Post- and pre-natal assessment of alpha-L-iduronidase deficiency with a radiolabelled natural substrate

Genotype-phenotype correlations in mucopolysaccharidosis type I using enzyme kinetics, immunoquantification and in vitro turnover studies

Fibroblasts from 16 patients with known alpha-L-iduronidase gene mutations and different clinical phenotypes of mucopolysaccharidosis type I (MPS I) were investigated in order to establish genotype/phenotype correlations. Enzyme kinetic studies were performed using the specific alpha-L-iduronidase substrate iduronosyl anhydro[1-3H]mannitol-6-sulfate. Specific residual enzyme activities were estimated using the kinetic parameters and an immunoquantification assay which determines levels of alpha-L-iduronidase protein. Cells were cultured in the presence of [35S]sulfate and the in vivo degradation of accumulated labelled glycosaminoglycans measured after different chase times. Residual enzyme activity and different amounts of residual enzyme protein were present in extracts from 9 of 16 cell lines covering a wide spectrum of clinical severity. Catalytic capacity, calculated as the product of kcat/Km and ng iduronidase protein per mg cell protein, was shown in most cases to be directly related to the severity of clinical phenotype, with up to 7% of normal values for patients with the attenuated form of MPS I (Scheie) and less than 0.13% for severely affected patients (Hurler) In vitro turnover studies allowed further refinement of correlations between genotype and phenotype. Scheie disease compared to Hurler disease patients were shown to accumulate smaller amounts of glycosaminoglycans that were also turned over faster. A combination of turnover and residual enzyme data established a correlation between the genotype, the biochemical phenotype and the clinical course of this lysosomal storage disorder.

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.

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 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.

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.

alpha-L-Iduronidase deficiency in mucopolysaccharidosis type I against a radiolabelled sulfated disaccharide substrate derived from dermatan sulfate

alpha-L-Iduronidase activity was assayed by incubation of a radiolabelled disaccharide, O-(alpha-L-idopyranosyluronic acid)-(1----3)-2,5 anhydro-D-[1,3H]-talitol 4-sulfate (IdoA-anT4S) derived from dermatan sulfate, with homogenates of leucocytes, cultured amniotic cells and skin fibroblasts from normal individuals and patients affected with an alpha-L-iduronidase-deficiency disorder (mucopolysaccharidosis type I, MPS I), parents of such patients and patients affected with other mucopolysaccharidoses. The assay clearly distinguished affected homozygotes from normal controls, heterozygotes and other mucopolysaccharidosis types. Preliminary results show that fibroblast homogenates from patients with the MPS I Hurler phenotype were virtually unable to hydrolyse IdoA-anT4S, whereas fibroblast homogenates from a patient with a relatively mild (Scheie) phenotype exhibited a residual activity with Vmax value of 2.5 pmol/min/mg protein and an apparent Km of 21 mumol/l compared to a range of 1020-2105 pmol/min/mg for Vmax and 12-35 mumol/l for Km for fibroblasts from normal controls.

Detection of the Sanfilippo D syndrome by the use of a radiolabeled monosaccharide sulfate as the substrate for the estimation of N-acetylglucosamine-6-sulfate sulfatase

N-Acetylglucosamine-6-sulfate sulfatase activity was assayed by incubation of the radiolabeled monosaccharide N-acetylglucosamine [1-14C]6-sulfate (GlcNAc6S) with homogenates of leukocytes and cultured skin fibroblasts and concentrates of urine derived from normal individuals, patients affected with N-acetylglucosamine-6-sulfate sulfatase deficiency (Sanfilippo D syndrome, mucopolysaccharidosis type IIID), and patients affected with other mucopolysaccharidoses. The assay clearly distinguished affected homozygotes from normal controls and other mucopolysaccharidosis types. The level of enzymatic activity toward GlcNAc6S was compared with that toward a sulfated disaccharide and a sulfated trisaccharide prepared from heparin. The disaccharide was desulfated at the same rate as the monosaccharide and the trisaccharide at 30 times that of the monosaccharide. Sulfatase activity toward glucose 6-sulfate and N-acetylmannosamine 6-sulfate was not detected. Sulfatase activity in fibroblast homogenates with GlcNAc6S exhibited a pH optimum at pH 6.5, an apparent Km of 330 mumol/liter, and inhibition by both sulfate and phosphate ions. The use of radiolabeled GlcNAc6S substrate for the assay of N-acetylglucosamine-6-sulfate sulfatase in leukocytes and skin fibroblasts for the routine enzymatic detection of the Sanfilippo D syndrome is recommended.

Selective depolymerisation of dermatan sulfate: production of radiolabelled substrates for alpha-L-iduronidase, sulfoiduronate sulfatase, and beta-D-glucuronidase

Radiolabelled disaccharide substrates for alpha-L-iduronidase, beta-D-glucuronidase, and sulfoiduronate sulfatase have been prepared from dermatan sulfate by application in sequence of N-deacetylation, deaminative cleavage, and reduction with NaBT4. The yield of disaccharides was approximately 87% of the total oligosaccharide fraction. Five disaccharides were isolated and tentatively identified. The major disaccharide, O-(alpha-L-idopyranosyluronic acid)-(1 leads to 3)-2,5-anhydro-D-[1-3H]talitol 4-sulfate (IdoA-anT4S), represented approximately 75% of the total disaccharide fraction. The other disaccharides were O-(alpha-L-idopyranosyluronic acid 2-sulfate)-(1 leads to 3)-2,5-anhydro-D-[1-3H]talitol 4-sulfate (IdoA2S-anT4S), O-(beta-D-glucopyranosyluronic acid)-(1 leads to 3)-2,5-anhydro-D-[1-3H]talitol 4-sulfate (GlcA-anT4S), O-(beta-D-glucopyranosyluronic acid)-(1 leads to 3)-2,5-anhydro-D-[1-3H]talitol 6-sulfate (GlcA-anT6S), and O-(alpha-L-idopyranosyluronic acid)-(1 leads to 3)-2,5-anhydro-D-[1-3H]talitol (IdoA-anT), which represented approximately 4.5, 11.2, 1.0, and 1.8%, respectively, of the total disaccharide fraction. When incubated with cultured skin-fibroblasts from normal controls, IdoA-anT4S was shown to be a sensitive substrate for alpha-L-iduronidase to produce 2,5-anhydro-D-talitol 4-sulfate (anT4S). Activity toward IdoA-anT4S was not observed with fibroblast homogenates from alpha-L-iduronidase-deficient patients (Mucopolysaccharidosis Type I). Similarly, normal-fibroblast homogenates degraded GlcA-anT6S to anT6S, and GlcA-anT4S to anT4S, at a rate 6 to 8 times greater than found for fibroblasts from beta-D-glucuronidase-deficient patients (Mucopolysaccharidosis Type VII). IdoA-anT4S was hydrolysed by alpha-L-iduronidase at a rate 365 times greater than that for IdoA-anT. Sulfation of the anhydro-D-[1-3H]talitol residues is an important structural determinant in the mechanism of action of alpha-L-iduronidase on disaccharide substrates. IdoA2S-anT4S was degraded to IdoA-anT4S and then to anT4S by normal-fibroblast homogenates, whereas fibroblasts from alpha-L-iduronidase-deficient and sulfoiduronate sulfatase-deficient (Mucopolysaccharidosis Type II) patients produced considerably decreased levels of anT4s and IdoA-anT4S (and anT4S), respectively.

Selective depolymerisation of keratan sulfate: production of radiolabelled substrates for 6-O-sulfogalactose sulfatase and beta-D-galactosidase

Keratan sulfate (KS) was N-deacetylated with anhydrous hydrazine and then degraded with nitrous acid, and the products were reduced with NaBT4. Radiolabelled disaccharides constituted at least 76% of the total oligosaccharide fraction. Three major disaccharides were isolated and identified. Of the total disaccharide isolated from bovine intervertebral-disc and human costal-cartilage, 91 and 79%, respectively, was identified as a disulfated disaccharide, O-(beta-D-galactopyranosyl 6-sulfate)-(1 leads to 4)-2,5-anhydro-D-[1-3H]mannitol 6-sulfate (Gal6S-anM6S). The disaccharide fraction isolated from bovine-cornea KS contained only 14% of Gal6S-anM6S. The yield of monosulfated disaccharide, identified as O-beta-D-galactopyranosyl-(1 leads to 4)-2,5-anhydro-D-[1-3H]mannitol 6-sulfate, was 9, 17, and 84% of the total KS-disaccharide fraction isolated from intervertebral disc, costal cartilage, and cornea, respectively. For each of the KS type studied, the yield of unsulfated disaccharide was less than 4% of the total disaccharide-fraction. The tetrasaccharides were fractionated, on the basis of their sulfate content, into at least four species by paper electrophoresis, and some tentative structures are proposed. Disaccharide and tetrasaccharide species were evaluated as substrates for beta-D-galactosidase and 6-O-sulfogalactose sulfatase.

Detection of the Sanfilippo type B syndrome using radiolabelled oligosaccharides as substrates for the estimation of alpha-N-acetylglucosaminidase

1. The following radiolabelled disaccharides were prepared from heparin and evaluated as substrates for alpha-N-acetylglucosaminidase present in cultured skin fibroblasts: O-(alpha-3-acetamido-2-deoxy-D-glucopyranosyl)-(1 leads to 4)-L-[6,3H]idose (GlcNAc-Ido), O-(alpha-2-acetamido-2-deoxy-D-glucopyranosyl)-(1 leads to 4)-1,6 anhydro-L-[6,3H]idose (GlcNAc-anIdo), O-(alpha-2-acetamido-2-deoxy-D-glucopyranosyl)-(1 leads to 4)-L-[6,3H]idose 2-sulfate (GlcNAc-Ido(OS)), O-(alpha 2-acetamido-2-deoxy-D-glucopyranosyl)-(1 leads to 3)-L-[6,3H]idonic acid (GlcNAc-IdOA). 2. Alpha-N-Acetylglucosaminidase activity assessed with GlcNAc-IdOA was 12 times higher than the values obtained using GlcNAc-Ido, GlcNAc-anIdo and GlcNAc-Ido(OS). Less than 5% of normal activity resulted when these substrates were incubated with fibroblasts from Sanfilippo B patients. These results demonstrate that a C6 carboxyl group on the adjacent residue to the N-acetylglucosaminide moiety is an important structural requirement in the mechanism of action or binding of alpha-N-acetylglucosaminidase toward alpha-linked N-acetylglucosaminide residues. The presence of a C2 sulfate group on the adjacent residue had no effect on enzyme activity. 3. Alpha-N-Acetylglucosaminidase activity in leucocyte and fibroblast homogenates assayed using GlcNAc-IdOA as substrate clearly distinguished Sanfilippo B patients from normal controls, and Sanfilippo A, C and D patients.

Selective depolymerisation of heparin to produce radio-labelled substrates for sulfamidase, 2-acetamido-2-deoxy-alpha-D-glucosidase, acetyl-CoA:2-amino-2-deoxy-alpha-D-glucoside N-acetyltransferase, and 2-acetamido-2-deoxy-D-glucose 6-sulfate sulfatase

Heparin was carboxyl-reduced with NaBT4, and degraded under conditions of acid hydrolysis that selectively cleaved the 2-0-sulfo-L-idopyranosidic linkages. The resulting, radiolabelled-disaccharides and -tetrasaccharides were isolated by gel chromatography, and then fractionated by ion-exchange chromatography, paper chromatography, and paper electrophoresis. Of the nine disaccharides isolated and identified, eight were probably derived from the major repeating-disaccharide unit in heparin (2-deoxy-2-sulfoamino-D-glucosyl 6-sulfate leads to L-idosyluronic acid 2-sulfate). Sodium borotritide reduction and/or HNO2 deamination of these eight disaccharide fractions indicated four to contain L-idopyranose residues and the other four to contain 1,6-anhydro-L-idopyranose residues as terminal units. The latter, terminal unit probably represents a minor component formed during the acid hydrolysis. On the basis of N-acetylation, N-sulfation, and HNO2-deamination studies, and the known positions and configurations of the glycosidic and sulfate linkages in heparin, four disaccharides were identified as 0-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-(1 leads to 4)-L-[6-3H]idopyranose, 0-(2-amino-2-deoxy-alpha-D-glycopyranosyl)-(1 leads to 4)-L-[6-3H]idopyranose 2-sulfate, and 0-(2-amino-2-deoxy-alpha-D-glucopyranosyl 6-sulfate]-(1 leads to 4)-L-[6-3H]idopyranose 2-sulfate. A similar set of four disaccharides contained 1,6-anhydro-L-[6-3H]idopyranose residues in place of the L-[6-3H]idopyranose residues. The other disaccharide was tentatively identified as 0-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-(1 leads to 4)-L-[6-3H]idopyranose, the isolation of which suggests the presence of an IdA(OSO-3)-GlcNAc-IdA(OSO-3) sequence in the heparin preparation, which accounts for at least 1% of its total sequence. The tetrasaccharides were fractionated, on the basis of their sulfate content, into at least five species by ion-exchange chromatography or by paper electrophoresis. These were fractionated further into species with and without carboxyl groups, and with L-idopyranose or 1,6-anhydro-L-idopyranose residues as terminal units. Tentative structures for some of these tetrasaccharides are proposed. Disaccharide and tetrasaccharide species were evaluated before and after N-acetylation or N-sulfation, as substrates for sulfamidase, acetyl-CoA: 2-amino-2-deoxy-alpha-D-glucoside N-acetyl-transferase, 2-acetamido-2-deoxy-alpha-D-glucosidase, or 2-acetamido-2-deoxy-D-glucose 6-sulfate sulfatase in human-skin fibroblasts.

The diagnosis of the Sanfilippo C syndrome, using monosaccharide and oligosaccharide substrates to assay acetyl-CoA: 2-amino-2-deoxy-alpha-glucoside N-acetyltransferase activity

Glucosamine, galactosamine, mannosamine, several disaccharides and a tetrasaccharide were evaluated as substrates for the N-acetyltransferase involved in the pathogenesis of the Sanfilippo C syndrome. Glucosamine and alpha-D-glucosaminide disaccharides and a tetrasaccharide derived from heparin were exo-N-acetylated by homogenates of cultured skin fibroblast from normal individuals at pH 6.0 in the presence of acetyl-CoA, whereas fibroblast homogenates prepared from a Sanfilippo C patient failed to catalyse the N-acetyltransferase from acetyl-CoA to these substrates. The apparent Km values of the glucosamine and alpha-glucosaminide disaccharide N-acetyltransferase were 98 and 200 mumol/l respectively; the corresponding V values were 200 and 180 nmol.min-1.g-1 fibroblast whole cell homogenate protein respectively. Incubation of homogenates from normal individuals or the Sanfilippo C patient with glucosamine 6-phosphate and acetyl-CoA at pH 6.0 produced N-acetylglucosamine 6-phosphate. Acetyltransfer to glucosamine or glucosamine 6-phosphate in homogenates of normal fibroblasts was not inhibited by the addition of arylamines. It is proposed that N-acetyltransferase to glucosamine, glucosamine 6-phosphate and arylamines is carried out by separate enzymes. Glucosamine is a suitable substrate for the diagnostic assay of the enzyme involved in the exo-N-acetylation of alpha-glucosaminide residues at the non-reducing end of the heparan sulfate stored and excreted by Sanfilippo C patients.