Human liver sulphamate sulphohydrolase. Determinations of native protein and subunit Mr values and influence of substrate agylcone structure on catalytic properties

Pathogenic Roles of Heparan Sulfate and Its Use as a Biomarker in Mucopolysaccharidoses

Heparan sulfate (HS) is an essential glycosaminoglycan (GAG) as a component of proteoglycans, which are present on the cell surface and in the extracellular matrix. HS-containing proteoglycans not only function as structural constituents of the basal lamina but also play versatile roles in various physiological processes, including cell signaling and organ development. Thus, inherited mutations of genes associated with the biosynthesis or degradation of HS can cause various diseases, particularly those involving the bones and central nervous system (CNS). Mucopolysaccharidoses (MPSs) are a group of lysosomal storage disorders involving GAG accumulation throughout the body caused by a deficiency of GAG-degrading enzymes. GAGs are stored differently in different types of MPSs. Particularly, HS deposition is observed in patients with MPS types I, II, III, and VII, all which involve progressive neuropathy with multiple CNS system symptoms. While therapies are available for certain symptoms in some types of MPSs, significant unmet medical needs remain, such as neurocognitive impairment. This review presents recent knowledge on the pathophysiological roles of HS focusing on the pathogenesis of MPSs. We also discuss the possible use and significance of HS as a biomarker for disease severity and therapeutic response in MPSs.

Decoding the consecutive lysosomal degradation of 3-O-sulfate containing heparan sulfate by Arylsulfatase G (ARSG)

The lysosomal degradation of heparan sulfate is mediated by the concerted action of nine different enzymes. Within this degradation pathway, Arylsulfatase G (ARSG) is critical for removing 3-O-sulfate from glucosamine, and mutations in ARSG are causative for Usher syndrome type IV. We developed a specific ARSG enzyme assay using sulfated monosaccharide substrates, which reflect derivatives of its natural substrates. These sulfated compounds were incubated with ARSG, and resulting products were analyzed by reversed-phase HPLC after chemical addition of the fluorescent dyes 2-aminoacridone or 2-aminobenzoic acid, respectively. We applied the assay to further characterize ARSG regarding its hydrolytic specificity against 3-O-sulfated monosaccharides containing additional sulfate-groups and N-acetylation. The application of recombinant ARSG and cells overexpressing ARSG as well as isolated lysosomes from wild-type and Arsg knockout mice validated the utility of our assay. We further exploited the assay to determine the sequential action of the different sulfatases involved in the lysosomal catabolism of 3-O-sulfated glucosamine residues of heparan sulfate. Our results confirm and extend the characterization of the substrate specificity of ARSG and help to determine the sequential order of the lysosomal catabolic breakdown of (3-O-)sulfated heparan sulfate.

Targeting the Root Cause of Mucopolysaccharidosis IIIA with a New scAAV9 Gene Replacement Vector

No treatment is available to address the unmet needs of mucopolysaccharidosis (MPS) IIIA patients. Targeting the root cause, we developed a new self-complementary adeno-associated virus 9 (scAAV9) vector to deliver the human N-sulfoglucosamine sulfohydrolase (hSGSH) gene driven by a miniature cytomegalovirus (mCMV) promoter. In pre-clinical studies, the vector was tested at varying doses by a single intravenous (i.v.) infusion into MPS IIIA mice at different ages. The vector treatments resulted in rapid and long-term expression of functional recombinant SGSH (rSGSH) enzyme and elimination of lysosomal storage pathology throughout the CNS and periphery in all tested animals. Importantly, MPS IIIA mice treated with the vector at up to 6 months of age showed significantly improved behavior performance in a hidden task in the Morris water maze, as well as extended lifespan, with most of the animals surviving within the normal range, indicating that the vector treatment can prevent and reverse MPS IIIA disease progression. Notably, 2.5 × 1012 vector genomes (vg)/kg was functionally effective. Furthermore, the vector treatment did not lead to detectable systemic toxicity or adverse events in MPS IIIA mice. These data demonstrate the development of a safe and effective new gene therapy product for treating MPS IIIA, which further support the extended clinical relevance of platform recombinant AAV9 (rAAV9 gene delivery for treating broad neurogenetic diseases.

Lysosomal sulfatases: a growing family

Sulfatases constitute a family of enzymes that specifically act in the hydrolytic degradation of sulfated metabolites by removing sulfate monoesters from various substrates, particularly glycolipids and glycosaminoglycans. A common essential feature of all known eukaryotic sulfatases is the posttranslational modification of a critical cysteine residue in their active site by oxidation to formylglycine (FGly), which is mediated by the FGly-generating enzyme in the endoplasmic reticulum and is indispensable for catalytic activity. The majority of the so far described sulfatases localize intracellularly to lysosomes, where they act in different catabolic pathways. Mutations in genes coding for lysosomal sulfatases lead to an accumulation of the sulfated substrates in lysosomes, resulting in impaired cellular function and multisystemic disorders presenting as lysosomal storage diseases, which also cover the mucopolysaccharidoses and metachromatic leukodystrophy. Bioinformatics analysis of the eukaryotic genomes revealed, besides the well described and long known disease-associated sulfatases, additional genes coding for putative enzymes with sulfatases activity, including arylsulfatase G as well as the arylsulfatases H, I, J and K, respectively. In this article, we review current knowledge about lysosomal sulfatases with a special focus on the just recently characterized family members arylsulfatase G and arylsulfatase K.

Robust LC-MS/MS methods for analysis of heparan sulfate levels in CSF and brain for application in studies of MPS IIIA

Aim: Accumulation of heparan sulfate (HS) is associated with the neurodegenerative disorder Mucopolysaccharidosis type IIIA (MPS IIIA). Here, we compare HS levels in brain and cerebrospinal fluid (CSF) of MPS IIIA mice after treatment with a chemically modified sulfamidase (CM-rhSulfamidase). Materials & methods: Two LC-MS/MS methods were adapted from literature methodology, one to measure HS metabolites (HS_met), the other to measure digests of HS after heparinase treatment (HS_dig). Results: The HS_met and HS_dig methods showed similar relative reduction of HS in brain after CM-rhSulfamidase administration to MPS IIIA mice and the reduction was reflected also in CSF. Conclusion: The results of the two methods correlated and therefore the HS_dig method can be used in clinical studies to determine HS levels in CSF from patients with MPS IIIA.

Detection of mucopolysaccharidosis III-A (Sanfilippo Syndrome-A) in dried blood spots (DBS) by tandem mass spectrometry

BACKGROUND

With ongoing efforts to develop improved treatments for Sanfilippo Syndrome Type A (MPS-IIIA), a disease caused by the inability to degrade heparan sulfate in lysosomes, we sought to develop an enzymatic activity assay for the relevant enzyme, sulfamidase, that uses dried blood spots (DBS).

METHODS

We designed and synthesized a new sulfamidase substrate that can be used to measure sulfamidase activity in DBS using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

Sulfamidase activity was readily detected in DBS using the new substrate and LC-MS/MS. Sulfamidase activity showed acceptable linearity proportional to the amount of enzyme and reaction time. Sulfamidase activity in 238 random newborns was well elevated compared to the range of activities measured in DBS from 8 patients previously confirmed to have MPS-IIIA.

CONCLUSIONS

This is the first report of an assay capable of detecting sulfamidase in DBS. The new assay could be useful in diagnosis and potentially for newborn screening of MPS-IIIA.

Functional correction of neurological and somatic disorders at later stages of disease in MPS IIIA mice by systemic scAAV9-hSGSH gene delivery

The reversibility of neuropathic lysosomal storage diseases, including MPS IIIA, is a major goal in therapeutic development, due to typically late diagnoses and a large population of untreated patients. We used self-complementary adeno-associated virus (scAAV) serotype 9 vector expressing human N-sulfoglucosamine sulfohydrolase (SGSH) to test the efficacy of treatment at later stages of the disease. We treated MPS IIIA mice at 1, 2, 3, 6, and 9 months of age with an intravenous injection of scAAV9-U1a-hSGSH vector, leading to restoration of SGSH activity and reduction of glycosaminoglycans (GAG) throughout the central nervous system (CNS) and somatic tissues at a dose of 5E12 vg/kg. Treatment up to 3 months age improved learning ability in the Morris water maze at 7.5 months, and lifespan was normalized. In mice treated at 6 months age, behavioral performance was impaired at 7.5 months, but did not decline further when retested at 12 months, and lifespan was increased, but not normalized. Treatment at 9 months did not increase life-span, though the GAG storage pathology in the CNS was improved. The study suggests that there is potential for gene therapy intervention in MPS IIIA at intermediate stages of the disease, and extends the clinical relevance of our systemic scAAV9-hSGSH gene delivery approach.

Sanfilippo syndrome: causes, consequences, and treatments

Sanfilippo syndrome, or mucopolysaccharidosis (MPS) type III, refers to one of five autosomal recessive, neurodegenerative lysosomal storage disorders (MPS IIIA to MPS IIIE) whose symptoms are caused by the deficiency of enzymes involved exclusively in heparan sulfate degradation. The primary characteristic of MPS III is the degeneration of the central nervous system, resulting in mental retardation and hyperactivity, typically commencing during childhood. The significance of the order of events leading from heparan sulfate accumulation through to downstream changes in the levels of biomolecules within the cell and ultimately the (predominantly neuropathological) clinical symptoms is not well understood. The genes whose deficiencies cause the MPS III subtypes have been identified, and their gene products, as well as a selection of disease-causing mutations, have been characterized to varying degrees with respect to both frequency and direct biochemical consequences. A number of genetic and biochemical diagnostic methods have been developed and adopted by diagnostic laboratories. However, there is no effective therapy available for any form of MPS III, with treatment currently limited to clinical management of neurological symptoms. The availability of animal models for all forms of MPS III, whether spontaneous or generated via gene targeting, has contributed to improved understanding of the MPS III subtypes, and has provided and will deliver invaluable tools to appraise emerging therapies. Indeed, clinical trials to evaluate intrathecally-delivered enzyme replacement therapy in MPS IIIA patients, and gene therapy for MPS IIIA and MPS IIIB patients are planned or underway.

Molecular characterization of arylsulfatase G: expression, processing, glycosylation, transport, and activity

Arylsulfatase G (ARSG) is a recently identified lysosomal sulfatase that was shown to be responsible for the degradation of 3-O-sulfated N-sulfoglucosamine residues of heparan sulfate glycosaminoglycans. Deficiency of ARSG leads to a new type of mucopolysaccharidosis, as described in a mouse model. Here, we provide a detailed molecular characterization of the endogenous murine enzyme. ARSG is expressed and proteolytically processed in a tissue-specific manner. The 63-kDa single-chain precursor protein localizes to pre-lysosomal compartments and tightly associates with organelle membranes, most likely the endoplasmic reticulum. In contrast, proteolytically processed ARSG fragments of 34-, 18-, and 10-kDa were found in lysosomal fractions and lost their membrane association. The processing sites and a disulfide bridge between the 18- and 10-kDa chains could be roughly mapped. Proteases participating in the processing were identified as cathepsins B and L. Proteolytic processing is dispensable for hydrolytic sulfatase activity in vitro. Lysosomal transport of ARSG in the liver is independent of mannose 6-phosphate, sortilin, and Limp2. However, mutation of glycosylation site N-497 abrogates transport of ARSG to lysosomes in human fibrosarcoma cells, due to impaired mannose 6-phosphate modification.

Structure of sulfamidase provides insight into the molecular pathology of mucopolysaccharidosis IIIA

Mucopolysaccharidosis type IIIA (Sanfilippo A syndrome), a fatal childhood-onset neurodegenerative disease with mild facial, visceral and skeletal abnormalities, is caused by an inherited deficiency of the enzyme N-sulfoglucosamine sulfohydrolase (SGSH; sulfamidase). More than 100 mutations in the SGSH gene have been found to reduce or eliminate its enzymatic activity. However, the molecular understanding of the effect of these mutations has been confined by a lack of structural data for this enzyme. Here, the crystal structure of glycosylated SGSH is presented at 2 Å resolution. Despite the low sequence identity between this unique N-sulfatase and the group of O-sulfatases, they share a similar overall fold and active-site architecture, including a catalytic formylglycine, a divalent metal-binding site and a sulfate-binding site. However, a highly conserved lysine in O-sulfatases is replaced in SGSH by an arginine (Arg282) that is positioned to bind the N-linked sulfate substrate. The structure also provides insight into the diverse effects of pathogenic mutations on SGSH function in mucopolysaccharidosis type IIIA and convincing evidence for the molecular consequences of many missense mutations. Further, the molecular characterization of SGSH mutations will lay the groundwork for the development of structure-based drug design for this devastating neurodegenerative disorder.

Mannose 6-phosphate receptor-mediated transport of sulfamidase across the blood-brain barrier in the newborn mouse

Mucopolysaccharidosis type IIIA (MPS IIIA), which is a lysosomal storage disorder (LSD) caused by inherited deficiency of sulfamidase, is characterized by severe, progressive central nervous system (CNS) dysfunction. Enzyme replacement therapy (ERT) to treat CNS storage is challenging, because the access of enzymes to the brain is restricted by the blood-brain barrier (BBB). In a prior study, we found that phosphorylated beta-glucuronidase (P-GUS) could be transcytosed across the BBB in newborn mice by the mannose 6-phosphate (M6P) receptor. In order to determine whether sulfamidase can utilize this pathway, we examined brain influx and the specificity of uptake of sulfamidase after intravenous (i.v.) injection in 2-day-old and 8-week-old mice. [(131)I]Sulfamidase was transported across the BBB in neonates at rates higher than that of simultaneously injected [(125)I]albumin. In contrast, the transport of [(131)I]sulfamidase was negligible in 8-week-old mice, thereby showing that the BBB transport mechanism is developmentally downregulated. Capillary depletion revealed that 83.7% of the [(131)I]sulfamidase taken up by the brain was in the parenchyma, demonstrating transfer across the capillary wall. The uptake of [(131)I]sulfamidase into the brain was significantly reduced by co-injections of M6P and P-GUS. That is, the transport of sulfamidase into the brain parenchyma in early postnatal life is mediated by the M6P receptor, which is shared with P-GUS and is likely accessible to other M6P-containing lysosomal enzymes.

Sulfatases: Structure, Mechanism, Biological Activity, Inhibition, and Synthetic Utility

Sulfatases, which cleave sulfate esters in biological systems, play a key role in regulating the sulfation states that determine the function of many physiological molecules. Sulfatase substrates range from small cytosolic steroids, such as estrogen sulfate, to complex cell-surface carbohydrates, such as the glycosaminoglycans. The transformation of these molecules has been linked with important cellular functions, including hormone regulation, cellular degradation, and modulation of signaling pathways. Sulfatases have also been implicated in the onset of various pathophysiological conditions, including hormone-dependent cancers, lysosomal storage disorders, developmental abnormalities, and bacterial pathogenesis. These findings have increased interest in sulfatases and in targeting them for therapeutic endeavors. Although numerous sulfatases have been identified, the wide scope of their biological activity is only beginning to emerge. Herein, accounts of the diversity and growing biological relevance of sulfatases are provided along with an overview of the current understanding of sulfatase structure, mechanism, and inhibition.

Expression and functional characterization of human mutant sulfamidase in insect cells

Mucopolysaccharidosis IIIA (MPS IIIA; Sanfilippo syndrome) is an autosomal recessive lysosomal disorder caused by the deficiency of sulfamidase (EC 3.10.1.1), required for the degradation of the mucopolysaccharide heparan sulfate. The molecular defects of 26 unrelated Spanish MPS IIIA patients were recently reported by our group. Here we describe the heterologous expression, using a baculovirus system, of the cDNAs corresponding to eight out of the 14 mutant alleles present in this patient group and the characterization of the corresponding mutant enzymes. In particular, we expressed the following alleles: p.S66W, p.R74H, p.Q85R, p.R206P, p.L386R, p.R433W, p.R433Q, and c.1079delC (previously named as c.1091delC), and the two variants of the polymorphism p.R456H. The expression of the mutant alleles and the characterization of the corresponding enzymes revealed that their activity was severely compromised. Only mutations p.S66W and p.R206P retained low levels of residual activity. However, Western blot analysis showed in all cases the presence of the expected two forms of the sulfamidase, the precursor and the mature proteins, indicating a normal processing of the mutant enzyme.

Purification and characterization of recombinant murine sulfamidase

Mucopolysaccharidosis type IIIA (MPS IIIA) is a lysosomal storage disorder caused by a deficiency in the lysosomal enzyme sulfamidase, which is required for the degradation of heparan sulfate. The disease is characterized by neurological dysfunction but relatively mild somatic manifestations. A naturally occurring mouse model to MPS IIIA exhibits a similar disease progression to that observed in patients. Disease in the mice results from a base substitution at codon 31 in the sulfamidase gene, altering an aspartic acid to an asparagine (D31N). This aspartic 31 is involved in binding of the divalent metal ion needed for catalytic function, and as such reduces the specific activity of the enzyme to about 3% of that of wild-type. The mutant protein has decreased stability and shows increased degradation over a 24 h chase period when compared to wild-type mouse sulfamidase. Mouse sulfamidase that was purified using a two-step ion exchange procedure was shown to have similar kinetic properties to that of purified human sulfamidase. Recombinant murine sulfamidase was able to correct the storage phenotype of MPS IIIA fibroblasts after endocytosis via the mannose-6-phosphate receptor.

Enzyme-replacement therapy from birth delays the development of behavior and learning problems in mucopolysaccharidosis type IIIA mice

Mucopolysaccharidosis type IIIA (MPS IIIA; Sanfilippo syndrome) is a lysosomal storage disorder characterized by severe CNS degeneration, resulting in behavioral abnormalities and loss of learned abilities. Early treatment is vital to prevent long-term clinical pathology in lysosomal storage disorders. We have used naturally occurring MPS IIIA mice to assess the effects of long-term enzyme-replacement therapy initiated either at birth or at 6 wk of age. MPS IIIA and normal control mice received weekly i.v. injections of 1 mg/kg recombinant human sulfamidase until 20 wk of age. Sulfamidase is able to enter the brain until the blood-brain barrier completely closes at 10-14 d of age. MPS IIIA mice that were treated from birth demonstrated normal weight, behavioral characteristics, and ability to learn. MPS IIIA mice that were treated from birth performed significantly better in the Morris water maze than MPS IIIA mice that were treated from 6 wk of age or left untreated. A reduction in storage vacuoles in cells of the CNS in MPS IIIA mice that were treated from birth is consistent with the improvements observed. These data suggest that enzyme that enters the brain in the first few weeks of life, before the blood-brain barrier matures, is able to delay the development of behavior and learning difficulties in MPS IIIA mice.

Transport, enzymatic activity, and stability of mutant sulfamidase (SGSH) identified in patients with mucopolysaccharidosis type III A

Mucopolysaccharidosis type IIIA (MPSIIIA) is an autosomal recessive lysosomal storage disease caused by mutations in the N-sulfoglucosamine sulfohydrolase gene (SGSH; encoding sulfamidase, also sulphamidase) leading to the lysosomal accumulation and urinary excretion of heparan sulfate. Considerable variation in the onset and severity of the clinical phenotype is observed. We report here on expression studies of four novel mutations: c.318C>A (p.Ser106Arg), c.488T>C (p.Leu163Pro), c.571G>A (p.Gly191Arg), and c.1207_1209delTAC (p.Tyr403del), and five previously known mutations: c.220C>T (p.Arg74Cys), c.697C>T (p.Arg233X), c.1297C>T (p.Arg433Trp), c.1026dupC (p.Leu343fsX158), and c.1135delG (p.Val379fsX33) identified in MPSIIIA patients. Transient expression of mutant sulfamidases in BHK or CHO cells revealed that all the mutants were enzymatically inactive with the exception of c.318C>A (p.Ser106Arg), which showed 3.3% activity of the expressed wild-type enzyme. Western blot analysis demonstrated that the amounts of expressed mutant sulfamidases were significantly reduced compared with cells expressing wild type. No polypeptides were immunodetectable in extracts of cells transfected with the cDNA carrying the c.697C>T (p.Arg233X) nonsense mutation. In vitro translation and pulse-chase experiments showed that rapid degradation rather than a decrease in synthesis is responsible for the low, steady-state level of the mutant proteins in cells. The amounts of secreted mutant precursor forms, the cellular stability, the proteolytic processing, and data from double-label immunofluorescence microscopy suggest that the degradation of the majority of newly synthesized c.220C>T (p.Arg74Cys), c.571G>A (p.Gly191Arg), c.1297C>T (p.Arg433Trp), c.1026dupC (p.Leu343fsX158), and c.1135delG (p.Val379fsX33) mutant proteins probably occurs in the ER, whereas c.488T>C (p.Leu163Pro) mutant protein showed instability in the lysosomes.

Mouse sulphamidase gene: characterization of the promoter region of the gene and expression in mouse tissues

Sulphamidase is a lysosomal enzyme necessary for the degradation of heparan sulphate. The deficiency of this hydrolase causes a disorder known as mucopolysaccharidosis type IIIA, characterized by a profound neurological deterioration. Human and mouse exon-intron structures were reported without any characterization of their promoter regions [DNA Res. 3 (1996) 269; Mamm. Genome 11 (2000) 436]. The promoter region was isolated and characterized to understand the factors affecting the expression of mouse sulphamidase. The 5'-flanking region was shown to contain a GC-rich region and putative binding sites for the transcription factors SRY, MZF1 and Nkx-2.5 with no TATA or CAAT boxes present. The 5' region had promoter activity to drive luciferase gene expression in transfected COS cells. The transcription initiation site of mouse sulphamidase was mapped to a single adenine residue 355 bases upstream of ATG codon. Northern blot analysis revealed differential expression of a major transcript of 4.5 kb in all tissues examined. Finally, the 3'-untranslated region of the mouse sulphamidase gene was isolated and found to be longer than the region identified in the human gene.

Synthesis of 7-O-(2-deoxy-2-sulfamido-alpha-D-glucopyranosyl)-4-methylcoumarin sodium salt: a fluorogenic substrate for sulfamidase

The title compound, useful for testing the efficacy of heparin sulfamidase, was synthesized in good yield starting from 2-azido-2-deoxy-3,4,6-tri-O-acetyl-D-glucopyranosyl fluoride, reducing the azido group efficiently with SnCl(2)-PhSH-Et(3)N reagent and finally crystallizing the N-sulfated product from methanol after deacetylation.

Molecular genetics of mucopolysaccharidosis type IIIA and IIIB: Diagnostic, clinical, and biological implications

Mucopolysaccharidosis (MPS) types IIIA, B, C, and D are a group of autosomal recessive lysosomal storage diseases caused by mutations in one of four genes which encode enzyme activities required for the lysosomal degradation of heparan sulfate. The progressive lysosomal storage of heparan sulfate eventually results in the clinical onset of disease, which is predominantly characterized by severe central nervous system degeneration. MPS-IIIA and MPS-IIIB involve deficiencies of heparan sulfate sulfamidase (SGSH) and alpha-N-acetylglucosaminidase (NAGLU), respectively. Both the SGSH and NAGLU genes have been cloned and characterized, thereby permitting mutation analysis of MPS-IIIA and MPS-IIIB patients. A total of 62 mutations have now been defined for MPS-IIIA consisting of 46 missense/nonsense mutations, 15 small insertions/deletions, and one splice site mutation. A total of 86 mutations have been identified in the NAGLU gene of MPS-IIIB patients; 58 missense/nonsense mutations, 27 insertions/deletions, and one splice site mutation. Most of the identified mutations in the SGSH and NAGLU genes are associated with severe clinical phenotypes. Many of the missense, nonsense, and insertion/deletion mutations have been expressed in mammalian cell lines to permit the characterization of their effects on SGSH and NAGLU activity and intracellular processing and trafficking. For MPS-IIIA and MPS-IIIB many of the reported mutations are unique making screening the general population difficult. However, molecular characterization of MPS-IIIA patients has revealed a high incidence of particular mutations of different geographical origins, which will be beneficial for the molecular diagnosis of MPS-IIIA.

Heparan N-sulfatase: cysteine 70 plays a role in the enzyme catalysis and processing

Sulfatases are members of a highly conserved family of enzymes that catalyze the hydrolysis of sulfate ester bonds from a variety of substrates. The functional correlation reflects a high degree of amino acid sequence similarity along the entire length, in particular in the active site where the C(X)PSR consensus sequence is present. Cysteine undergoes an important co- or post-translation modification essential for the accomplishment of catalytic activity: conversion in formylglycine. In this work, the cysteine of heparan N-sulfatase (NS) was replaced either by a serine (C70S) or by a methionine (C70M) using site-directed mutagenesis. C70S and C70M mutant cDNAs were expressed and analyzed in COS cells; both mutations caused a loss of NS activity; however, while C70S showed a normal precursor form undergoing processing to a reduced mature form within the lysosomes, C70M was poorly synthesized and formed a complex with the molecular chaperone immunoglobulin binding protein.

Heparan N-sulfatase: in vitro mutagenesis of potential N-glycosylation sites

Heparan N-sulfatase cDNA contains five potential N-glycosylation sites at Asn positions 41, 142, 151, 264, and 413. We used site-directed mutagenesis, substituting the codon of asparagine for glutamine, to eliminate selected glycosylation sites and then performed expression studies in COS-7 cells to determine the influence on the catalytic activity, lysosomal targeting, and glycosylation-phosphorylation of the enzyme. Elimination of site 5 did not affect significantly enzyme activity; elimination of sites 2 and 4 gave a partial reduction, while elimination of sites 1 and 3 resulted in drastic reduction of catalytic activity (25 and 14%, respectively, of normal values), indicating that glycosylation of asparagine 41 and asparagine 151 is essential for catalysis and/or enzyme stability. Wild type enzyme produced in the presence of tunicamycin was also inactive, indicating that glycosylation is required for acquisition of enzyme activity and/or for enzyme stability. Metabolic labeling of each mutant cDNA, transiently transfected into COS cells, showed that enzyme from mutants N142Q, N264Q, and N413Q appeared to be properly folded, as judged by its ability to be proteolytically processed to a lower molecular weight form, while enzyme from mutants N41Q and N151Q did not reach lysosomes. These studies confirm that the five glycosylation sites of heparan N-sulfatase are all functional and show that Asn 41 and Asn 151 have a role in protein folding and/or stability.

Expression and characterization of wild type and mutant recombinant human sulfamidase. Implications for Sanfilippo (Mucopolysaccharidosis IIIA) syndrome

Mucopolysaccharidosis IIIA (MPS-IIIA) is an autosomal recessive lysosomal storage disorder caused by the deficiency of sulfamidase (NS; EC 3.10.1.1), resulting in defective degradation and storage of heparan sulfate. This paper reports the production and characterization of monoclonal and polyclonal antibodies against recombinant human sulfamidase (rhNS) to quantitate and characterize normal and mutant sulfamidase produced from the wild type NS expression vector. Glycosylation and phosphorylation studies of immunoprecipitated rhNS show that all five potential glycosylation sites are utilized, with three high mannose/hybrid oligosaccharides and two simpler chains, with at least one functional mannose 6-phosphate group. An NS quantification system was developed to determine the effect of the three most common and severe patient mutations: S66W (Italy), R74C (Poland), and R245H (The Netherlands). The quantity and specific activity of expressed mutant rhNS was significantly lower than expressed normal rhNS, with 0.3, 0.2, and 0.05% of normal rhNS produced and 15, 17, and 83% of normal specific activity for S66W, R74C, and R245H observed, respectively. The recent structural elucidation of N-acetylgalactosamine-4-sulfatase was utilized to postulate the effect on the structure-function relationship of NS. The characterization of normal and mutated rhNS has relevance for efficient diagnosis and therapeutic developments for MPS-IIIA patients.

Sulphamidase

Sulphamidase is one of four lysosomal proteins whose deficiency clinically manifests as Sanfilippo syndrome. Deficiency of sulphamidase results in the lysosomal storage of the glycosaminoglycan (GAG) heparan sulphate (HS) and is termed mucopolysaccharidosis type IIIA (MPS IIIA). Sulphamidase catalyses the hydrolysis of an N-linked sulphate from the nonreducing terminal glucosaminide residue of HS (Fig. 1). It is unique among the known lysosomal sulphatases involved in GAG degradation in that it is an N-sulphatase, all the others being O-sulphatases. Purification of sulphamidase from human liver has facilitated the amino-terminal sequencing of the protein and hence the isolation of cDNA and genomic clones for sulphamidase. This has in turn made possible a range of further studies aimed at better diagnosis, treatment and understanding of MPS IIIA.

Human platelet heparanase: purification, characterization and catalytic activity

Heparan sulphate (HS) is an important component of the extracellular matrix (ECM) and the vasculature basal lamina (BL) which functions as a barrier to the extravasation of metastatic and inflammatory cells. Platelet-tumour cell aggregation at the capillary endothelium results in activation and degranulation of platelets. Cleavage of HS by endoglycosidase or heparanase activity produced in relatively large amounts by the platelets and the invading cells may assist in the disassembly of the ECM and BL, and thereby facilitate cell migration. Using a recently published rapid, quantitative assay for heparanase activity towards HS [Freeman, C. and Parish, C.R. (1997), Biochem. J., 325, 229-237], human platelet heparanase has now been purified 1700-fold to homogeneity in 19% yield by a five column procedure, which consists of concanavalin A-Sepharose, Zn2+-chelating-Sepharose, Blue A-agarose, octyl-agarose and gel filtration chromatography. The enzyme, which was shown to be an endoglucuronidase that degrades both heparin and HS, has a native molecular mass of 50 kDa when analysed by gel filtration chromatography and by SDS/PAGE. Platelet heparanase degraded porcine mucosal HS in a stepwise fashion from a number average molecular mass of 18.5 to 13, to 8 and finally to 4.5 kDa fragments as determined by gel filtration analysis. Bovine lung heparin was degraded from 8.9 to 4.8 kDa while porcine mucosal heparin was degraded from 8.1 kDa to 3.8 and finally to 2.9 kDa fragments. Studies of the enzyme's substrate specificity using modified heparin analogues showed that substrate cleavage required the presence of carboxyl groups, but O- and N-sulphation were not essential. Inhibition studies demonstrated an absolute requirement for the presence of O-sulphate groups. Platelet heparanase was inhibited by heparin analogues which also inhibited tumour heparanase, suggesting that sulphated polysaccharides which inhibit tumour metastasis may act to prevent both tumour cell and platelet heparanase degradation of endothelial cell surface HS and the basal laminar.

Recombinant human sulphamidase: expression, amplification, purification and characterization

Mucopolysaccharidosis type IIIA (MPS IIIA, Sanfilippo A syndrome) is a lysosomal storage disease that causes a profound neurological deterioration. The disorder is caused by a deficiency of the lysosomal enzyme sulphamidase which is a requisite for the degradation of heparan sulphate. To facilitate the development of enzyme-replacement strategies for MPS IIIA patients, we have constructed a high-level expression system for recombinant human sulphamidase in Chinese hamster ovary (CHO) cells. An expression construct containing a methotrexate-resistant dihydrofolate reductase (DHFR) gene allowed amplification of expression levels from less than 1 mg of sulphamidase per litre of culture medium to approx. 15 mg/l. Unlike many cell lines made by gene amplification in DHFR-deficient CHO cells, and utilizing the normal DHFR gene, these cell lines appeared to be stable in the absence of selective pressure. Recombinant human sulphamidase was purified from unamplified and amplified cell lines. The native enzyme was found to be a dimer of 115 kDa. Denaturing and reducing SDS/PAGE revealed a subunit size of 62 kDa. Kinetic analysis demonstrated that the recombinant enzyme had broadly similar kinetic characteristics to sulphamidase purified from liver. Recombinant human sulphamidase was able to correct the storage phenotype of MPS IIIA fibroblasts after endocytosis via the mannose-6-phosphate receptor.

A rapid quantitative assay for the detection of mammalian heparanase activity

Heparan sulphate (HS) is an important component of the extracellular matrix and the vasculature basal laminar which functions as a barrier to the extravasation of metastatic and inflammatory cells. Cleavage of HS by endoglycosidase or heparanase activity produced by invading cells may assist in the disassembly of the extracellular matrix and basal laminar, and thereby facilitate cell migration. Heparanase activity has previously been shown to be related to the metastatic potential of murine and human melanoma cell lines [Nakajima, Irimura and Nicolson (1988) J. Cell. Biochem. 36, 157-167]. To determine heparanase activity, porcine mucosal HS was partially de-N-acetylated and re-N-acetylated with [3H]acetic anhydride to yield a radiolabelled substrate. This procedure prevented the masking of, or possible formation of, new heparanase-sensitive cleavage sites as has been observed with previous methods of radiolabelling. Heparanase activity in a variety of tissues and cell homogenates including human platelets, colonic carcinoma cells, umbilical vein endothelial cells and rat mammary adenocarcinoma cells (both metastatic and non-metastatic variants) and liver homogenates all degraded the substrate in a stepwise fashion from 18.5 to approximately 13, 8 and finally to 4.5 kDa fragments, as assessed by gel-filtration analysis, confirming the substrate as suitable for the detection of heparanase activity present in a variety of cells and tissues. A rapid quantitative assay was developed with the HS substrate using a novel method for separating degradation products from the substrate by taking advantage of the decreased affinity of the heparanase-cleaved products for the HS-binding plasma protein chicken histidine-rich glycoprotein (cHRG). Incubation mixtures were applied to cHRG-Sepharose columns, with unbound material corresponding to heparanase-degradation products. Heparanase activity was determined for a variety of human, rat and murine cell and tissue homogenates. The highly metastatic rat mammary adenocarcinoma and murine lung carcinoma cell lines had four to ten times the heparanase activity of non-metastatic variants, confirming the correlation of heparanase activity with metastatic potential. Human cancer patients had twice the serum heparanase levels of normal healthy adults. The assay will be valuable for the determination of heparanase activity from a variety of tissue and cell sources, as a diagnostic tool for the determination of heparanase potential, and for the development of specific inhibitors of heparanase activity and metastasis.

A fluorimetric enzyme assay for the diagnosis of Sanfilippo disease type A (MPS IIIA)

4-Methylumbelliferyl-alpha-D-N-sulphoglucosaminide (MU-alpha-GlcNS) was synthesized and shown to be a substrate for the lysosomal heparin sulphamidase. Sanfilippo A patients' fibroblasts (n = 42) and lymphocytes (n = 1) showed 0-3% of mean normal heparin sulphamidase activity; in total leukocytes from patients (n = 8) sulphamidase activity was clearly deficient. In fibroblasts from obligate heterozygotes for Sanfilippo A, the sulphamidase activity was reduced in 9 out of 10 cases. Heparin sulphamidase desulphates MU-alpha GlcNS to MU-alpha GlcNH2 and further hydrolysis during a second incubation is required to liberate 4-methylumbelliferone, which can be measured. Yeast alpha-glucosidase, which has low but sufficient alpha-glucosaminidase activity, was used to hydrolyse the reaction intermediate MU-alpha GlcNH2 to release 4-methylumbelliferone and free glucosamine.

Cloning of the sulphamidase gene and identification of mutations in Sanfilippo A syndrome

Sanfilippo A syndrome is one of four recognised Sanfilippo sub-types (A, B, C and D) that result from deficiencies of different enzymes involved in the lysosomal degradation of heparan sulphate; patients suffer from severe neurological disorders. The Sanfilippo syndrome sub-types are also known as mucopolysaccharidosis (MPS) type III (MPS-IIIA, B, C and D), and are part of the large group of lysosomal storage disorders. Each of the MPS-III types is inherited as an autosomal recessive disorder with considerable variation in severity of clinical phenotype. The incidence of Sanfilippo syndrome has been estimated at 1:24,000 in The Netherlands with MPS IIIA (MIM #252900) the most common. MPS-IIIA is the predominant MPS-III in the United Kingdom, and has a similar high incidence to that found in The Netherlands (E. Wraith, personal communication). There is a particularly high incidence of a clinically severe form of MPS-IIIA in the Cayman Islands with a carrier frequency of 0.1 (ref. 4). Due to the mild somatic disease compared to other MPS disorders there is difficulty in diagnosing mild cases of MPS-III, hence Sanfilippo syndrome may be underdiagnosed, especially in patients with mild mental retardation. Here, we report the isolation, sequence and expression of cDNA clones encoding the enzyme sulphamidase (EC 3.10.1.1). In addition, we report the chromosomal localisation of the sulphamidase gene as being 17q25.3. An 11-bp deletion, present in sulphamidase cDNA from two unrelated Sanfilippo A patients, is described.

A cDNA clone for human glucosamine-6-sulphatase reveals differences between arylsulphatases and non-arylsulphatases

Glucosamine-6-sulphatase is an exo-hydrolase required for the lysosomal degradation of heparan sulphate and keratan sulphate. Deficiency of glucosamine-6-sulphatase activity leads to the lysosomal storage of the glycosaminoglycan, heparan sulphate and the monosaccharide sulphate N-acetylglucosamine 6-sulphate and the autosomal recessive genetic disorder mucopolysaccharidosis type IIID. Glucosamine-6-sulphatase can be classified as a non-arylsulphatase since, relative to arylsulphatase B, it shows negligible activity toward 4-methylumbelliferyl sulphate. We have isolated human cDNA clones and derived amino acid sequence coding for the entire glucosamine-6-sulphatase protein. The predicted sequence has 552 amino acids with a leader peptide of 36 amino acids and contains 13 potential N-glycosylation sites, of which it is likely that 10 are used. Glucosamine-6-sulphatase shows strong sequence similarity to other sulphatases such as the family of arylsulphatases, although the degree of similarity is not as high as that between members of the arylsulphatase family. This pattern of inter- and intra-family similarity delineates regions and amino acid residues that may be critical for sulphatase function and substrate specificity.

Human alpha-L-iduronidase. Catalytic properties and an integrated role in the lysosomal degradation of heparan sulphate

The kinetic parameters (Km and kcat) of human liver alpha-L-iduronidase were determined with a variety of heparin-derived disaccharide and tetrasaccharide substrates. More structurally complex substrates, in which several aspects of the aglycone structure of the natural substrates heparin and heparan sulphate were maintained, were hydrolysed with catalytic efficiencies up to 255 times that observed for the simplest disaccharide substrate to be hydrolysed. The major aglycone structure that influenced both substrate binding and enzyme activity was the presence of a C-6 sulphate ester on the residue adjacent to the iduronic acid residue being hydrolysed. Sulphate ions and a number of substrate and product analogues were potent inhibitors of enzyme activity. Human liver alpha-L-iduronidase activity towards 4-methylumbelliferyl alpha-L-iduronide at pH 4.8 had two Km values of 37 microM and 1.92 mM with corresponding kcat. values of 299 and 650 mol of product formed/min per mol of enzyme respectively, which may explain the wide range of Km values previously reported for alpha-L-iduronidase activity toward its substrate. Skin fibroblast alpha-L-iduronidase activity towards the heparin-derived oligosaccharides was influenced by the same substrate aglycone structural features as was observed for the human liver enzyme. A comparison was made of the effect of substrate aglycone structure upon catalytic activities of the enzymes which act to degrade the highly sulphated regions of heparan sulphate. A model was proposed whereby the substrate is directed from alpha-L-iduronidase to subsequent enzyme activities to ensure the efficient degradation of heparan sulphate.

Human glucosamine-6-sulphatase deficiency. Diagnostic enzymology towards heparin-derived trisaccharide substrates

Glucosamine-6-sulphatase (6S) activity towards a series of radiolabelled heparin-derived trisaccharide substrates was determined in cultured human skin fibroblast and leucocyte homogenates, and in urine supernatants of normal individuals and patients affected with 6S deficiency [Sanfilippo D syndrome; mucopolysaccharidosis (MPS) type IIID]. The N-sulphated and N-acetylated derivatives of the trisaccharide substrate O-(alpha-glucosamine 6-sulphate)-(1----4)-L-O-(alpha-iduronic acid 2-sulphate)-(1----4)-D-O-2,5-anhydro[1-3H]mannitol 6-sulphate (GlcNH6S-IdoA2S-anM6S) were prepared by enzymic digestion of a pentasulphated tetrasaccharide isolated following the HNO2 deamination of heparin. Purified lysosomal enzymes and MPS-patient skin fibroblasts were used along with chemical degradation to confirm the structure of each of the substrates that were utilized to study the interaction of the enzyme activities required to degrade the highly sulphated regions of heparan sulphate. Human liver, skin fibroblast and urine 6S activities were separated by chromatofocusing into at least four and possibly up to six individual activities. 6S activities present in each of the tissues generally had similar catalytic properties, including Km values, pH optima and inhibition with NaCl, Na2SO4 and NaH2PO4. Leucocyte and skin fibroblast 6S activities towards GlcNAc6S-IdoA2S-anM6S were maximal at pH 4.1 and 3.9 respectively, with Km values of 2.8 microM and 0.9-1.7 microM respectively. Urine 6S activity towards GlcNAc6S-IdoA2S-anM6S was stimulated 30-fold by BSA at pH 3.9, which shifted the pH optimum from 5.1 to 4.2 and decreased the Km value at pH 4.2 from 4.0 microM to 0.5 microM. Residual 6S activity present in the skin fibroblast homogenates from MPS IIID patients was characterized for activity towards GlcNAc6S-IdoA2S-anM6S and observed to have similar pH optima and Km values to normal skin fibroblast 6S activities, although the residual 6S activity was less than 1% of the normal control range.

Human liver N-acetylgalactosamine 6-sulphatase. Purification and characterization

Human N-acetylgalactosamine 6-sulphatase (EC 3.1.6.14), which is involved in the lysosomal degradation of the glycosaminoglycans keratan sulphate and chondroitin 6-sulphate, was purified more than 130,000-fold in 2.8% yield from liver by an eight-step column procedure. One major form was identified with a pI of 5.7 and a native molecular mass of 62 kDa by gel filtration. When analysed by SDS/PAGE, dithioerythritol-reduced enzyme contained polypeptides of molecular masses 57 kDa, 39 kDa and 19 kDa, whereas non-reduced enzyme contained a major polypeptide of molecular mass 70 kDa. It is proposed that active enzyme contains either the 57 kDa polypeptide or disulphide-linked 39 kDa and 19 kDa polypeptides. Minor amounts of other enzyme forms separated during the chromatofocusing step and the Blue A-agarose step were not further characterized. Purified N-acetylgalactosamine 6-sulphatase was inactive towards 4-methylumbelliferyl sulphate, but was active, with pH optima of 3.5-4.0, towards 6-sulphated oligosaccharide substrates. Km values of 12.5 and 50 microM and Vmax. values of 1.5 and 0.09 mumol/min per mg were determined with oligosaccharide substrates derived from chondroitin 6-sulphate and keratan sulphate respectively. Sulphate, phosphate and chloride ions were inhibitors of enzyme activity towards both substrates, with 50 microM-Na2SO4 giving 50% inhibition towards the chondroitin 6-sulphate trisaccharide substrate.

Glucuronate-2-sulphatase activity in cultured human skin fibroblast homogenates

The optimization of the assay conditions to detect glucuronate-2-sulphatase (GS) activity present in cultured human skin fibroblast homogenates towards a heparin-derived disaccharide substrate O-(beta-D-glucuronic acid 2-sulphate)-(1----4)-D-O-2,5-anhydro[l-3H]mannitol 6-sulphate (GSMS) has shown that a complex relationship exists between pH, buffer composition, ionic strength and the influence of added BSA and salts (NaCl, Na2SO4, CuCl2 and ZnCl2) to achieve maximum sulphatase activity. Whereas albumin stimulated GS activity by more than 2-fold over the pH range 2.7-5.7, CuCl2 stimulated GS activity over the narrow pH range 3.0-4.2, and inhibited GS activity at higher pH. ZnCl2 stimulated GS activity more than 3-fold at pH 3.0 and by more than 10-fold at pH 4.8. NaCl inhibited GS activity at pH 3.0, while activity between pH 4.2 and 4.8 was stimulated by up to 10-fold, resulting in a shift in the observed pH optimum from 3.0 to 4.8 in the presence of 315 mM-NaCl. Skin fibroblast GS activity toward GSMS had apparent Km values of 0.5-1.2 microM at pH 3.0, and 27.0-33.2 microM at pH 4.8. Albumin stimulated GS activity at both low and high pH by an increase in the apparent Vmax. values without significant alteration in the respective Km values. At pH 4.8, NaCl stimulated GS activity as a result of an increase in Vmax. values. These observations raise the possibility that two forms of GS activity are present in skin fibroblast homogenates: a low-Km form that has a pH optimum of 3.0 and is stimulated by BSA and a high-Km form with a pH optimum of 4.8 which is stimulated by NaCl.

Sanfilippo D syndrome: correction of glucosamine-6-sulphatase deficiency following fibroblast culture in Chang's media

The de-O-sulphation of alpha-linked glucosamine-6-sulphate residues in heparan sulphate requires a specific sulphatase, glucosamine-6-sulphatase, which has been shown to be deficient in tissues of Sanfilippo D, or mucopolysaccharidosis type IIID (MPS IIID), patients. MPS IIID fibroblasts cultured in Basal Eagle's medium supplemented with either fetal calf serum or heat-inactivated fetal calf serum, MDCB or Ultraserg media had residual glucosamine-6-sulphatase activities towards a heparin-derived trisaccharide substrate, O-(alpha-N-acetylglucosamine-6-sulphate)-(1----4)-L-O-(alpha- iduronic acid-2-sulphate)-(1----4)-D-O-2,5-anhydro[1-3H]mannitol-6- sulphate, GlcNAc6S-IdoA2S-anM6S, which were less than 1 per cent of the normal range for fibroblasts cultured in Basal Eagle's medium supplemented with fetal calf serum. However, the glucosamine-6-sulphatase activities of MPS IIID fibroblasts grown in Chang's medium were similar to the activities in normal control fibroblasts which were cultured in Basal Eagle's medium. These results indicate that caution is required for prenatal diagnosis of MPS IIID patients using chorionic villi or amniotic cells cultured in Chang's medium.

Human liver iduronate-2-sulphatase. Purification, characterization and catalytic properties

Human iduronate-2-sulphatase (EC 3.1.6.13), which is involved in the lysosomal degradation of the glycosaminoglycans heparan sulphate and dermatan sulphate, was purified more than 500,000-fold in 5% yield from liver with a six-step column procedure, which consisted of a concanavalin A-Sepharose-Blue A-agarose coupled step, chromatofocusing, gel filtration on TSK HW 50S-Fractogel, hydrophobic separation on phenyl-Sepharose CL-4B and size separation on TSK G3000SW Ultrapac. Two major forms were identified. Form A and form B, with pI values of 4.5 and less than 4.0 respectively, separated at the chromatofocusing step in approximately equal amounts of recovered enzyme activity. By gel-filtration methods form A had a native molecular mass in the range 42-65 kDa. When analysed by SDS/PAGE, dithioerythritol-reduced and non-reduced form A and form B consistently contained polypeptides of molecular masses 42 kDa and 14 kDa. Iduronate-2-sulphatase was purified from human kidney, placenta and lung, and form A was shown to have similar native molecular mass and subunit components to those observed for liver enzyme. Both forms of liver iduronate-2-sulphatase were active towards a variety of substrates derived from heparin and dermatan sulphate. Kinetic parameters (Km and Kcat) of form A were determined with a variety of substrates matching structural aspects of the physiological substrates in vivo, namely heparan sulphate, heparin and dermatan sulphate. Substrate with 6-sulphate esters on the aglycone residue adjacent to the iduronic acid 2-sulphate residue being attack were hydrolysed with catalytic efficiencies up to 200 times above that observed for the simplest disaccharide substrate without a 6-sulphated aglycone residue. The effect of incubation pH on enzyme activity towards the variety of substrates evaluated was complex and dependent on substrate aglycone structure, substrate concentration, buffer type and the presence of other proteins. Sulphate and phosphate ions and a number of substrate and product analogues were potent inhibitor of form A and form B enzyme activities.

Human liver glucuronate 2-sulphatase. Purification, characterization and catalytic properties

Human glucuronate 2-sulphatase (GAS), which is involved in the degradation of the glycosaminoglycans heparan sulphate and chondroitin 6-sulphate, was purified almost 2,000,000-fold to homogeneity in 8% yield from liver with a four-step six-column procedure, which consists of a concanavalin A-Sepharose/Blue A-agarose coupled step, a DEAE-Sephacel/octyl-Sepharose coupled step, CM-Sepharose chromatography and gel-permeation chromatography. Although more than 90% of GAS activity had a pI of greater than 7.5, other forms with pI values of 5.8, 5.3, 4.7 and less than 4.0 were also present. The pI greater than 7.5 form of GAS had a native molecular mass of 63 kDa. SDS/polyacrylamide-gel-electrophoretic analysis resulted in two polypeptide subunits of molecular mass 47 and 19.5 kDa. GAS was active towards disaccharide substrates derived from heparin [O-(beta-glucuronic acid 2-sulphate)-(1----4)-O-(2,5)-anhydro[1-3H]mannitol 6-sulphate (GSMS)] and chondroitin 6-sulphate [O-(beta-glucuronic acid 2-sulphate-(1----3)-O-(2,5)-anhydro[1-3H]talitol 6-sulphate (GSTS)]. GAS activity towards GSMS and GSTS was at pH optima of 3.2 and 3.0 respectively with apparent Km values of 0.3 and 0.6 microM respectively and corresponding Vmax values of 12.8 and 13.7 mumol/min per mg of protein respectively. Sulphate and phosphate ions are potent inhibitors of enzyme activity. Cu2+ ions stimulated, whereas EDTA inhibited enzyme activity. It was concluded that GAS is required together with a series of other exoenzyme activities in the lysosomal degradation of glycosaminoglycans containing glucuronic acid 2-sulphate residues.

Immunopurification and characterization of human alpha-L-iduronidase with the use of monoclonal antibodies

alpha-L-Iduronidase from human liver was purified by a three-step five-column procedure and by immunoaffinity chromatography with a monoclonal antibody raised against purified enzyme. Seven bands identified by staining with Coomassie Blue had molecular masses of 74, 65, 60, 49, 44, 18 and 13 kDa and were present in both preparations of the liver enzyme. However, relative to the immunopurification procedure, alpha-L-iduronidase purified by the five-column procedure was considerably enriched in the 65 kDa polypeptide band. The seven bands were identified by Western-blot analysis with two different monoclonal antibodies raised against alpha-L-iduronidase. The chromatographic behaviour of alpha-L-iduronidase on the antibody column was dependent upon the quantity of enzyme loaded. Above a particular load concentration a single peak of enzyme activity was eluted, whereas at load concentrations below the critical value alpha-L-iduronidase was eluted in two peaks of activity, designated form I (eluted first) and form II (eluted second). The following properties of the two forms of alpha-L-iduronidase were determined. (1) The two forms from liver were composed of different proportions of the same seven polypeptides. (2) When individually rechromatographed on the antibody column, each form from liver shifted to a more retarded elution position but essentially retained its chromatographic behaviour relative to the other form. (3) Forms I and II of liver alpha-L-iduronidase showed no difference in their activities towards disaccharide substrates derived from two glycosaminoglycan sources, heparan sulphate and dermatan sulphate. (4) The native molecular size of forms I and II of liver alpha-L-iduronidase was 65 kDa as determined by gel-permeation chromatography. (5) Immunoaffinity chromatography of extracts of human lung and kidney resulted in the separation of alpha-L-iduronidase into two forms, each with different proportions of the seven common polypeptide species. (6) Lung forms I and II were taken up readily into cultured skin fibroblasts taken from a patient with alpha-L-iduronidase deficiency. Liver forms I and II were not taken up to any significant extent. Lung form II gave intracellular contents of alpha-L-iduronidase that were more than double those of normal control fibroblasts, whereas lung form I gave contents approximately equal to normal control values. We propose that all seven polypeptides are derived from a single alpha-L-iduronidase gene product, and that different proportions of these polypeptides can function as a single alpha-L-iduronidase entity.(ABSTRACT TRUNCATED AT 400 WORDS)

Sanfilippo D syndrome: estimation of N-acetylglucosamine-6-sulfatase activity with a radiolabeled monosulfated disaccharide substrate

N-Acetylglucosamine-6-sulfatase activity was assayed by incubation of the radiolabeled disaccharide O-(a-N-acetylglucosamine-6-sulfate)-(1----3)-L-[6-3H]-idonic acid (GlcNAc6S-IdOA), with homogenates of leucocytes, cultured fibroblasts, and urine from normal individuals, patients affected with N-acetylglucosamine-6-sulfatase-deficiency (Sanfilippo D syndrome, mucopolysaccharidosis type IIID), and patients affected with other mucopolysaccharidoses and lysosomal storage disorders. The assay clearly distinguished affected homozygotes from their obligate heterozygotes and normal controls and other lysosomal storage disorders. Sulfatase activity in fibroblasts, leucocytes, and urine toward GlcNAc6S-IdOA exhibited a pH optimum at 4.2, 4.5, and 5.1, respectively. Sulfatase activity in fibroblasts had an apparent Km of 7.2 microM and was significantly inhibited by both sulfate and phosphate ions. The action of fibroblast or leucocyte N-acetylglucosamine-6-sulfatase activity toward GlcNAc6S-IdOA is recommended for the routine enzymatic detection and classification of mucopolysaccharidosis type IIID patients.

Human N-acetylgalactosamine-4-sulphate sulphatase. Purification, monoclonal antibody production and native and subunit Mr values

Initial purification of N-acetylgalactosamine-4-sulphate sulphatase from human liver homogenates containing approx. 1 mg of enzyme in 26 g of soluble proteins was achieved by a six-column chromatography procedure and yielded approx. 40 micrograms of a single major protein species. Enzyme thus prepared was used to produce N-acetylgalactosamine-4-sulphate sulphatase-specific monoclonal antibodies. The use of a monoclonal antibody linked to a solid support facilitated the purification of approx. 0.5 mg of N-acetylgalactosamine-4-sulphate sulphatase from a similar liver homogenate. Moreover the enzyme isolated contained a single protein species, shown by SDS/polyacrylamide-gel electrophoresis to have an Mr of 57,000, which dissociated into subunits of Mr 43,000 and 13,000 in the presence of reducing agents. Essentially identical enzyme preparations were isolated from homogenates of human kidney and lung and from concentrated human urine. The native protein Mr of enzyme from human liver and kidney was assessed by gel-permeation chromatography to be 43,000 on Ultrogel AcA and Bio-Gel P-150. The liver N-acetylgalactosamine-4-sulphate sulphatase was shown to have pH optima of approx. 4 and 5.5 with the oligosaccharide substrate (GalNAc4S-GlcA-GalitolNAc4S) and fluorogenic substrate (methylumbelliferyl sulphate) respectively. Km values of 60 microM and 4 mM and Vmax. values of 2 and 20 mumol/min per mg were determined with the oligosaccharide and fluorogenic substrates respectively.

Human liver N-acetylglucosamine-6-sulphate sulphatase. Catalytic properties

Kinetic parameters (Km and kcat.) of the two major forms (A and B) and a minor form (C) of human liver N-acetylglucosamine-6-sulphate sulphatase [Freeman, Clements & Hopwood (1987) Biochem. J. 246, 347-354] were determined with a variety of substrates matching structural aspects of the physiological substrates in vivo, namely heparin, heparan sulphate and keratan sulphate. Enzyme activity is highly specific towards glucosamine 6-sulphate or glucose 6-sulphate residues. More structurally complex substrates, in which several aspects of the aglycone structure of the natural substrate were maintained, are hydrolysed with catalytic efficiencies up to 3900 times above that observed for the monosaccharide substrate N-acetylglucosamine 6-sulphate. Forms A and B both desulphate substrates derived from keratan sulphate and heparin. Aglycone structures that influence substrate binding and/or enzyme activity were penultimate-residue 6-carboxy and 2-sulphate ester groups for heparin-derived substrates and penultimate-residue 6-sulphate ester groups for keratan sulphate-derived substrates. The 4-hydroxy group of the N-acetylglucosamine 6-sulphate or the 2-sulphaminoglucosamine 6-sulphate under enzymic attack is involved in the catalytic mechanism. The presence of a 2-amino group in place of a 2-acetamido or a 2-sulphoamino group considerably decreases the catalytic efficiency of the sulphatase, particularly in the absence of a penultimate-aglycone-residue 6-carboxy group. Both forms A and B are exo-enzymes, since activity towards internal sulphate ester bonds was not observed. The effect of incubation pH on enzyme activity towards the variety of substrates evaluated was complex and dependent on substrate aglycone structure. The presence of aglycone 2-sulphate ester, 6-carboxy group and 6-sulphate ester groups on the glucosamine 6-sulphate residue under attack considerably affects the pH response. Sulphate and phosphate ions are potent inhibitors of enzyme activity.

Human liver N-acetylglucosamine-6-sulphate sulphatase. Purification and characterization

Human N-acetylglucosamine-6-sulphate sulphatase was purified at least 50,000-fold to homogeneity in 78% yield from liver with a simple three-step four-column procedure, which consists of a concanavalin A-Sepharose/Blue A-agarose coupled step, chromatofocusing and Cu2+-chelating Sepharose chromatography. In all, four forms were isolated and partially characterized. Forms A and B, both with a pI greater than 9.5 and representing 30% and 60% respectively of the recovered enzyme activity, were separated by hydroxyapatite chromatography of the enzyme preparation obtained from the Cu2+-chelating Sepharose step. Both forms A and B had native molecular masses of 75 kDa. When analysed by SDS/polyacrylamide-gel electrophoresis, form A consists of a single polypeptide of molecular mass 78 kDa, whereas form B contained 48 kDa and 32 kDa polypeptide subunits. Neither form A nor form B was taken up from the culture medium into cultured human skin fibroblasts. The two other forms (C and D), with pI values of 5.8 and 5.4 respectively, represented approx. 7% and 3% of the total recovered enzyme activity. The native molecular masses of forms C and D were 94 kDa and approx. 75 kDa respectively. Form C contained three polypeptides with molecular masses of 48, 45 and 32 kDa. N-Acetylglucosamine-6-sulphate sulphatase activity was measured with a radiolabelled disaccharide substrate derived from heparin. The development of this substrate enabled the isolation and characterization of N-acetylglucosamine-6-sulphate sulphatase to proceed efficiently. Forms A, B and C had pH optima of 5.0, Km values of 11.7, 14.2 and 11.1 microM respectively and Vmax. values of 105, 60 and 53 nmol/min per mg of protein respectively. The molecular basis of the multiple forms of this sulphatase is not known. It is postulated that the differences in structure and properties of the four enzyme forms are due to differences in the state of processing of a large subunit.

Diagnosis of Maroteaux-Lamy syndrome by the use of radiolabelled oligosaccharides as substrates for the determination of arylsulphatase B activity

The kinetic parameters (Km and V) of human arylsulphatase B (4-sulpho-N-acetylgalactosamine sulphatase) activity in cultured skin fibroblasts were determined with a variety of substrates matching structural aspects of the physiological substrates in vivo chondroitin 4-sulphate and dermatan sulphate. More structurally complex substrates, in which several aspects of the aglycone structure of the natural substrate were maintained, were desulphated up to 4400 times faster than the minimum arylsulphatase-B-specific substrate, namely the monosaccharide N-acetylgalactosamine 4-sulphate. Aglycone structures that influence substrate binding and/or enzyme activity were an adjacent-residue C-6 carboxy group and a second but internal N-acetylgalactosamine 4-sulphate residue. Arylsulphatase B activity in fibroblast homogenates assayed with O-(beta-N-acetylgalactosamine 4-sulphate)-(1----4)-O-D-(beta-glucuronic acid)-(1----3)-O-D-N-acetyl[1-3H] galactosaminitol 4-sulphate derived from chondroitin 4-sulphate as substrate clearly distinguished Maroteaux-Lamy-syndrome patients from normal controls and other mucopolysaccharidosis patients. We recommend the use of the above trisaccharide substrate for both postnatal and prenatal diagnosis of Maroteaux-Lamy syndrome.