Purification and properties of a heat-stable glucocerebrosidase activating factor from control and Gaucher spleen

Experimental abdominal aortic aneurysm growth is inhibited by blocking the JAK2/STAT3 pathway

BACKGROUND

The JAK/STAT pathway is a vital transcription signaling pathway that regulates gene expression and cellular activity. Our recently published study highlighted the role of IL-17A in abdominal aortic aneurysm (AAA) formation and rupture. IL-17A has been proven to upregulate vascular endothelial growth factor (VEGF) expression in some diseases. However, no study has demonstrated the relationships among JAK2/STAT3, IL-17A and VEGF. Therefore, we hypothesized that IL-17A may up-regulate VEGF expression via the JAK2/STAT3 signaling pathway to amplify the inflammatory response, exacerbate neovascularization, and accelerate AAA progression.

METHODS

To fully verify our hypothesis, two separate studies were performed: i) a study investigating the influence of JAK2/STAT3 on AAA formation and progression. ii) a study evaluating the relationship among IL-17A, JAK2/STAT3 and VEGF. Human tissues were collected from 7 AAA patients who underwent open surgery and 7 liver transplantation donors. All human aortic tissues were examined by histological and immunohistochemical staining, and Western blotting. Furthermore, mouse aortic tissues were also examined by histological and immunohistochemical staining and Western blotting, and the mouse aortic diameters were assessed by high-resolution Vevo 2100 microimaging system.

RESULTS

Among human aortic tissues, JAK2/STAT3, IL-17A and VEGF expression levels were higher in AAA tissues than in control tissues. Group treated with WP1066 (a selective JAK2/STAT3 pathway inhibitor), IL-17A, and VEGF groups had AAA incidences of 25%, 40%, and 65%, respectively, while the control group had an incidence of 75%. Histopathological analysis revealed that the IL-17A- and VEGF-related inflammatory responses were attenuated by WP1066. Thus, blocking the JAK2/STAT3 pathway with WP1066 attenuated experimental AAA progression. In addition, in study ii, we found that IL-17A siRNA seemed to attenuate the expression of IL-17A and VEGF in vivo study; treatment with VEGF siRNA decreased the expression of VEGF, while IL-17A expression remained high. In an in vitro study, rhIL-17A treatment increased JAK2/STAT3 and VEGF expression in macrophages in a dose-dependent manner.

CONCLUSION

Blocking the JAK2/STAT3 pathway with WP1066 (a JAK2/STAT3 specific inhibitor) attenuates experimental AAA progression. During AAA progression, IL-17A may influence the expression of VEGF via the JAK2/STAT3 signaling pathway. This potential mechanism may suggest a novel strategy for nonsurgical AAA treatment.

X-ray and biochemical analysis of N370S mutant human acid β-glucosidase

Gaucher disease is caused by mutations in the enzyme acid β-glucosidase (GCase), the most common of which is the substitution of serine for asparagine at residue 370 (N370S). To characterize the nature of this mutation, we expressed human N370S GCase in insect cells and compared the x-ray structure and biochemical properties of the purified protein with that of the recombinant human GCase (imiglucerase, Cerezyme®). The x-ray structure of N370S mutant acid β-glucosidase at acidic and neutral pH values indicates that the overall folding of the N370S mutant is identical to that of recombinant GCase. Subtle differences were observed in the conformation of a flexible loop at the active site and in the hydrogen bonding ability of aromatic residues on this loop with residue 370 and the catalytic residues Glu-235 and Glu-340. Circular dichroism spectroscopy showed a pH-dependent change in the environment of tryptophan residues in imiglucerase that is absent in N370S GCase. The mutant protein was catalytically deficient with reduced V(max) and increased K(m) values for the substrate p-nitrophenyl-β-D-glucopyranoside and reduced sensitivity to competitive inhibitors. N370S GCase was more stable to thermal denaturation and had an increased lysosomal half-life compared with imiglucerase following uptake into macrophages. The competitive inhibitor N-(n-nonyl)deoxynojirimycin increased lysosomal levels of both N370S and imiglucerase 2-3-fold by reducing lysosomal degradation. Overall, these data indicate that the N370S mutation results in a normally folded but less flexible protein with reduced catalytic activity compared with imiglucerase.

Lysosomal degradation on vesicular membrane surfaces. Enhanced glucosylceramide degradation by lysosomal anionic lipids and activators

According to a recent hypothesis (Sandhoff, K., and Kolter, T. (1996) Trends Cell Biol. 6, 98-103), glycolipids, which originate from the plasma membrane, are exposed to lysosomal degradation on the surface of intralysosomal vesicles. Taking the interaction of membrane-bound lipid substrates and lysosomal hydrolases as an experimental model, we studied the degradation of glucosylceramides with different acyl chain lengths by purified glucocerebrosidase in a detergent-free liposomal assay system. Our investigation focused on the stimulating effect induced by lysosomal components such as sphingolipid activator protein C (SAP-C or saposin C), anionic lysosomal lipids, bis(monoacylglycero)phosphate, and dolichol phosphate, as well as degradation products of lysosomal lipids, e.g. dolichols and free fatty acids. The size of the substrate-containing liposomal vesicles was varied in the study. Enzymatic hydrolysis of glucosylceramide carried by liposomes made of phosphatidylcholine and cholesterol was rather slow and only weakly accelerated by the addition of SAP-C. However, the incorporation of anionic lipids such as bis(monoacylglycero)phosphate, dolichol phosphate, and phosphatidylinositol into the substrate carrying liposomes stimulated glucosylceramide hydrolysis up to 30-fold. Dolichol was less effective. SAP-C activated glucosylceramide hydrolysis under a variety of experimental conditions and was especially effective for the increase of enzyme activity when anionic lipids were inserted into the liposomes. Glucosylceramides with short acyl chains were found to be degraded much faster than the natural substrates. Dilution experiments indicated that the added enzyme molecules associate at least partially with the membranes and act there. Surface plasmon resonance experiments demonstrated binding of SAP-C at concentrations up to 1 microM to liposomes. At higher concentrations (2.5 microM SAP-C), liposomal lipids were released from the liposome coated chip. A model for lysosomal glucosylceramide hydrolysis is discussed.

Activator proteins and topology of lysosomal sphingolipid catabolism

The lysosomal degradation of several sphingolipids by acid hydrolases is dependent on small non-enzymic cofactors, called sphingolipid activator proteins some of which have been identified as sphingolipid binding proteins. This review summarizes the information available on the structure, function, biosynthesis, gene organization and pathobiochemistry of the known sphingolipid activator proteins. It also offers models for their mode of action and for the topology of lysosomal digestion of glycolipids.

Reabsorption and intracellular transport of cytochrome c and lysozyme in rat kidney

Renal uptake and degradation of cytochrome c and lysozyme were investigated, using preparations that were labelled by means of covalent coupling of either protein to iodinated tyramine-cellobiose. Following proteolytic digestion, the label remains 'trapped' within intracellular organelles. Within 15 min after intravenous injection, 43% of the [125I]tyramine-cellobiose-cytochrome c and 29% of the [131I]tyramine-cellobiose-lysozyme were recovered in the kidneys. Isopycnic sucrose-gradient fractionation indicates that the two proteins initially exhibit closely similar intracellular distributions, being associated with vesicles of an equilibrium density slightly lower than that of plasma membranes. However, within 5 min after injection, the two proteins exhibit distinctly different distribution profiles. The [125I]tyramine-cellobiose-cytochrome c is localized predominantly in the lysosomal fraction of the gradient. The [131I]tyramine-cellobiose-lysozyme is also translocated to the lysosomal fraction, but at a much lower rate. For both proteins, the rates of intracellular degradation correlate with their rates of translocation. The observed difference in their kinetics of intracellular movement suggests that the two proteins are translocated at different rates into transport vesicles.

Acid beta-glucosidase: enzymology and molecular biology of Gaucher disease

Human lysosomal beta-glucosidase (D-glucosyl-acylsphingosine glucohydrolase, EC 3.2.1.45) is a membrane-associated enzyme that cleaves the beta-glucosidic linkage of glucosylceramide (glucocerebroside), its natural substrate, as well as synthetic beta-glucosides. Experiments with cultured cells suggest that in vivo this glycoprotein requires interaction with negatively charged lipids and a small acidic protein, SAP-2, for optimal glucosylceramide hydrolytic rates. In vitro, detergents (Triton X-100 or bile acids) or negatively charged ganglioside or phospholipids and one of several "activator proteins" increase hydrolytic rate of lipid and water-soluble substrates. Using such in vitro assay systems and active site-directed covalent inhibitors, kinetic and structural properties of the active site have been elucidated. The defective activity of this enzyme leads to the variants of Gaucher disease, the most prevalent lysosomal storage disease. The nonneuronopathic (type 1) and neuronopathic (types 2 and 3) variants of this inherited (autosomal recessive) disease but panethnic, but type 1 is most prevalent in the Ashkenazi Jewish population. Several missense mutations, identified in the structural gene for lysosomal beta-glucosidase from Gaucher disease patients, are presumably casual to the specifically altered posttranslational oligosaccharide processing or stability of the enzyme as well as the altered in vitro kinetic properties of the residual enzyme from patient tissues.

Sphingolipid activator protein deficiency in a 16-week-old atypical Gaucher disease patient and his fetal sibling: biochemical signs of combined sphingolipidoses

We describe a patient who presented shortly after birth with hyperkinetic behaviour, myoclonia, respiratory insufficiency and hepatosplenomegaly. Gaucher-like storage cells were found in bone marrow. A liver biopsy showed massive lysosomal storage morphologically different to that in known lipid storage disorders. Biochemically, the patient had partial deficiencies of beta-galactocerebrosidase, beta-glucocerebrosidase and ceramidase in skin fibroblast extracts, but the sphingomyelinase activity was normal. Glucosyl ceramide and ceramide were elevated in liver tissue. Loading of cultured fibroblasts with radioactive sphingolipid precursors indicated a profound defect in ceramide catabolism. Immunological studies in fibroblasts showed a total absence of cross-reacting material to sphingolipid activator protein 2 (SAP-2). The patient died at 16 weeks of age. The fetus from his mother's next pregnancy was similarly affected. The possibility that the disorder results from a primary defect at the level of SAP-2 is discussed. We have named this unique disorder SAP deficiency.

Intracellular activity of lysosomal glucosylceramidase measured with 4-nonylumbelliferyl beta-glucoside

Enzymatic activity of lysosomal glucosyl-ceramidase was determined in intact murine hybridoma and macrophage cells with the synthetic substrate nonylumbeliferyl-beta-glucoside (NUG). The substrate was applied as complex with bovine serum albumin (two binding sites, Kd 2.2 +/- 0.3 microM). The transport of the artificial substrate from medium to the enzyme was explored by measurements of substrate concentrations in cellular membranes and of endocytosis rate relative to substrate hydrolysis. The results indicated that, after enrichment in the plasma membrane, the substrate is mainly transported by simple diffusion. Release of nonylumberlliferone monitored fluorimetrically after disintegration of the cells in borate buffer containing Triton X-100 at pH 9.5 showed that 10(8) cells of both cell lines hydrolysed 1-1.5 nmol substrate/min at a total concentration of 0.1 mM NUG in the medium. Substrate hydrolysis was prevented by preincubating the cells with conduritol B epoxide (CBE), a specific active site-directed inhibitor of lysosomal glucosylceramidase. The substrate concentration at the site of the enzyme and maximal activity were evaluated by the inhibiting effect of the substrate on the inactivation rate by conduritol B epoxide. The rate of inhibitor uptake measured with bromo-[3H]conduritol B epoxide was shown to be not rate-limiting for the inactivation reaction. The molar concentration of the enzyme was determined by labeling with bromo-[3H]conduritol B epoxide. Comparison of the maximal intracellular activity with that of the enzyme after disintegration and activation by taurocholate showed a 20-fold lower activity in the native environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Saposin D: a sphingomyelinase activator

Saposin D, a newly discovered heat-stable, 10 kDa glycoprotein, was isolated from Gaucher spleen and purified to homogeneity. Chemical sequencing from its amino terminus demonstrated colinearity between its amino acid sequence and the deduced amino acid sequence of the fourth domain of prosaposin, the precursor of saposin proteins. Saposin D specifically stimulates acid sphingomyelinase but has no significant effect on the other hydrolases tested.

The carbohydrate moiety of the activator protein for glucosylceramide beta-glucosidase

SAP-2 is a family of heat-stable, acidic glycoproteins which stimulate enzymatic hydrolysis of glucosylceramide. We studied the carbohydrate moieties of a ConA-binding form of SAP-2. The protein contained glucosamine, galactose, mannose, and fucose; galactosamine and sialic acid were not detectable. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining showed three bands of 6.5, 8.5, and 10 kDa. After deglycosylation with peptide N-glycosidase, SAP-2 eluted more slowly from the C4 column and showed a single band of 4 kDa. From carbohydrate analysis it was evident that deglycosylation had removed more than 90% of the sugars. These data indicate that SAP-2 possesses N-linked complex or hybrid type oligosaccharide chains. The specific activity of the deglycosylated protein in the glucosidase stimulation assay was unaffected.

Heterogeneity in human acid beta-glucosidase revealed by cellulose-acetate electrophoresis

Cellulose-acetate gel electrophoresis, a technique commonly used for the separation of human acid hydrolases, was applied to study heterogeneity in acid beta-glucosidase (EC 3.2.1.45). With this technique, three forms of beta-glucosidase were distinguishable in extracts of several tissues. The most anodic beta-glucosidase activity (band 3) represents the broad-specificity beta-glucosidase that is not deficient in Gaucher disease and is not inhibited by conduritol B-epoxide (CBE). The beta-glucosidase activity was deficient in Gaucher disease. A third beta-glucosidase activity with an intermediate mobility (band 2) was also inhibited by CBE and deficient in Gaucher disease. Band 1 and band 2 beta-glucosidase thus represent different forms of glucocerebrosidase. By adding phosphatidylserine and sphingolipid activator protein (SAP-2), monomeric glucocerebrosidase could be completely converted into a form that comigrated with band 2 beta-glucosidase of tissue extracts. The addition of phosphatidylserine only also resulted in a changed mobility of the monomeric enzyme, but the migration in this case differed from that of band 2 beta-glucosidase of tissue extracts. The electrophoretic profile of beta-glucosidase activity of tissue extracts changed upon ethanol/chloroform extraction: the two glucocerebrosidase forms were converted into a band with a mobility identical to that of band 1 beta-glucosidase. Our findings indicate that the interaction of glucocerebrosidase with phospholipid and SAP-2 has major effects on the mobility of the enzyme in the cellulose-acetate gel electrophoresis system. The findings with the cellulose-acetate gel electrophoretic system are discussed in relation to the heterogeneity in glucocerebrosidase observed with sucrose density gradient analysis, immunochemical methods and isoelectric focussing studies.

The precursor of sulfatide activator protein is processed to three different proteins

The enzymic degradation of a number of sphingolipids in the lysosomes is stimulated by small acid glycoproteins named activator proteins. We purified and sequenced a new protein, called component C, which seems to be related to sulfatide activator and to a recently described activator of glucosylceramidase (A1 activator) (Kleinschmidt, T., Christomanou, H. & Braunitzer, G. (1987) Biol. Chem. Hoppe-Seyler 368, 1571-1578). It consists of 78 amino acids and carries one carbohydrate chain at aparagine 20. Component C shows 21.5% sequence homology to sulfatide activator and 34.2% homology to A1 activator. Structural similarities between these three proteins have also been detected. Recently the cDNA sequence of the sulfatide activator precursor has been published (Dewji, N.N., Wenger, D.A. & O'Brien, J.S. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8652-8656). We could align the protein sequences of sulfatide activator, A1 activator and component C with that of this large precursor protein. After minor corrections of the DNA sequence we obtained total fit. Thus it seems that three different proteins are derived from the sulfatide activator precursor by proteolytic processing. Possible processing sites were found on the precursor at sites adjacent to the N-termini and C-termini of the mature proteins. The processing of sulfatide activator was studied by Fujibayashi and Wenger (Fujibayashi, S. & Wenger, D.A. (1986) Biochim. Biophys. Acta 875, 554-562). Their data support our assumption that processing occurs by simultaneous cleavage at all possible sites.

Activation of human spleen glucocerebrosidases by monoacylglycol sulfates and diacylglycerol sulfates

The study of the acidic lipid requirement of human spleen glucocerebrosidase was extended to include two new series of acidic lipids, namely, monoacylglycol sulfates and diacylglycerol sulfates. Lysosomal glucocerebrosidase was extracted with sodium cholate and 1-butanol to render its beta-glucosidase activity dependent upon exogenous lipids. Maximum reactivation of control glucocerebrosidase was obtained with nonanoylglycol sulfate (NGS) and diheptanoylglycerol sulfate (DHGS). However, the effects of these lipids were markedly dependent on the nature of buffer used in the assay medium; specifically, 0.2 M sodium citrate-phosphate (pH 5.5) was much more effective than 0.2 M sodium acetate (pH 5.5) in permitting these lipids to reactivate glucocerebrosidase. In contrast, the marked activation of glucocerebrosidase by phosphatidylserine and galactocerebroside 3-sulfate (sulfatide) that was achievable in the sodium acetate buffer was totally inhibited by citrate or phosphate ions. The effects of NGS and DHGS on the kinetic parameters of control glucocerebrosidase were to lower the Km for the substrate, 4-methylumbelliferyl-beta-D-glucoside from 5.5 mM to approximately 2 mM (in sodium citrate-phosphate buffer) and markedly increase the Vmax. Furthermore, with DHGS, significant activation was achieved at concentrations below the lipid's critical micellar concentration. None of the monoacylglycol- or diacylglycerol sulfates were capable of stimulating mutant glucocerebrosidases from either type 1 (Ashkenazi-Jewish) or type 2 Gaucher's disease patients. Like control glucocerebrosidase, the type 1 glucocerebrosidase was unresponsive to phosphatidylserine and sulfatide when the beta-glucosidase assay was conducted in 0.2 M sodium citrate-phosphate buffer. Based on the differential action of these lipid activators in the two buffers and their effects on the mutant enzymes, we propose that, with regard to the lipid requirement of glucocerebrosidase, there are two classes of acidic lipids--one comprised of phosphatidylserine and sulfatide and the other comprised of the likes of NGS, DHGS, or sodium taurodeoxycholate. It appears that control glucocerebrosidase and the mutant enzyme of the patient with type 1 Gaucher's disease is reconstitutable with the first class of lipids whereas the glucocerebrosidase of the type 2 patient is not. The observations in this report are interpreted in terms of a model which postulates that normal glucocerebrosidase possesses at least two distinct lipid binding domains.

Presence of activator proteins for the enzymatic degradation of glucosylceramide in several human tissues

Glucosylceramidase (EC 3.2.1.45) protein activators, similar to the 'placental factor' previously identified by us in human placenta, have also been found in human liver, normal and Gaucher fibroblasts and Gaucher spleen. They stimulate enzymatic hydrolysis of the natural substrate, glucosylceramide, but not that of the artificial substrate, 4-MU-beta-D-glucopyranoside. They are present in the tissues over the minimum amount necessary for full activation of the enzyme and must be eliminated from crude enzyme preparations in order to observer their effect on glucosylceramidase activity. The factors are not tissue-specific in that the factors from any one of the sources can activate glucosylceramidase from either placenta or liver. The presence of taurocholate or phosphatidylserine in the assay is essential for the factor efficiency. A normal level of the activator proteins was found in fibroblasts from subjects affected with Gaucher disease type I, type II and type III.

Effect of a heat-stable factor in human placenta on glucosylceramidase, glucosylsphingosine glucosyl hydrolase, and acid beta-glucosidase activities

A new protein activator of glucosylceramidase has recently been found in human placenta. In the present work, it has been compared with a previously reported glucosylceramidase activator, the Gaucher factor. The two activators showed different properties. The Gaucher factor stimulated 100% the 4-methylumbelliferyl-beta-D-glucopyranoside hydrolysis while the placental factor inhibited it 50%. Furthermore, the placental factor neither decreased the Michaelis constant, Km, nor increased the degree of inactivation by conduritol-beta-epoxide as the Gaucher factor does. From these results it has been concluded that the two activators are different substances. The activating effect of the placental factor is specific for the hydrolysis of glucosylceramide; neither the hydrolysis of glucosylsphingosine nor that of the 4-methylumbelliferyl derivative are enhanced by this protein. Owing to its specificity and high level in a human tissue, the placental factor is likely to have a physiological role in the catabolism of glucosylceramide.

Hydrolysis of galactosylsphingosine and lactosylsphingosine by beta-galactosidases in human brain and cultured fibroblasts

Enzymatic properties of beta-galactosidases with galactosylsphingosine (psychosine) and lactosylsphingosine as the substrates were examined. Although bile salts were stimulatory on the hydrolysis of the glycolipids in normal brain and cultured fibroblasts, the hydrolytic activities could be readily assayed, without detergents. The in vitro hydrolysis of lactosylsphingosine in cultured fibroblast homogenates was catalyzed by two enzymes, as is the case with the hydrolysis of galactosylceramide and lactosylceramide. Lactosylsphingosine beta-galactosidase activities assayed in the absence and the presence of taurocholate (probably lactosylceramidase I) were deficient in fibroblasts from patients with globoid cell leukodystrophy, while the activity assayed with sodium cholate (probably lactosylceramidase II) was deficient in GM1 gangliosidosis fibroblasts. In contrast, galactosylsphingosine beta-galactosidase was not activated by cholate and the enzyme activities assayed with the no-additive and taurocholate systems were deficient in brain and fibroblasts from patients with globoid cell leukodystrophy, thereby indicating that the hydrolysis of galactosylsphingosine is catalyzed by one enzyme, galactosylceramidase I. Exogenous lipids and an activator protein purified from normal spleen activated galactosylsphingosine beta-galactosidase but they were inhibitory to lactosylsphingosine beta-galactosidase. Because the Km values of lactosylsphingosine beta-galactosidase assayed with cholate were several magnitude higher than those obtained with the no-additive system and because lactosylsphingosine is readily hydrolyzed with the no-additive system in vitro, it is likely that the in vivo hydrolysis of the lipid is catalyzed by only one enzyme, lactosylceramidase I.

The binding of glucosylceramidase to glucosylceramide is promoted by its activator protein

A protein activator of glucosylceramidase (EC 3.2.1.45) has been previously identified by us in human placenta [(1985) Biochim. Biophys. Acta 836, 157-166]. In the present paper we report that its function in vitro is to stimulate the binding of the enzyme to its substrate, glucosylceramide. After the purification step which frees the enzyme of most of its activator protein (octyl-Sepharose 4B chromatography), the capacity of glucosylceramidase to bind to the glucosylceramide micelles is dramatically decreased. The addition of the activator protein to the purified enzyme restores this binding.

Relationship between the two immunologically distinguishable forms of glucocerebrosidase in tissue extracts

Extracts of human spleen contain two immunologically distinguishable forms of glucocerebrosidase: form I is precipitable by polyclonal or monoclonal anti-(placental glucocerebrosidase) antibodies, whereas form II is not [Aerts, J. M. F. G., Donker-Koopman, W. E., Van der Vliet, M. F. K., Jonsson, L. M. V., Ginns, E. I., Murray, G. J., Barranger, J. A., Tager, J. M. & Schram, A. W. (1985) Eur. J. Biochem. 150, 565-574]. The proportion of form II glucocerebrosidase was high in extracts of spleen, liver and kidney and low in extracts of brain, placenta and fibroblasts. Furthermore, the proportion of form II enzyme was higher in a detergent-free aqueous extract of spleen than in a Triton X-100 extract of total spleen or splenic membranes. When form II glucocerebrosidase in a splenic extract was separated from form I enzyme by immunoaffinity chromatography and stored at 4 degrees C, a gradual conversion to form I enzyme occurred. The conversion was almost immediate if 30% (v/v) ethylene glycol was present. In the denatured state both forms of glucocerebrosidase reacted with anti-(placental glucocerebrosidase) antibodies. Form I glucocerebrosidase was stimulated by sodium taurocholate or sphingolipid-activator protein 2 (SAP-2), whereas form II enzyme exhibited maximal activity in the absence of the effectors. The pH activity profile of form II glucocerebrosidase was almost identical to that of form I enzyme in the presence of SAP-2. In the native state, form I glucocerebrosidase had a molecular mass of 60 kDa whereas that of form II glucocerebrosidase was about 200 kDa. After gel-permeation high-performance liquid chromatography of splenic extracts, the fractions with form II glucocerebrosidase contained material cross-reacting with both anti-(placental glucocerebrosidase) and anti-(SAP-2) antibodies. Preincubation of form I glucocerebrosidase with SAP-2 at pH 4.5 led to masking of the epitope on glucocerebrosidase reacting with monoclonal anti-(placental glucocerebrosidase) antibody 2C7. Furthermore, preincubation of form I glucocerebrosidase with monoclonal antibody 2C7 prevented activation of the enzyme by SAP-2. We propose that form I glucocerebrosidase is a monomeric form of the enzyme, whereas form II glucocerebrosidase is a high-Mr complex of the enzyme in association with sphingolipid-activator protein 2.

Specificity of human glucosylceramide beta-glucosidase towards synthetic glucosylsphingolipids inserted into liposomes. Kinetic studies in a detergent-free assay system

The behaviour of highly purified glucosylceramide beta-glucosidase (glucosylceramidase, EC 3.2.1.45) from human placenta [Furbish, F. S., Blair, H. E., Shiloach, J., Pentchev, P. G. & Brady, R. B. (1977) Proc. Natl Acad. Sci. USA 74, 3560-3563] was investigated in the absence of detergents with structurally modified glucosylceramides inserted into unilamellar liposomes. The reaction between the water-soluble enzyme and the liposomal substrates was significantly dependent on the structure of the lipophilic aglycon moiety of glycolipids: glucosyl-N-acetyl-sphingosines (D-erythro and L-threo) were better substrates than the corresponding glucosylceramides. The L-threo derivatives were poorer substrates with higher apparent Km values than the corresponding D-erythro derivatives. For glucosyl-3-keto-ceramide and glucosyl-dihydro-ceramide (D-erythro), higher Km values were found than for glucosylceramide. Sphingosine, glucosylsphingosine and glucosyl-N-acetyl-sphingosine were the most effective inhibitors of the hydrolysis of glucosylceramide. D-erythro-Ceramide and D-galactosyl-N-acetyl-D-erythro-sphingosine inhibited the hydrolysis of amphiphilic glucosylceramide but not that of water-soluble 4-methyl-umbelliferyl-beta-glucoside, suggesting a hydrophobic binding site of the enzyme for the aglycon moiety of its membrane-bound substrate. Dilution experiments suggested that at least a fraction of the enzyme associates with the liposomes and degrades the lipid substrate even in the absence of activator proteins. Acidic phospholipids incorporated into liposomes caused a powerful stimulation (30-40-fold) of the glucosylceramide beta-glucosidase, whereas acidic sphingolipids (sulphatide, gangliosides GM1 and GD1a) incorporated into liposomes stimulated this enzyme only moderately (3-10-fold).

Immunochemical characterization of two activator proteins stimulating enzymic sphingomyelin degradation in vitro. Absence of one of them in a human Gaucher disease variant

Two nonenzymic activator proteins shown previously to strongly stimulate enzymic sphingomyelin degradation in vitro were purified from human Gaucher type 1 and control spleen. Activator A1 (molecular mass 6,500 Da) had affinity for ConA-Sepharose, while activator A2 (molecular mass 3,500 Da) did not. Monospecific antibodies to each activator protein were prepared in rabbits by immunization with protein purified from type 1 Gaucher spleen. A1 and A2 activators from Gaucher type 1 spleen were shown to be immunochemically identical to A1 and A2 activators from control spleen. However, A1 and A2 activators, whether isolated from Gaucher type 1 or control spleen, were shown to be distinct proteins. Immunochemical examination of all collected fractions during the purification revealed the existence of a third activator (molecular mass 6,000 Da), which was antigenically identical to A1 activator but had no affinity for ConA-Sepharose. The two forms of A1 activator showed similar mobility on immunoelectrophoresis differing from that of A2 activator. Fibroblast extracts from controls and patients with different variants of Gaucher disease were investigated using immunodiffusion against antisera to A1 or A2 activator. In contrast to normal and Gaucher (types 1, 2 and 3) cell extracts, those of a Gaucher patient with normal glucosylceramidase activity had no visible precipitin line towards the antiserum against the two forms of A1 activator. The lack of crossreacting material to antibodies against A1 activator was confirmed by radial immunodiffusion and rocket immunoelectrophoresis. A1 activator stimulated the basal glucosylceramidase activity 5-6 fold in fibroblasts from this patient, whereas the normal effect was only a 1.2-1.5-fold stimulation. The immunological results together with the biochemical data provide evidence for the lack of an activator protein in a variant form of human Gaucher disease for the first time.

Sulfogalactocerebroside and bis-(monoacylglyceryl)-phosphate as activators of spleen glucocerebrosidase

Sequential extraction of human spleen membranes with sodium cholate and n-butanol removes endogenous lipids and renders glucocerebrosidase activity dependent upon exogenous acidic lipids (e.g., phosphatidylserine, gangliosides) and a heat-stable activator protein (HSF). In the present report, we show that two previously untested lysosomal acidic lipids, namely sulfogalactocerebroside and bis-(monoacylglyceryl)-phosphate (BMP), also activate normal human glucocerebrosidase. In addition, sulfogalactocerebroside also markedly enhanced the activity of glucocerebrosidase isolated from a patient with type 1 (non-neuronopathic) Gaucher's disease, resulting in a specific activity which was 60-80% that of control glucocerebrosidase. Furthermore, when the sulfolipid was used as the activator, glucocerebrosidase from the type 1 patient was 30 times more active than the corresponding glucocerebrosidase from a person with type 2 (neuronopathic) Gaucher's disease. In contrast, the two BMPs, one rich in C26 saturated fatty acid and another rich in C18 unsaturated fatty acids, were relatively poor activators of both mutant glucocerebrosidases while providing excellent reconstitution of control activity.

Comparison of the ability of phospholipids from rat liver lysosomes to reconstitute glucocerebrosidase

The in situ lipid activator of rat liver glucocerebrosidase was investigated. Rat liver lysosomes were purified (42.9-fold relative to the crude homogenate) by sequential isopycnic centrifugation in sucrose and metrizamide gradients. Lipids were extracted with chloroform:methanol (2:1) and phospholipids were separated by one-dimensional thin-layer chromatography. The phospholipid content of the lysosome preparation was 0.28 mumol lipid phosphorus/mg protein. Phosphatidylcholine was present as the major nonacidic phospholipid (39.3%). Of the acidic phospholipids, phosphatidylinositol and phosphatidylserine were present in the greatest amounts (12.0 and 19.7%, respectively). The resolved phospholipids were tested separately and in the presence of a heat-stable factor from Gaucher spleen for their ability to reconstitute butanol-delipidated rat liver glucocerebrosidase activity. Alone or in the presence of the heat-stable factor, phosphatidylserine and phosphatidylinositol were the most effective activators of glucocerebrosidase. Bis(monoacylglyceryl) phosphate derived from rat liver tritosomes or rabbit lung macrophages was also effective in reconstituting beta-glucosidase activity.

Comparison of N-acyl phosphatidylethanolamines with different N-acyl groups as activators of glucocerebrosidase in various forms of Gaucher's disease

The acidic phospholipid requirement of the predominant particulate beta-glucosidase of mammalian spleen and liver was investigated using a series of N-acyl derivatives of dioleoyl phosphatidylethanolamine (PE). The PE, a neutral phospholipid, was converted to an acidic lipid, (N-acyl)-phosphatidylethanolamine (NAPE) by acylation of the amino group with different fatty acyl chains. Lysosomal beta-glucosidases from rat liver and spleens of controls and patients with various types of Gaucher's disease were solubilized and delipidated by extraction with sodium cholate and 1-butanol. All members of the NAPE series tested were effective activators of the delipidated rat liver beta-glucosidase, and the stimulatory power of the NAPE family increased with increasing chain length of the fatty acid substitution. In contrast, dioleoyl-PE had no effect on beta-glucosidase activity. A heat-stable factor (HSF) purified from the spleen of a patient with Gaucher's disease significantly increased the sensitivity of the rat liver beta-glucosidase to all of the NAPE derivatives. The maximum stimulation achieved in the presence of HSF was independent of N-acyl chain length. Compared to the potent activator, phosphatidylserine (PS), (N-acetyl)-PE and (N-linoleoyl)-PE were half as effective as activators of beta-glucosidase from control human spleen. PS stimulated the beta-glucosidase of type 1 nonneurologic Gaucher's disease, but none of the NAPE compounds activated it. Neither PS nor any of the (N-acyl)-PE compounds could activate a delipidated preparation of beta-glucosidase obtained from the spleen of a neurologic case. These results indicate that although the presence of a net negative charge on a phospholipid confers upon it an ability to reconstitute beta-glucosidase activity to the normal, nonmutant enzyme, it is insufficient to permit differentiation of the various types of Gaucher's disease.

An endogenous activator protein in human placenta for enzymatic degradation of glucosylceramide

An endogenous, heat-stable and pronase-sensitive activator for enzymatic hydrolysis of glucosylceramide was detected in the crude lysosome-mitochondria fraction of human placenta. Its properties differ distinctly in several important respects from those of the previously described glucosylceramidase activator. The activator reported here had no effect on crude glucosylceramidase with either glucosylceramide or 4-methylumbelliferyl-beta-D-glucopyranoside as the substrate in the presence of either sodium taurocholate or phosphatidylserine. On the contrary, glucosylceramide hydrolysis by the enzyme partially purified through Octyl-Sepharose 4B chromatography was stimulated by this activator 6-9-fold in the presence of either sodium taurocholate or phosphatidylserine. The Km for glucosylceramide in the presence of the activator was 1/3 of that without the activator. In the crude enzyme fraction, the activator was present in a 16-fold excess over the minimum amount necessary for full activation of the enzyme. Hydrolysis of the fluorogenic substrate by the post-Octyl-Sepharose enzyme, however, was not stimulated by the activator. Similarly, hydrolysis of galactosylceramide by galactosylceramidase obtained from the same Octyl-Sepharose chromatography was not stimulated. Our observations are consistent with the idea that glucosylceramidase is saturated by, or perhaps tightly associated with, this activator in the placenta and that they are dissociated by the Octyl-Sepharose chromatography. In fact, the properties of the combined post-Octyl-Sepharose enzyme and activator closely mimic those of the crude enzyme without added activator.

Sucrose gradient analysis of phospholipid-activated beta-glucosidase in type 1 and type 2 Gaucher's disease

Using sucrose density gradients, differences in delipidated lysosomal beta-glucosidase isolated from control spleen and spleen from patients with nonneurologic (type 1) and neurologic (type 2) Gaucher's disease have been examined. The three enzymes differ in sedimentation properties as well as in their responsiveness to activation by phosphatidylserine and heat-stable factor. The control beta-glucosidase sedimented as an apparent 45,000-Da species whose activity was dependent upon the inclusion of exogenous sodium taurodeoxycholate in the assay medium. Preincubation with a mixture of phosphatidylserine and heat-stable factor converted the control enzyme to a faster-sedimenting form which exhibited considerable activity in the absence of exogenous bile salt. Spleen beta-glucosidase from a patient with type 1 Gaucher's disease exhibited an apparent molecular weight of 154,000 on sucrose gradients. Like the control enzyme, the activity of this form was bile salt dependent. Upon preincubation with phosphatidylserine and heat-stable factor, beta-glucosidase from the type 1 case was also converted to a faster-sedimenting form which was more active in the absence of sodium taurodeoxycholate than in the presence of the bile salt. Spleen beta-glucosidase from the patient with type 2 Gaucher's disease sedimented as a broad peak of activity in the most dense regions of the sucrose gradients, appearing to be much larger than the beta-glucosidase from either the control or the type 1 Gaucher's disease patient. The activity of this large species was strongly dependent upon bile salt, and was not affected by preincubation of the enzyme with phosphatidylserine and heat-stable factor. Using the chaotropic salt, sodium thiocyanate (0.15 M), the spleen beta-glucosidase isolated from the type 1 Gaucher's disease case was converted to a slower-sedimenting species. The control enzyme sedimented slightly farther into the sucrose gradients upon treatment with the NaSCN. Thiocyanate treatment had no effect on the spleen beta-glucosidase isolated from the case of type 2 Gaucher's disease.

Studies on a sphingolipid activator protein (SAP-2) in fibroblasts from patients with lysosomal storage diseases, including Niemann-Pick disease Type C

Sphingolipid activator protein-2 (SAP-2) has been found to stimulate the enzymatic hydrolysis of at least three sphingolipids, glucosylceramide, galactosylceramide and sphingomyelin. Using monospecific antibodies against SAP-2 the level of SAP-2 was determined in cultured skin fibroblasts by rocket immunoelectrophoresis. Extracts from 14 controls had 1.03 +/- 0.28 micrograms cross-reactive material/mg solubilized protein and extracts from 46 patients with Niemann-Pick disease Type C had 1.12 +/- 0.26. Extracts from other lysosomal storage diseases, including Gaucher disease, Krabbe disease and Niemann-Pick disease Types A, B and D, had normal or slightly elevated SAP-2 concentrations, while extracts from patients with I-Cell disease had half normal SAP-2 concentration. When the fibroblast extracts were subjected to sodium dodecylsulfate-polyacrylamide gel electrophoresis followed by electroblotting and immunochemical staining two major SAP-2 bands with estimated molecular weights of 9000 and 10000 were found. Extracts from patients with I-Cell disease showed only a faint higher molecular weight band. Isoelectric focusing followed by electroblotting and immunochemical staining demonstrated no significant difference in the charge of SAP-2 obtained from different cell lines. In this study we could not demonstrate any change in concentration, size or charge of SAP-2 in fibroblast extracts from Niemann-Pick disease Type C, and we provided evidence that SAP-2 might be subject to post-translational processing similar to that of lysosomal enzymes.

Isolation of two forms of an activator protein for the enzymic sphingomyelin degradation from human Gaucher spleen

Two activator proteins for sphingomyelin degradation were isolated from heat-treated extracts of human Gaucher spleen. The separation was based on the degree of affinity of the activators for ConA-Sepharose. Activator A1, which had affinity for ConA-Sepharose, was purified 1 430-fold, and activator A2, which had no affinity for ConA-Sepharose, 2 140-fold as compared with the original heat-treated extracts. The molecular masses of activator A1 and activator A2 were 6 000 and 3 500 Da, respectively, as determined by dodecyl sulfate electrophoresis, and approximately 5 000 Da as measured in the presence of 8M urea. The two activators had similar properties and a similar but not identical amino-acid composition. Both were shown to form a complex with sphingomyelin and stimulate the degradation of sphingomyelin by normal fibroblast homogenates and by an approximately 1 430-fold purified sphingomyelin phosphodiesterase ("acid sphingomyelinase") from normal human urine. This stimulation was greatly reduced after incubation with pronase E. The enzymic degradation of glucosylceramide and galactosylceramide was not affected by these activators.

Characterization of human glucosylsphingosine glucosyl hydrolase and comparison with glucosylceramidase

Properties of glucosylsphingosine (gluco-psychosine) glucosyl hydrolase were studied in detail in cultured human fibroblasts and placenta and were compared with those of glucosylceramidase. The two activities, that are deficient in tissues of Gaucher patients, showed minor but consistent differences. The pH optima were 4.8 for psychosine hydrolysis and 5.3 for glucosylceramide hydrolysis. In the presence of oleic acid, taurocholate activated glucosylceramidase more than 10-fold, while it activated psychosine hydrolysis only by about 30%. Triton X-100 was stimulatory for glucosylceramidase but was strongly inhibitory for psychosine hydrolysis. Phospholipids, that increase many times glucosylceramidase activity, were moderately inhibitory to enzymatic hydrolysis of psychosine. The psychosine hydrolase activity was slightly more heat-stable than the glucosylceramidase activity. The Km values for the two substrates were similar; 1.7 X 10(-5) M for psychosine and 2.7 X 10(-5) M for glucosylceramide. The V for glucosylceramide was, however, 100-times that for psychosine hydrolysis. Psychosine acted as a potent non-competitive inhibitor (Ki = 1.8 X 10(-5) M), while glucosylceramide was a weak inhibitor against psychosine hydrolysis. Within the limit of glucosylceramide solubility, psychosine hydrolysis could not be inhibited by more than 50%. Furthermore, the Dixon plot of glucosylceramide inhibition showed an anomalous slope. The ratio of the two activities was similar in fibroblasts, in the placenta mitochondria-lysosomal fraction and in a partially purified placental preparation. These findings are best explained by the hypothesis that, although the two substrates are hydrolyzed by a single enzyme, they share an overlapping but not identical catalytic site while binding to hydrophobic sites unique for the respective substrates.

Gaucher disease (type 1): physical and kinetic properties of liposomal and soluble 'acid' beta-glucosidase

'Acid' beta-glucosidase of human spleen, from either normal controls or patients with type 1 (adult) Gaucher disease, was incorporated into phosphatidylcholine liposomes. The non-incorporated (soluble) Gaucher-enzyme had a higher apparent molecular weight than had the corresponding control. Liposomal 'acid' beta-glucosidase prepared from Gaucher-spleen was more thermostable than was the corresponding normal enzyme; it was also stimulated by acidic lipids to a much lesser extent. The results suggest that the genetic mutation in type 1 (adult) Gaucher disease has multiple effects on the glycoprotein form of 'acid' beta-glucosidase.

Further studies on the activation of glucocerebrosidase by a heat-stable factor from Gaucher spleen

Using Sephadex G-75 and DEAE-cellulose column chromatography, an 8270-Da glycopeptide (designated Fragment II) has been isolated from a cyanogen bromide-formic acid digest of a heat-stable factor from Gaucher spleen which activates a lipid-depleted preparation of lysosomal glucocerebrosidase from human liver. Fragment II contains all of the activity present in the native heat-stable factor. Compared with the parent factor, Fragment II contains four fewer cysteine and methionine residues and one less of each of the following: aspartic acid, threonine, serine, valine, isoleucine, and leucine. Nearly all of the monosaccharides present in the parent heat-stable factor can be accounted for in Fragment II, including three glucosamine, three mannose, one sialic acid, and one fucose. By itself, Fragment II has little or no stimulatory activity; its major effect is to markedly increase the sensitivity of glucocerebrosidase to activation by phosphatidylserine. A mixture of 1 microgram phosphatidylserine and 2 micrograms of the cyanogen bromide fragment activates the lipid-depleted preparation of glucocerebrosidase 50% more than 30 micrograms phosphatidylserine alone. Analysis of the Km and Vmax of glucocerebrosidase at various hydrogen ion concentrations revealed that the heat-stable factor and phosphatidylserine together dramatically increase the catalytic efficiency (Vmax/Km) of glucocerebrosidase while making apparent three ionizable groups that participate in the catalysis. Phosphatidylserine alone recruits two ionizable groups, but catalytic efficiency is lower than when heat-stable factor is also present. Heat-stable factor alone has no discernable effect on the ionization of functional groups on the enzyme or on catalytic efficiency. By sucrose density gradient ultracentrifugation, it was shown that preincubation of rat liver glucocerebrosidase with phosphatidylserine and heat-stable factor shifted the enzyme completely from a 56,600-Da form to a 188,100-Da form. The activity of the slower sedimenting form of glucocerebrosidase was totally dependent upon exogenous bile salt activator, whereas the faster sedimenting form exhibited the same activity in the presence or absence of sodium taurocholate. It appears that the heat-stable factor promotes the transfer of phosphatidylserine to glucocerebrosidase, which, in turn, results in an increase in both the catalytic efficiency and size of the enzyme.

Determination of the glucosidase-stimulating proteins by competitive enzyme-linked immunoassay

A procedure for the immunoassay of cohydrolase sphingolipid-I in mouse tissue is described. This cohydrolase (actually a mixture of at least four related proteins) stimulates or activates the beta-glucosidase which hydrolyzes ceramide glucoside, a widely occurring glycosphingolipid. The method involves extraction of cohydrolase from tissue homogenate with a salt-buffer solution, removal of proteins by adjustment to pH 6, further removal of proteins by heating, and removal of interfering materials with a small size exclusion column. Antibodies were raised to bovine cohydrolase in rabbits and purified with an affinity column made from cohydrolase. The immunoassay involves binding of antibody by the cohydrolase sample (20-200 pg) in competition with cohydrolase that has been chemically linked to horseradish peroxidase. The mixture is treated with particle-linked second antibody and centrifuged; the pellet is then assayed fluorometrically for peroxidase content. Initial application of the method showed that cohydrolase was present in all mouse tissues studied and that its concentration paralleled that of glucocerebrosidase relatively closely. Changes with age (14 and 92 days) occurred in a similar fashion for the two substances.

Human lysosomal beta-glucosidase: purification by affinity chromatography

Two Sepharose-bound substrate analogs, 6'-aminohexanoyl-(2-N-sphingosyl-O-beta-D-glucoside) and 6'-aminohexyl-dodecanedioyl-1-(2-N-sphingosyl-1-O-beta-D-glu coside), were synthesized and used sequentially for the affinity purification of lysosomal beta-glucosidase (N-acyl-sphingosyl-1-O-beta-D-glucoside:glucohydrolase, EC 3.2.1.45). The capacities of these nondegradable affinity supports were 0.1 and 0.15 mg enzyme/ml settled gel, respectively. The purified enzyme had a specific activity of 75 mumol min-1 mg-1. The preparation had a single protein band with a molecular weight of 67,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, evidencing its apparent homogeneity. Isoelectric focusing on granular gels revealed four molecular forms of the enzyme with pI values of 4.0, 4.5, 4.7, and 5.8 to 6.2. The purified enzyme hydrolyzed glucosyl ceramide and 4-methylumbelliferyl-beta-D-glucoside with Km and Vmax values of 0.6 and 2.5 mM, and 101 and 26.1 mumol min-1 mg-1, respectively. The enzyme also hydrolyzed octyl beta-glucoside, a linear mixed-type inhibitor of the enzyme. Binding constants (Ki) were determined for the inhibitors, sphingosyl-1-O-beta-D-glucoside (Ki = 20 microM) and its N-hexyl derivative (Ki = 0.3 microM). The enzyme had a half-life of 65 and 30 min at 50 degrees C and pH 5.0 or 6.0, respectively. In addition, two other classes of ligands were used for the purification of lysosomal beta-glucosidase, and their capacities and specificities were compared to those of the substrate analog affinity supports. These included (i) the alkyl amine inhibitors octylamine, decylamine, and tetradecylamine; and (ii) the inhibitors, 6-aminohexanoyl-beta-glucosylamine and aminododecanoyl-1-(2-N-sphingosyl-1-O-beta-D-glucoside). Compared to these other ligand columns, the substrate analog affinity supports had about 100- to 1000-fold greater capacities or afforded 8- to 40-fold greater purification of human lysosomal beta-glucosidase.

Human lysosomal beta-glucosidase: kinetic characterization of the catalytic, aglycon, and hydrophobic binding sites

Three binding sites on highly purified lysosomal beta-glucosidase from human placenta were identified by studies of the effects of interactions of various enzyme modifiers. The negatively charged lipids, taurocholate and phosphatidylserine, were shown to be noncompetitive, nonessential activators of 4-methylumbelliferyl-beta-D-glucoside hydrolysis. Similar results were observed using the natural substrate, glucosyl ceramide, and low concentrations of taurocholate (less than 1.8 mM) or phosphatidylserine (0.5 mM). However, higher concentrations resulted in a complex partial inhibition of glucosyl ceramide hydrolysis. Increasing concentrations of phosphatidylserine obviated the effects of taurocholate, suggesting that these compounds compete for a common binding site on the enzyme. Glucosyl sphingosine and its N-hexyl derivative were potent noncompetitive inhibitors of the enzyme activity using either substrate. Taurocholate (or phosphatidylserine) and glucosyl sphingosine were shown to be mutually exclusive, indicating competition for a common binding site. In contrast, octyl- and dodecyl-beta-glucosides were linear-mixed-type inhibitors of glucosyl ceramide or 4-methylumbelliferyl-beta-D-glucoside hydrolysis, indicating at least two binding sites on the enzyme. Inhibition by these alkyl beta-glucosides was observed only in the presence of taurocholate or phosphatidylserine. The competitive component [Ki (slope)] for the two alkyl beta-glucosides decreased with increasing alkyl chain length, and was unaffected by increasing taurocholate or phosphatidylserine concentration. The noncompetitive component [Ki (intercept)] was nearly identical for both alkyl beta-glucosides and was decreased by increasing taurocholate or phosphatidylserine concentration. These results indicated that the negatively charged lipids and alkyl beta-glucosides were not mutually exclusive, but interacted with different binding sites on the enzyme. Gluconolactone was shown to protect the enzyme from inhibition by the catalytic site-directed covalent inhibitor, conduritol B indicating an interaction at a common binding site. In the presence of substrate, taurocholate facilitated the inhibition of gluconolactone or conduritol B epoxide. These studies indicated that lysosomal beta-glucosidase had at least three binding sites: (i) a catalytic site which cleaves the beta-glucosidic moiety, (ii) an aglycon site which binds the acyl or alkyl moieties of substrates and some inhibitors, and (iii) a hydrophobic site which interacts with negatively charged lipids and facilitates enzyme catalysis.

Studies on the activation of the enzymatic hydrolysis of sphingomyelin liposomes

Two pH optima were observed for the hydrolysis of sphingomyelin liposomes by brain and fibroblast extracts; one at pH 4.2-4.5, the other at pH 7-8. The proportion of the acidic activity in fibroblasts was affected greatly by the culturing conditions. Both the acidic and neutral enzyme activities were deficient in Niemann-Pick Type A fibroblasts, suggesting that both were genetically related. Partially purified activators from normal as well as Gaucher disease spleen stimulated the hydrolysis of sphingomyelin, at both pH values, by fibroblast and brain extracts. After further purification by DE-52 and Sephacryl 200 column chromatography the Gaucher activator retained its ability to stimulate sphingomyelinase and was active as well towards beta-glucocerebrosidase and beta-galactocerebrosidase.