case represents the first animal form similar to the activator deficiency (or defect) of Type AB GM2 gangliosidosis in humans.

Characterization of a new model of GM2-gangliosidosis (Sandhoff's disease) in Korat cats

We have detected a disorder in Korat cats (initially imported from Thailand) that is analogous to human Sandhoff's disease. Pedigree analysis indicates that this disease in an autosomal recessive disorder in the American Korat. Postmortem studies on one affected cat showed hepatomegaly that was not reported in the only other known feline model of GM2-gangliosidosis type II. Histologic and ultra-structural evaluation revealed typical storage vacuoles. There was a marked deficiency in the activity of hexosaminidase (HEX) A and B in affected brain and liver as compared to controls. Electrophoresis of a liver extract revealed a deficiency of normal HEX A and B in the affected animals. The blocking primary enzyme immunoassay verified the presence of antigenically reactive HEX present in affected cat livers in quantities slightly elevated with respect to the normal HEX concentration in control cats. In leukocytes, obligate heterozygotes had intermediate levels of total HEX activity with a slight increase in the percent activity due to HEX A. Indeed, 4 of 11 phenotypically normal animals in addition to four obligate heterozygotes appear to be carriers using this assay. Affected brain and liver compared with control brain and liver contained a great excess of bound N-acetylneuraminic acid in the Folch upper-phase solids; thin-layer chromatography showed a marked increase in GM2-ganglioside. In summary, we have characterized the pedigree, pathology, and biochemistry of a new feline model of GM2-gangliosidosis which is similar to but different from the only other known feline model.

Purification and properties of beta-N-acetyl-D-hexosaminidase from boar seminal plasma

beta-N-Acetyl-D-hexosaminidase has been purified ca. 190-fold to homogeneity from boar seminal plasma. It catalyzed the hydrolysis of the p-nitrophenyl-N-acetyl derivatives of both beta-D-glucosaminide and beta-D-galactosaminide but was inactive with the o- or p-nitrophenyl glycosides of other monosaccharides. Its pH optimum was 4.5 and its KM was 1.5 mM with p-nitrophenyl-N-acetyl-beta-D-glucosamide as substrate. The enzyme was inhibited by mercuribenzoate compounds but not by iodoacetamide, 2,2'-dipyridyl disulfide, methylmethane thiosulfonate, nor N-ethylmaleimide. The active enzyme had mol. wt ca. 250,000 by Sephacryl S-300 chromatography. SDS electrophoresis showed single bands corresponding to subunit mol. wts ca. 62,000 and 107,000 depending on whether the enzyme had been denatured in the presence of 2-mercaptoethanol or not. These data suggest that the enzyme is a tetramer of identical subunits, pairs of which are held together by disulfide bonds.

Prenatal diagnosis of atypical Tay-Sachs disease by chorionic villi sampling

We studied a family at risk for atypical TSD in which the index case showed, clinically, a late onset and a gradual psychomotor deterioration and biochemically, a residual hex. A activity in leucocytes. Two prenatal diagnoses of affected fetuses were made in this family. The first one on amniotic cells, the second one on trophoblast biopsy samples. Both of them were confirmed after abortion on cultured cells. Prenatal diagnosis of TSD, even of some atypical forms is possible using trophoblast biopsy, but formal confirmation should be obtained on cultured trophoblasts.

Effects of antisera raised against native and denatured human alpha-glucosidase and beta-hexosaminidases on native enzyme activity

Antisera were raised in rabbits against native and sodium dodecylsulfate denatured forms of human acid alpha-glucosidase and beta-hexosaminidases A and B. Anti-native enzyme antisera were able to precipitate all or nearly all enzyme activity from cell extracts, and to eliminate all stainable activity on electrophoresis. Antisera prepared against denatured enzymes precipitated only a minor part of enzyme activity. Electrophoretic analysis showed that these antisera were able to bind to the enzyme molecule. The result was a slowing down of the anodic migration but not immobilization. The use of variants with hexosaminidase deficiencies helped to clarify the action of the antisera on the various hexosaminidase isozymes.

High frequency of beta-hexosaminidase deficiency in lymphoblastoid cell lines

The activity of seven lysosomal enzymes was determined in 25 lymphoblastoid cell lines. These lines included normal controls transformed with Epstein-Barr virus, Burkitt's lymphomas and other lymphomas with or without EBV genome. Four lines were deficient in total beta-hexosaminidase activity. The deficiency was as severe as that of the variant O (Sandhoff's disease) of clinical beta-hexosaminidase deficiency. The electrophoretic pattern was also similar to that observed in Sandhoff's disease. The possible mechanisms explaining the high frequency of beta-hexosaminidase deficiency in lymphoblastoid cell lines are discussed.

A new form of residual hexosaminidase activity in infantile Tay Sachs disease fibroblasts

Fibroblast cell lines obtained from five patients with the early onset form of Tay Sachs disease (TSD) possess a species of beta-N-acetylhexosaminidase (Hex) which is more anionic than Hex B but which is stable to heating under conditions which completely inactivate Hex A. This species, which comprised between 3 and 20% of the total hexosaminidase activity in homozygous TSD fibroblasts, appeared to be unstable and upon isoelectric focussing produced a mixture of Hex B (pI = 7.2) and an isozyme with a pI of 6.2. This intermediate form of hexosaminidase was not seen in two normal fibroblast cell lines but was observed following anion exchange chromatography of extracts of fibroblast cell lines obtained from two obligate heterozygotes. A species of hexosaminidase with the same chromatographic properties, thermostability and isoelectric point as the intermediate form found in fibroblasts with the TSD genotypes can be recovered after anion exchange chromatography of a partially purified preparation of human liver Hex A that had been treated with merthiolate. We hypothesize that in TSD cells a form of the beta subunit which is usually incorporated into Hex A accumulates due to the absence of alpha subunits. This form of the beta subunit is more anionic than the beta subunit found in Hex B. In the absence of alpha subunits these anionic beta subunits form tetramers with a pI = 6.2. This form of the enzyme is unstable in the presence of cellular proteases and may be modified to Hexosaminidase B.

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.

Purification and partial characterization of the carbohydrate structure of lysosomal N-acetyl-beta-D-hexosaminidases from bovine brain

1. The lysosomal forms A and B, and an intermediate form I of N-acetyl-beta-D-hexosaminidase (EC 3.2.1.30) were isolated from bovine brain, resulting in the following purification factors and specific activities: hexosaminidase A 20255, 103 U mg-1; hexosaminidase B 34715, 134 U mg-1; hexosaminidase I 15241, 78 U mg-1. 2. The molecular weights of the polypeptide chains were identical for each isoenzyme: two bands of 50 and 53 k daltons were found. 3. Carbohydrate analysis showed the presence of mannose, galactose, N-acetylglucosamine and sialic acid. This composition, and the absence of N-acetylgalactosamine, indicated that only N-glycosidically linked oligosaccharide chains are present. 4. The amino-acid composition showed no substantial differences for the three isoenzymes.

Effect of sodium tetrathionate on the activities of some enzymes in kidney and urine

The activities of lactate dehydrogenase, glutamate dehydrogenase, aspartate aminotransferase, beta-galactosidase, N-acetyl-beta-D-glucosaminidase, leucine aminopeptidase, gamma-glutamyltransferase and alkaline phosphatase in renal tissue and urine of rats treated with sodium tetrathionate were determined. A decrease of enzyme activities in renal tissue and an increase in urine were observed. The largest decrease in the glutamate dehydrogenase of renal tissue amounted to 0.7 times the control value, and was correlated with an appropriate increase in the urine. Increases in urinary enzyme activity were especially marked for beta-galactosidase and N-acetyl-beta-D-glucosaminidase (3 and 6 times the control values, respectively). The increase in enzyme activities was not accompanied by a corresponding change in the urinary protein. Characterization of urinary lactate dehydrogenase and N-acetyl-beta-D-glucosaminidase isoenzymes also indicates the renal origin of these enzymes. The abnormally high enzyme activities of the urine correlated with the nature and degree of renal damage shown by electron microscopy.

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.

Neutral hydrolases of rat brain. Preliminary characterization and developmental changes of neutral beta-N-acetylhexosamindases

The bulk of rat brain neutral beta-N-acetylhexosaminidases (2-acetamido-2-deoxy-beta-D-hexoside acetamidodeoxyhexohydrolase, EC 3.2.1.52) were present in the cytosol fraction. They were not bound by concanavalin A-Sepharose while the acid beta-N-acetylhexosaminidases were all bound. The neutral beta-N-acetylgalactosaminidase had a pH optimum of 5.2 and Km of 0.57 mM, while the neutral beta-N-acetylgalactosaminidase had the highest reaction rate at lost more than 90% of the activity in 30 min at 50 degrees C. The galactosaminidase pH 6.0 with a Km of 0.12 mM. No divalent ions activated either of the enzymes. The galactosaminidase was heat-stable and lost only 10--20% of its activity after 3 h at 50 degrees C. The neutral glucosaminidase was inhibited by free N-acetylglucosamine but not by N-acetylgalactosamine. The reverse was found for the neutral beta-galactosaminidase. Two enzymes were separated almost completely by hydroxyapatite chromatography. Heat stability of the separated activity peaks suggested that the neutral beta-N-acetylgalactosaminidase, which was not bound to hydroxyapatite, may be specific to the galactosaminide substrate. The neutral beta-N-acetylglucosaminidase may, on the other hand, have some activity toward the galactosaminide substrate. Both of the neutral enzyme activities were highest during the first postnatal week in rat brain in contrast to the acidic enzyme which showed peak activities during the second and third weeks. These results confirmed and expanded earlier observations by Frohwein and Gatt in calf brain. The relationship of these enzymes to the hexosaminidase C in human tissues is less certain at the present time.

Purification and some properties of liver and brain beta-N-acetyl-hexosaminidase S

beta-N-Acetyl-hexosaminidase S (2-acetamido-2-deoxy-beta-hexoside acetamido-deoxyhexohydrolase, EC 3.2.1.52) was purified from liver and brain of a patient deceased of type O GM2 gangliosidosis (Sandhoff's disease). Brain beta-N-acetyl-hexosaminidase S was further purified by preparative polyacrylamide gel electrophoresis. The pH optimum of the purified liver and brain enzyme was 5.0 and Km values were 0.8--0.9 mM and 0.3--0.4 mM with 4-methylumbelliferyl-beta-D-N-acetylglucosamine and beta-D-N-acetylgalactosaminide derivatives, respectively. beta-N-Acetyl-hexosaminidase S was thermolabile losing most of its activity after 50 min at 50 degrees C. The apparent molecular weights of the purified liver and brain enzymes were 154 000 and 152 000, respectively. Hexosamines activated beta-N-acetyl-hexosaminidase S whereas the isoenzyme A and B were inhibited. The glycoprotein nature of beta-N-acetyl-hexosaminidase S was suggested by its affinity towards Concanavalin A-Sepharose.

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.

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.

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).