The patterns of arylsulphatases A and B in human normal and metachromatic leucodystrophy tissues and their relationship to the cerebroside sulphatase activity

Novel patient cell-based HTS assay for identification of small molecules for a lysosomal storage disease

Small molecules have been identified as potential therapeutic agents for lysosomal storage diseases (LSDs), inherited metabolic disorders caused by defects in proteins that result in lysosome dysfunctional. Some small molecules function assisting the folding of mutant misfolded lysosomal enzymes that are otherwise degraded in ER-associated degradation. The ultimate result is the enhancement of the residual enzymatic activity of the deficient enzyme. Most of the high throughput screening (HTS) assays developed to identify these molecules are single-target biochemical assays. Here we describe a cell-based assay using patient cell lines to identify small molecules that enhance the residual arylsulfatase A (ASA) activity found in patients with metachromatic leukodystrophy (MLD), a progressive neurodegenerative LSD. In order to generate sufficient cell lines for a large scale HTS, primary cultured fibroblasts from MLD patients were transformed using SV40 large T antigen. These SV40 transformed (SV40t) cells showed to conserve biochemical characteristics of the primary cells. Using a specific colorimetric substrate para-nitrocatechol sulfate (pNCS), detectable ASA residual activity were observed in primary and SV40t fibroblasts from a MLD patient (ASA-I179S) cultured in multi-well plates. A robust fluorescence ASA assay was developed in high-density 1,536-well plates using the traditional colorimetric pNCS substrate, whose product (pNC) acts as “plate fluorescence quencher” in white solid-bottom plates. The quantitative cell-based HTS assay for ASA generated strong statistical parameters when tested against a diverse small molecule collection. This cell-based assay approach can be used for several other LSDs and genetic disorders, especially those that rely on colorimetric substrates which traditionally present low sensitivity for assay-miniaturization. In addition, the quantitative cell-based HTS assay here developed using patient cells creates an opportunity to identify therapeutic small molecules in a disease-cellular environment where potentially disrupted pathways are exposed and available as targets.

Metachromatic leukodystrophy in Greece: observations on 4 cases

We report our findings in four cases of metachromatic leukodystrophy diagnosed in Greece during the last 4 years. The age of onset and the clinical symptoms were those described for the late infantile form of the disease. However, one patient retained his speech and mental abilities despite his pronounced motor regression and neurological involvement. This was combined with high residual arylsulphatase A activity in white blood cell homogenates even in the 0 degrees C incubation assay.

Biochemical characterization of arylsulfatases detected in granulomatous inflammation

Arylsulfatases A and B were measured in the liver of mice infected with Schistosoma mansoni. The increase of total arylsulfatases paralleled enlargement of the granulomas. It began at 7 weeks after infection and reached a maximum at 10 to 14 weeks when the enzyme activity became about 2.5 times that of normal liver. The elevated enzyme activity was due to granulomatous tissue, because when granulomas were separated from hepatic cells, the former contained the increased activity but the latter did not. Arylsulfatase A, arylsulfatase B, and arylsulfatase Bv, in both normal liver and granulomas, were separated by anion-exchange column chromatography and differences in net charges of these enzymes were demonstrated by polyacrylamide gel electrophoresis. Biochemical properties were indistinguishable between arylsulfatase B and arylsulfatase Bv while they differed from arylsulfatase A. Granulomas at 8 weeks after infection showed 3.0-, 3.5-, and 5.0-fold increases in activity for arylsulfatase A, B, and Bv, respectively. As the granulomas enlarged, by 12 weeks, arylsulfatases B and Bv activities further increased but the arylsulfatase A value remained the same as that of 8 weeks. The finding suggests that arylsulfatases are involved in granuloma development and arylsulfatases B and Bv activities may reflect functions of macrophages and other cells including fibroblasts.

Leukocyte sulfatidase for the reliable diagnosis of metachromatic leukodystrophy

A simple assay technique for the determination of sulfatidase activity in leukocytes has been developed for the reliable diagnosis of metachromatic leukodystrophy (MLD). Sulfatide is tritiated in sphingosine and fatty acid by reduction with [3H]sodium borohydride in alkali in the presence of palladium chloride. This labeled natural substrate for aryl sulfatase A (AsA) is hydrolyzed by normal human leukocytes in 25 mM-acetate buffer, pH 5.0, in the presence of 0.3% sodium taurodeoxycholate. The enzyme activity is greatly improved after dialysis, exhibiting better linearity with protein concentration. It is stimulated maximally by 5 mM-MnCl2 with an apparent Km of 0.17 mM for the substrate. Patients with MLD exhibited virtually no detectable sulfatidase activity although they had residual AsA activity that was measured with the synthetic substrate, p-nitrocatechol sulfate (NCS). Potential heterozygotes could be identified by the sulfatidase assay in instances where the NCS assay for AsA was inconclusive. Several individuals with levels of AsA activity characteristic of MLD, including a few healthy carriers and certain patients with unknown neurological diseases, were shown not to have MLD by the presence of measurable levels of sulfatidase in their leukocytes.

Arylsulphatase A and B in juvenile metachromatic leukodystrophy

A series of five living patients with juvenile metachromatic leukodystrophy (MLD), ten first-degree relatives, and a number of controls were subjected to biochemical investigations including quantitative determination of arylsulphatase A (ASA) and B (ASB) activities in peripheral leukocytes and polyacrylamide disc gel elctrophoresis of arylsulphatases. Five relatives were family members of four previously deceased patients with juvenile MLD. The mean ASA activity of the patients was 1.3 nmol of p-nitrocatechol sulphate hydrolysed in 30 min per mg protein. It was 84 nmol in the relatives, 129 nmol in other neurological patients and 136 nmol in normal controls. The corresponding ASB activity was 38 nmol in the patients, 49 nmol in the relatives, and 99 nmol in normal controls. An extremely low ASB activity, 3.4 nmol, was found in one relative. No ASA band could be visualised in the enzyme electrophoretic patterns of the patients' leukocytes but the bands representing ASB appeared normal. Seven relatives showed ASA bands weaker than normal, and the relative with low ASB activity exhibited very weak ASB band. The low ASB activity in the patients and heterozygotes may be a characteristic feature of the slowly progressive juvenile type MLD diagnosed in the present series.

A sensitive fluorescence assay for the simultaneous and separate determination of arylsulphatases A and B

A sensitive fluorometric assay utilizing 4-methylumbelliferyl sulphate has been developed for the simultaneous determination of arylsulphatases A and B from leucocytes, based on the differential effect of silver ions on the two enzymes. The procedure allows discrimination between normal cases and those with metachromatic leucodystrophy.

Arylsulfatase of human tissue. Studies on a form of arylsulfatase B found predominantly in brain

The distribution of soluble arylsulfatase (aryl-sulfate sulfohydrolases, EC 3.1.6.1) in human tissues was investigated by DEAE-cellulose chromatography, All tissues examined contained arylsulfatase A and arylsulfatase B. In addition, brain singularly contained significant quantities (15-25% of total arylsulfatase) of a minor anionic arylsulfatase from designated arylsulfatase Bm, whereas only trace amounts of arylsulfatase Bm were found in liver, kidney, testis and placenta. Arylsulfatase B and arylsulfatase Bm had equal activity toward methyl-umbelliferyl sulfate, nitrocatechol sulfate and a physiological substrate UDP-N-acetylgalactosamine 4-sulfate, but both forms were inactive toward the arylsulfatase A substrates cerebroside sulfate and ascorbic acid 2-sulfate. Purified preparations of placental arylsulfatase B, brain arylsulfatase Bm, and urinary arylsulfatase A did not hydrolyze estrone sulfate, dehydroepiandrosterone sulfate or pregnenolone sulfate. The physico-chemical properties of arylsulfatase Band arylsulfatase Bm differed with respect to thermal lability, DEAE-cellulose chromatography, polyacrylamide gel electrophoresis and isoelectric focussing. In the latter technique, utilizing thin polyacrylamide slab gels, the isoelectric point for placental arylsulfatase B was 8.2, while brain arylsulfatase Bm resolved into 3 activity bands with pI values 6.8, 7.0 and 7.2. Although the physico-chemical properties differed, arylsulfatase B and arylsulfatase Bm appear to be functionally equivalent as well as generically related.

Microheterogeneity of arylsulfatase A from human tissues

Human arylsulfatase A (cerebroside-3-sulfate 3-sulfohydrolase, EC 3.1.6.8) exhibited microheterogeneity on isoelectric focusing in polyacrylamide gels. Pure urinary enzyme gave 3 bands of activity with pI values of 4.7, 4.8 and 4.9, whereas purified liver enzyme yielded six equally spaced bands from pI 4.4 to 4.9. Detection of enzyme in the gel was made by either methylumbelliferyl sulfate or nitrocatechol sulfate. Crude enzyme preparations from human liver, kindey, placenta, brain and testis showed the six-banded pattern with varying amounts of activity in the different bands. The banding pattern of cultured human fibroblast extracts was distinctive: in addition to activity in the area of Bands 1-6 a sharp band at pI 5.1 was observed with both enzyme stains. This latter band was also present in metachromatic leukodystrophy fibroblast extracts. However, in this case the band did not appear when the specific aryl-sulfatase A stain was used. Enzyme Bands 1, 2 and 3 from urine were isolated by extraction of the gel. The three bands refocused in their initial positions; showed nearly identical enzymatic activities toward methylumbelliferyl sulfate, mitrocatechol sulfate, cerebroside sulfate and ascorbic acid 2-sulfate; and demonstrated equivalent immunological competence by antibody titration. The banding pattern of urinary arylsulfatase A was unchanged with neuraminidase treatment, whereas Bands 4-6 of the liver enzyme were converted to Bands 1-3 by this treatment. It appears that Bands 4-6 are due to sialylation of aryl-sulfatase A but that Bands 1-3 are probably due to some other type of post-ribosomal protein modification.

Clinical and ultrastructural ocular histopathologic studies of adult-onset metachromatic leukodystrophy

A twin brother and sister with adult-onset metachromatic leukodystrophy developed progressive central acuity loss and optic disk pallor. Both had normal electroretinograms and fluorescein angiography. Postmortem examination of the sister's eyes by histochemistry and electron microscopy revealed ganglion cell loss and optic atrophy. Cerebroside sulfate had accumulated in optic nerve glial cells. Optic atrophy was more advanced than in previously reported cases of infantile-onset metachromatic leukodystrophy. The pathologic process seemed to be retrograde optic nerve degeneration due to abnormal myelin metabolism.

The activator of cerebroside sulphatase. Lysosomal localization

1) An activator protein necessary for the enzymic hydrolysis of cerebroside sulphate could be partially purified from unfractionated rat liver. This activator, which is similar to that of human origin, proved to be a heat-stable, non-dialyzable, low molecular weight protein with an isoelectric point of 4.1. Its activity could be destroyed by pronase. 2) For elucidation of the subcellular localization of the activator, rat liver was fractionated by differential centrifugation. The intracellular distribution of the cerebroside sulphatase activator was compared to the distribution patterns of marker enzymes for different cell organelles and found to coincide with the lysosomal arylsulphatase, thus indicating a lysosomal localization. 3) This was confirmed using highly purified secondary, i.e. iron-loaded, lysosomes. After disruption by osmotic shock, these organelles hydrolyzed cerebroside sulphate when incubations were performed under physiological conditions with endogenous as well as exogenous sulphatase A as enzyme. 4) After subfractionation of the disrupted secondary lysosomes into membrane and lysosol fractions by high speed centrifugation, it was found that the activator protein was exclusively associated with the lysosol, whereas the acid hydrolases were distributed differently between the two fractions. 5) The lysosol was further fractionated by semi-preparative electrophoresis on polyacrylamide gels. Two protein fractions were obtained: a high molecular weight fraction, containing the activator-free acid hydrolases, and a low molecular weight fraction, containing the enzyme-free activator of cerebroside sulphatase. 6) The significance of these findings for the hydrolysis of sphingolipids in the lysosomes is discussed.

Incorporation of inorganic sulfate in rat-liver Golgi

Partial subcellular distribution and specifically the incorporation of 35SO4 in the Golgi apparatus of rat liver was studied. The isolated Golgi fraction was placed on a further sucrose gradient for subfraction and in all samples the total incorporated label, the trichloroacetic-acid-soluble and insoluble, and chloroform/methanol-soluble and insoluble amounts of label were measured. Approximately 2.3% of the total injected 35S was found in the liver, of which 2% was localized in the isolated Golgi fraction. Further subfractionation of the Golgi has shown that the majority of the 35S is localized at densities 1.12-1.13 g/cm3 in the gradient. In the isolated Golgi fraction 85% of the label was present in the trichloroacetic-acid-soluble and 15% in the insoluble part. Approximately 1% of the label in the Golgi fraction was soluble in chloroform/methanol. On the bases of radioactivity per weight of protein and total radioactivity, the Golgi fractions had the highest activity, except the first supernatant which had higher radioactivity. Briefly, a significant amount of the 35SO4 can be found in the isolated rat liver Golgi 15 min after the injection of the label, and the majority of this label is localized in the material sedimenting at density 1.12-1.13 g/cm3 after subfractionation of the Golgi apparatus.

The activator of human cerebroside sulphatase. Activating effect on the acidic forms of the sulphatases from invertebrates

1) Acidic forms of the sulphatase were partially purified from the following invertebrate species: Tethya aurantium (Porifera), Patella vulgata (mollusca), Maja squinado (Arthropoda), Marthasterias glacialis (Echinodermata) and Microcosmus sulcatus (Tunicata). Enzyme preparations thus obtained cleaved cerebroside sulphates (sulphatides) only in the presence of either specific detergents (e.g. taurodeoxycholate) or an activator protein isolated from human liver. This corresponds to the findings on purified sulphatase A of human origin. 2) At low concentrations, the activating effect was proportional to the amount of activator protein applied; at higher concentrations, proportionality was obtained only in some cases. On a molar basis, less of the activator protein was required to achieve the same activation as taurodeoxycholate. At optimum concentrations of the detergent however, the activation was much higher. 3) The enzyme specificity of the activator and some evolutionary implications are discussed.

Sulfatide excreting heterozygous carrier of juvenile metachromatic leukodystrophy or asymptomatic patient of adult metachromatic leukodystrophy

In a family with juvenile metachromatic leukodystrophy (sulfatide lipidosis) 2 patients showed residual arysulfatase A activities of 5--6%. The patients' healthy father was characterized biochemically by a 39% normal activity of leukocyte plus plasma arylsulfatase A. The father was further characterized by a high sulfatide excretion (0.2--0.5 mg/I urine) and, paradoxically, by a normal sulfatide degrading enzyme activity in vitro. This special carrier is suspected to be heterozygous for a) arylsulfatase A deficiency and b) arylsulfatase A (sulfatidase) lability. This presumed additional genetic defect could be the cause of the sulfatide excretion which, in turn, would be a sign of the preclinical stage of an exceptional form of adult metachromatic leukodystrophy. The normal sulfatidase activity seems to be due to an in vitro effect.

Comparison of the cerebroside sulphatase and the arylsulphatase activity of human sulphatase A in the absence of activators

A cerebroside sulphatase (cerebroside-3-sulphate 3 sulphohydrolase, EC 3.1.6.8) assay based on radio thin-layer chromatography is described. The substrate was labelled by the catalytic addition of tritium to cerebroside sulphate. Using this assay the cerebroside sulphatase activity of sulphatase A (Aryl-sulphate sulphohydrolase, EC 3.1.6.1) from human liver and kidney in the absence of activators was investigated. The pH optimum of this reaction depends on the buffer concentration, being pH 4.5 at 50 mM and 5.3 at 10 mM sodium formate. With the latter concentration the apparent Km for cerebroside sulphate is 0.06 mM; SO2-4 and nitrocatechol sulphate inhibit noncompetitively with a Ki of 4.51 mM for Na2SO4 and 0.43 mM for nitrocatechol sulphate. The cerebroside sulphatase activity of sulphatase A is highly dependent on the ionic strength. The optimum sodium formate concentration is 10 mM, and the cerebroside suophatase activity decreases rapidly with increasing buffer concentration. The same concentration dependence is observed in the inhibitory effect of cerebroside sulphate on the arylsulphatase reaction. The inhibition decreases at increasing buffer concentrations, becoming an activation at 70 mM sodium formate. The progress curve of the cerebroside sulphatase reaction shows a deviation from linearity similar to that of the arylsulphatase reaction. Investigation of the effect of preincubation with cerebroside sulphate on the arylsulphatase activity of the enzyme shows that cerebroside sluphatase activity and inactivation of the enzyme by cerebroside sulphate occur simultaneously. These observations are interpreted as supporting the assumption that cerebroside suophate and arylsulphates are degraded at an identical active site on the same enzyme. Differences in the properties of the cerebroside sulphatase and the arylsulphatase reaction of the enzyme may be attributed to the differences in the physiocochemical state of the two substrates.