A protein activator of galactosylceramide beta-galactosidase

The protective role of prosaposin and its receptors in the nervous system

Prosaposin (also known as SGP-1) is an intriguing multifunctional protein that plays roles both intracellularly, as a regulator of lysosomal enzyme function, and extracellularly, as a secreted factor with neuroprotective and glioprotective effects. Following secretion, prosaposin can undergo endocytosis via an interaction with the low-density lipoprotein-related receptor 1 (LRP1). The ability of secreted prosaposin to promote protective effects in the nervous system is known to involve activation of G proteins, and the orphan G protein-coupled receptors GPR37 and GPR37L1 have recently been shown to mediate signaling induced by both prosaposin and a fragment of prosaposin known as prosaptide. In this review, we describe recent advances in our understanding of prosaposin, its receptors and their importance in the nervous system.

Cholesterol and glycosphingolipids of human trabecular meshwork and aqueous humor: comparative profiles from control and glaucomatous donors

PURPOSE

To determine the differential profiles of cholesterol and glycosphingolipid species and their quantitative differences between control and glaucomatous aqueous humor (AQH) and the trabecular meshwork (TM) derived from human donors.

METHODS

Control TM and selected primary open angle glaucoma (POAG) TM samples were collected from cadaveric donors. Other TM samples, glaucomatous AQH and control AQH were procured during intraocular surgery. Lipid extraction was performed using modifications of the Bligh and Dyer method. Protein concentration was estimated using the Bradford colorimetric assay. Cholesterol and glycosphingolipids were identified and subjected to ratiometric quantification utilizing precursor ion scan and neutral ion loss scan in positive ion mode using appropriate class specific lipid standards (Cholesterol and Psychosine) on a TSQ Quantum Access Max mass spectrometer.

RESULTS

Control and glaucomatous AQH demonstrated 7 and 4 unique cholesterol species, whereas the TM demonstrated 7 and 12 unique species, respectively. The control and POAG AQH showed 6 and 0 whereas TM samples showed 5 and 1 unique glycosphingolipids, respectively. A total of 65 and 62 common cholesterol species and 59 and 58 common glycosphingolipids were found in AQH and TM, respectively. Increased zymosterol and glucopyranosyl cholesterol levels were found in glaucomatous AQH. Significantly decreased levels of galactosylceramide, glucosylceramide in glaucomatous TM were found compared to control TM.

CONCLUSION

A high percentage of cholesterol and glycosphingolipid species was found to be common between control and POAG AQH and TM. Several cholesterol and glycosphingolipid species was found to be unique in a subset of POAG or controls. Glaucomatous aqueous humor and TM showed relatively higher levels of zymosterol (an intermediate precursor of cholesterol) and decreased glycoceramide levels, respectively.

Saposin A Mobilizes Lipids from Low Cholesterol and High Bis(monoacylglycerol)phosphate-containing Membranes

Saposin A (Sap-A) is one of five known sphingolipid activator proteins required for the lysosomal degradation of sphingolipids and for the loading of lipid antigens onto antigen-presenting molecules of the CD1 type. Sap-A assists in the degradation of galactosylceramide by galactosylceramide-beta-galactosidase in vivo, which takes place at the surface of intraendosomal/intralysosomal vesicles. Sap-A is believed to mediate the interaction between the enzyme and its membrane-bound substrate. Its dysfunction causes a variant form of Krabbe disease. In the present study we prepared glycosylated Sap-A free of other Saps, taking advantage of the Pichia pastoris expression system. Using liposomes and surface plasmon resonance spectroscopy, we tested the binding and lipid mobilization capacity of Sap-A under different conditions. Along the endocytic pathway, the pH value decreases, and the lipid composition of intraendosomal and intralysosomal membranes changes drastically. In the inner membranes the cholesterol concentration decreases, and that of the anionic phospholipid bis(monoacylglycero)phosphate increases. Here, we show that Sap-A is able to bind to liposomes and to mobilize lipids out of them at acidic pH values below pH 4.7. Low cholesterol levels and increasing concentrations of bis(monoacylglycero)phosphate favor lipid extraction significantly. Galactosylceramide as a bilayer component is not essential for lipid mobilization by Sap-A, which requires intact disulfide bridges for activity. We also show for the first time that glycosylation of Sap-A is essential for its lipid extraction activity. Variant Sap-A proteins, which cause storage of galactosylceramide in humans (Krabbe disease, Spiegel, R., Bach, G., Sury, V., Mengistu, G., Meidan, B., Shalev, S., Shneor, Y., Mandel, H., and Zeigler, M. (2005) Mol. Genet. Metab. 84, 160-166) and in mutant mice (Matsuda, J., Vanier, M. T., Saito, Y., Tohyama, J., and Suzuki, K. (2001) Hum. Mol. Genet. 10, 1191-1199) are deficient in lipid extraction capacity.

A short guided tour through functional and structural features of saposin-like proteins

SAPLIPs (saposin-like proteins) are a diverse family of lipid-interacting proteins that have various and only partly understood, but nevertheless essential, cellular functions. Their existence is conserved in phylogenetically most distant organisms, such as primitive protozoa and mammals. Owing to their remarkable sequence variability, a common mechanism for their actions is not known. Some shared principles beyond their diversity have become evident by analysis of known three-dimensional structures. Whereas lipid interaction is the basis for their functions, the special cellular tasks are often defined by interaction partners other than lipids. Based on recent findings, this review summarizes phylogenetic relations, function and structural features of the members of this family.

A mutation in the saposin A coding region of the prosaposin gene in an infant presenting as Krabbe disease: first report of saposin A deficiency in humans

A six-month-old infant girl presenting with progressive encephalopathy and abnormal myelination in the cerebral white matter was originally diagnosed as suffering from Krabbe disease. The diagnosis was based on a deficiency of galactocerebrosidase activity found in leukocytes isolated from whole blood. When cultured skin fibroblasts did not show a similar enzyme deficiency and sulphatide (stearoyl-1-14C) uptake indicated an abnormal storage of galactosylceramide, a deficiency of an activator was implied. A three base pair deletion was found in the saposin A coding sequence of the prosaposin gene leading to the deletion of a conserved valine at amino acid number 11 of the saposin A protein. This deletion in saposin A is proposed as the cause for the abnormal galactosylceramide metabolism in this infant. This is the first report of a saposin A mutation in humans leading to pathological consequences.

Analysis of recombinant human saposin A expressed by Pichia pastoris

Saposins (SAPs) are small glycoproteins required for activation of sphingolipid hydrolysis by lysosomal enzymes. Four SAPs, SAP-A, -B, -C, and -D, are proteolytically cleaved from a single gene product termed prosaposin. The mature coding sequence of human SAP-A tagged with 6-histidine was expressed in Pichia pastoris and the recombinant protein was purified from the culture supernatant by simple purification steps with an immobilized metal ion affinity column, a Concanavalin A column, and reversed-phase HPLC. Secreted SAP-A contained both glycosylated and nonglycosylated forms. Both forms of SAP-A activated galactocerebroside and 4-methylumbelliferyl beta-d-glucoside hydrolysis by galactocerebrosidase and glucocerebrosidase. SAP-A expressed in P. pastoris should be useful for further structural and functional analysis of this protein.

Biosynthesis and degradation of mammalian glycosphingolipids

Glycolipids are a large and heterogeneous family of sphingolipids that form complex patterns on eukaryotic cell surfaces. This molecular diversity is generated by only a few enzymes and is a paradigm of naturally occurring combinatorial synthesis. We report on the biosynthetic principles leading to this large molecular diversity and focus on sialic acid-containing glycolipids of the ganglio-series. These glycolipids are particularly concentrated in the plasma membrane of neuronal cells. Their de novo synthesis starts with the formation of the membrane anchor, ceramide, at the endoplasmic reticulum (ER) and is continued by glycosyltransferases of the Golgi complex. Recent findings from genetically engineered mice are discussed. The constitutive degradation of glycosphingolipids (GSLs) occurs in the acidic compartments, the endosomes and the lysosomes. Here, water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues from glycolipids. For glycolipid substrates with short oligosaccharide chains, the additional presence of membrane-active sphingolipid activator proteins (SAPs) is required. A considerable part of our current knowledge about glycolipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within this pathway. A new post-translational modification is the attachment of glycolipids to proteins of the human skin.

Correlation among genotype, phenotype, and biochemical markers in Gaucher disease: implications for the prediction of disease severity

Gaucher disease is a lysosomal storage disorder characterized by a deficiency of the enzyme acid beta-glucosidase. The clinical manifestations of Gaucher disease are highly variable, and although certain genotypes are often associated with mild or severe symptoms, a defined correlation between genotype and phenotype does not exist. Identification of biochemical markers characteristic of pathology may be of use in predicting the progression of the disease state. In this study the relationship among genotype, glycolipid substrates, lysosomal proteins, and the clinical manifestations of Gaucher disease has been evaluated. Plasma glycolipids were analyzed using electrospray ionization-tandem mass spectrometry. Lysosomal-associated membrane protein-1 (LAMP-1) and saposin C were determined by immunoquantification. Patients with Gaucher disease were shown to have an increased 16:0-glucosylceramide/16:0-lactosylceramide ratio and elevated concentrations of LAMP-1 and saposin C in plasma. A general relationship was found to exist among the 16:0-glucosylceramide/16:0-lactosylceramide ratio, LAMP-1 and saposin C levels, and patient phenotype, providing a refinement of the genotype-phenotype correlation. These findings have major implications for the diagnosis, prediction of disease severity, and monitoring of therapy in patients with Gaucher disease.

Saposins (sap) A and C activate the degradation of galactosylsphingosine

As previously shown for [(3)H-galactosyl]ceramide, the breakdown of [(3)H-galactosyl]sphingosine was reduced in prosaposin-deficient skin fibroblast homogenates. Galactosylsphingosine hydrolysis was also deficient in cell homogenates from Krabbe's disease (beta-galactocerebrosidase-deficient) patients, but not acid beta-galactosidase-deficient patients. Moreover, hydrolysis of galactosylsphingosine in the prosaposin-deficient cell homogenates could be partially restored by adding pure saposin A or C, thereby identifying these saposins as essential facilitators of galactosylsphingosine hydrolysis. By contrast, saposins B and D had little effect on galactosylsphingosine hydrolysis in the prosaposin-deficient cells. The reduced galactosylsphingosine turnover in prosaposin-deficiency suggests that there could be a pathogenetic cerebral accumulation of galactosylsphingosine in this disorder.

Identification of a novel sequence involved in lysosomal sorting of the sphingolipid activator protein prosaposin

Prosaposin is synthesized as a 53-kDa protein, post-translationally modified to a 65-kDa form and further glycosylated to a 70-kDa secretory product. The 65-kDa protein is associated to Golgi membranes and is targeted to lysosomes, where four smaller nonenzymatic saposins implicated in the hydrolysis of sphingolipids are generated by its partial proteolysis. The targeting of the 65-kDa protein to lysosomes is not mediated by the mannose 6-phosphate receptor. The Golgi apparatus appears to accomplish the molecular sorting of the 65-kDa prosaposin by decoding a signal from its amino acid backbone. This investigation deals with the characterization of the sequence involved in this process by deleting the saposin functional domains A, B, C, and D and the highly conserved N and C termini of prosaposin. The truncated cDNAs were subcloned into expression vectors and transfected to COS-7 cells. The destination of the mutated proteins was assessed by immunocytochemistry. Deletion of the C terminus did not interfere with the secretion of prosaposin but abolished its transport to lysosomes. Deletion of saposins and the N-terminal domain did not affect the lysosomal or secretory routing of prosaposin. A chimeric construct of albumin and the C terminus of prosaposin was not directed to lysosomes. However, albumin connected to the C terminus and one or more functional domains of prosaposin reached lysosomes, indicating that the C terminus and at least one saposin domain are required for this process. In summary, we are reporting a novel sequence involved in the targeting of prosaposin to lysosomes.

Molecular genetics of Krabbe disease (globoid cell leukodystrophy): diagnostic and clinical implications

Galactocerebrosidase (GALC) is a lysosomal beta-galactosidase responsible for the hydrolysis of the galactosyl moiety from several galactolipids, including galactosylceramide and psychosine. The deficiency of this enzyme results in the autosomal recessive disorder called Krabbe disease. It is also called globoid cell leukodystrophy (GLD), because of the characteristic storage cells found around cerebral blood vessels in the white matter of affected human patients and animal models. Although most patients present with clinical symptoms before 6 months of age, older patients, including adults, have been diagnosed by their severe deficiency of GALC activity. More than 40 mutations have been identified in patients with all clinical types of GLD. While some mutations clearly result in the infantile type if found homozygous or with another severe mutation, it is difficult to predict the phenotype of novel mutations or when mutations are found in the heterozygous state. A high incidence of polymorphic changes on apparent disease-causing alleles also complicates the interpretation of the effects of mutations. The detection of mutations has greatly improved carrier identification among family members and will permit preimplantation diagnosis for some families. The molecular characterization of the naturally occurring mouse, dog, and monkey models will permit their use in trials to evaluate different modes of therapy.

Cloning, expression and map assignment of chicken prosaposin

Prosaposin is the precursor of four small glycoproteins, saposins A-D, that activate lysosomal sphingolipid hydrolysis. A full-length cDNA encoding prosaposin from chicken brain was isolated by PCR. The deduced amino acid sequence predicted that, similarly to human and other mammalian species studied, chicken prosaposin contains 518 residues, including four domains that correspond to saposins A-D. There was 59% identity and 76% similarity of human and chicken prosaposin amino acid sequences. The basic three-dimensional structures of these saposins is predicted to be similar on the basis of the conservation of six cysteine residues and an N-glycosylation site. Identity of amino acid sequences was higher among saposins A, B and D than in saposin C. The predicted amino acid sequence of saposin B matched exactly that of purified chicken saposin B protein. The chicken prosaposin gene was mapped to a single locus, PSAP, in chicken linkage group E11C10 and is closely linked to the ACTA2 locus. This confirms the homology between chicken and human prosaposins and defines a new conserved segment with human chromosome 10q21-q24.

Saposins (sap) A and C activate the degradation of galactosylceramide in living cells

In loading tests using galactosylceramide which had been labelled with tritium in the ceramide moiety, living skin fibroblast lines derived from the original prosaposin-deficient patients had a markedly reduced capacity to degrade galactosylceramide. The hydrolysis of galactosylceramide could be partially restored in these cells, up to about half the normal rate, by adding pure saposin A, pure saposin C, or a mixture of these saposins to the culture medium. By contrast, saposins B and D had little effect on galactosylceramide hydrolysis in the prosaposin-deficient cells. Cells from beta-galactocerebrosidase-deficient (Krabbe) patients had a relatively high residual galactosylceramide degradation, which was similar to the rate observed for prosaposin-deficient cells in the presence of saposin A or C. An SV40-transformed fibroblast line from the original saposin C-deficient patient, where saposin A is not affected, showed normal degradation of galactosylceramide. The findings support the hypothesis, which was deduced originally from in vitro experiments, that saposins A and C are the in vivo activators of galactosylceramide degradation. Although the results with saposin C-deficient fibroblasts suggest that the presence of only saposin A allows galactosylceramide breakdown to proceed at a normal rate in fibroblasts, it remains to be determined whether saposins A and C can substitute for each other with respect to their effects on galactosylceramide metabolism in the whole organism.

Model SV40-transformed fibroblast lines for metabolic studies of human prosaposin and acid ceramidase deficiencies

Skin fibroblasts from patients with Farber disease (acid ceramidase deficiency) and from two siblings of the only known family affected with prosaposin deficiency were transformed by transfection with a plasmid carrying the SV40 large T antigen. The prosaposin-deficient transformed cell lines conserved their original metabolic defects, and in particular they were free of detectable immunoreactivity when using anti-saposin B and anti-saposin C antisera. Ultrastructurally, the cells contained heterogeneous lysosomal storage products. As found for their parental cell lines, the SV40-transformed fibroblasts exhibited deficient in vitro activities of lysosomal ceramidase and beta-galactosylceramidase, but a normal activity of acid sphingomyelinase. As observed for SV40-transformed fibroblasts from Farber disease, degradation of radioactive glucosylceramide or low density lipoprotein-associated radiolabelled sphingomyelin by the prosaposin-deficient cells in situ showed a clear impairment in the turnover of lysosomal ceramide. Ceramide storage in prosaposin-deficient cells was also demonstrated by ceramide mass determination. In contrast to acid ceramidase deficient cells, both the accumulation of ceramide and the reduced in vitro activity of acid ceramidase in cells from prosaposin deficiency could be corrected by addition of purified saposin D. The data confirm that prosaposin is required for lysosomal ceramide degradation, but not for sphingomyelin turnover. The SV40-transformed fibroblasts will be useful for pathophysiological studies on human prosaposin deficiency.

Formation of a ternary complex between GM2 activator protein, GM2 ganglioside and hexosaminidase A

The GM2 activator is a 17 kDa protein required for the hydrolysis of GM2 ganglioside by the lysosomal enzyme hexosaminidase A (HexA). The activator behaves as a substrate binding protein, solubilizing GM2 ganglioside monomers from micelles (in vitro) or membranes (in vivo). However, the activator also shows a high order of specificity for activation of lysosomal hydrolases and has been predicted to form a ternary complex with the heterodimeric enzyme (alphabeta) Hex A and GM2 ganglioside. We demonstrated a transient interaction between HexA and the GM2 activator. A chimeric protein containing the FLAG epitope sequence upstream of the GM2 activator was expressed in Escherichia coli and purified using the M1 immunoaffinity (anti-FLAG) column. Binding of the FLAG-GM2 activator (FLAG-AP) fusion protein to the M1 column led to the specific retardation of Hex A applied to the column. Other proteins were not retarded by the column nor did they compete with Hex A for binding to FLAG-AP. Hex A and GM2 ganglioside could be simultaneously bound to the column, but the binding of each ligand was independent of the other. The homodimeric (beta beta) isozyme Hex B did not bind to the immobilized activator. The alpha alpha homodimer, HexS, bound weakly, confirming that a hexosaminidase alpha subunit is required for interaction of enzyme and activator.

Specific recognition of N-acetylneuraminic acid in the GM2 epitope by human GM2 activator protein

GM2 Activator is a low molecular weight protein cofactor that stimulates the enzymatic conversion of GM2 into GM3 by human beta-hexosaminidase A and also the conversion of GM2 into GA2 by clostridial sialidase (Wu, Y.-Y., Lockyer, J.M., Sugiyama, E., Pavlova, N.V., Li, Y.-T., and Li, S.-C. (1994) J. Biol. Chem. 269, 16276-16283). Among the five known activator proteins for the enzymatic hydrolysis of glycosphingolipids, only GM2 activator is effective in stimulating the hydrolysis of GM2. However, the mechanism of action of GM2 activator is still not well understood. Using a unique disialosylganglioside, GalNAc-GD1a, as the substrate, we were able to show that in the presence of GM2 activator, GalNAc-GD1a was specifically converted into GalNAc-GM1a by clostridial sialidase, while in the presence of saposin B, a nonspecific activator protein, GalNAc-GD1a was converted into both GalNAc-GM1a and GalNAc-GM1b. Individual products generated from GalNAc-GD1a by clostridial sialidase were identified by thin layer chromatography, negative secondary ion mass spectrometry, and immunostaining with a monoclonal IgM that recognizes the GM2 epitope. Our results clearly show that GM2 activator recognizes the GM2 epitope in GalNAc-GD1a. Thus, GM2 activator may interact with the trisaccharide structure of the GM2 epitope and render the GalNAc and NeuAc residues accessible to beta-hexosaminidase A and sialidase, respectively.

Structural analysis of saposin C and B. Complete localization of disulfide bridges

Saposins A, B, C, and D are a group of homologous glycoproteins derived from a single precursor, prosaposin, and apparently involved in the stimulation of the enzymatic degradation of sphingolipids in lysosomes. All saposins have six cysteine residues at similar positions. In the present study we have investigated the disulfide structure of saposins B and C using advanced mass spectrometric procedures. Electrospray analysis showed that deglycosylated saposins B and C are mainly present as 79- and 80-residue monomeric polypeptides, respectively. Fast atom bombardment mass analysis of peptide mixtures obtained by a combination of chemical and enzymatic cleavages demonstrated that the pairings of the three disulfide bridges present in each saposin are Cys4-Cys77, Cys7-Cys71, Cys36-Cys47 for saposin B and Cys5-Cys78, Cys8-Cys72, Cys36-Cys47 for saposin C. We have recently shown that saposin C interacts with phosphatidylserine-containing vesicles inducing destabilization of the lipid surface (Vaccaro, A. M., Tatti, M., Ciaffoni, F., Salvioli, R., Serafino, A., and Barca, A. (1994) FEBS Lett. 349, 181-186); this perturbation promotes the binding of the lysosomal enzyme glucosylceramidase to the vesicles and the reconstitution of its activity. It was presently found that the effects of saposin C on phosphatidylserine liposomes and on glucosylceramidase activity are markedly reduced when the three disulfide bonds are irreversibly disrupted. These results stress the importance of the disulfide structure for the functional properties of the saposin.

Hydrolysis of lactosylceramide by human galactosylceramidase and GM1-beta-galactosidase in a detergent-free system and its stimulation by sphingolipid activator proteins, sap-B and sap-C. Activator proteins stimulate lactosylceramide hydrolysis

Two exo-beta-galactosidases are involved in the lysosomal degradation of glycosphingolipids: GM1-beta-galactosidase (EC 3.2.1.23) and galactosylceramidase (EC 3.2.1.46). Analyses were performed with both enzymes, using lactosylceramides with varying acyl chain lengths as substrates that were inserted into unilamellar liposomes and naturally occurring sphingolipid activator proteins sap-B and sap-C, rather than detergents, to stimulate the reaction. While sap-B was a better activator for the reaction catalyzed by GM1-beta-galactosidase, sap-C preferentially stimulated lactosylceramide hydrolysis by galactosylceramidase. The enzymic hydrolysis of liposome-integrated lactosylceramides was significantly dependent on the structure of the lipophilic aglycon moiety of the lactosylceramide decreasing with increasing length of its fatty acyl chain (C2 > C4 > C6 > C8 > C10 > C18). However, in the presence of detergents the degradation rates were independent of the acyl chain length. Hydrolysis of liposomal lactosylceramide was compared with sap-B-stimulated hydrolysis of liposomal ganglioside GM1 by GM1-beta-galactosidase and sap-C-stimulated degradation of liposomal galactosylceramide by galactosylceramidase. Kinetic and dilution experiments indicated that sap-B forms water-soluble complexes with both lactosylceramide and GM1. These complexes were recognized by GM1-beta-galactosidase as optimal substrates in the same mode, as postulated for the hydrolysis of sulfatides by arylsulfatase A [Fischer, G. and Jatzkewitz, H. (1977) Biochim. Biophys. Acta 481, 561-572]. GM1-beta-galactosidase was more active on these complexes than on glycolipids (GM1 and lactosylceramides) still residing in liposomal membranes. On the other hand, dilution experiments indicated that degradation of galactosylceramide and lactosylceramide by galactosylceramidase proceeds almost exclusively on liposomal surfaces: both activators, sap-C and sap-B, stimulated the hydrolysis of lactosylceramide analogues with long acyl chains more than the hydrolysis of lactosylceramides with short acyl chains.

Prosaposin deficiency: further characterization of the sphingolipid activator protein-deficient sibs. Multiple glycolipid elevations (including lactosylceramidosis), partial enzyme deficiencies and ultrastructure of the skin in this generalized sphingolipid storage disease

Sphingolipid activator protein (SAP) deficiency, previously described in two sibs and shown to be caused by the absence of the common saposin precursor (prosaposin), was further characterized by biochemical lipid and enzyme studies and by ultrastructural analysis. The 20-week-old fetal sib had increased concentrations of neutral glycolipids, including mono-, di-, tri- and tetrahexosylceramide, in liver, kidney and cultured skin fibroblasts compared with the controls. Glucosylceramide and lactosylceramide were particularly elevated. The kidney of the affected fetus showed additional increases in the concentration of sulphatide, galactosylceramide and digalactosylceramide. Free ceramide was stored in the liver and kidney, and GM3 and GM2 gangliosides were elevated in the liver, but not the brain, of the fetus. Phospholipids, however, were normal in the affected fetus. In the liver biopsy of the propositus, who later died at 16 weeks of age, only a few lipids could be studied. Glucosylceramide, dihexosylceramide and ceramide were elevated in agreement with our previous study. Enzyme studies were undertaken using detergent-free liposomal substrate preparations and fibroblast extracts. The sibs' beta-glucocerebrosidase and beta-galactocerebrosidase activities were clearly reduced, but their sphingomyelinase activities were normal. The normal activity of the latter enzyme and the almost normal tissue concentration of sphingomyelin in prosaposin deficiency suggest that the prosaposin-derived SAPs are not required for sphingomyelinase activity in vivo. In keeping with the biochemical findings, skin biopsies from the sibs showed massive lysosomal storage with a vesicular and membranous ultrastructure. The function of SAPs in sphingolipid degradation and the role of SAPs for enzyme activity in vitro are discussed. In addition, the similarity in neutral glycolipid accumulations in Niemann-Pick disease type C and in prosaposin deficiency are noted. The phenotype of the prosaposin deficient sibs resembled acute neuronopathic (type 2) Gaucher disease more than Farber disease in several aspects, but their genotype was unique.

Synthesis and characterization of a bioactive 82-residue sphingolipid activator protein, saposin C

The sphingolipid activator protein, saposin C (also termed SAP 2), was chemically synthesized, purified, and characterized. The fully protected 82-residue protein was synthesized by automated solid-phase methods, with multiple recoupling steps resulting in a high average coupling efficiency of 98.8%. The overall yield was estimated to be approx 40%. Deprotection and cleavage of the peptide from the resin was followed by folding in the absence of chaotropic agents at pH 8.5. The protein was purified by reversed-phase high pressure liquid chromatography (HPLC) and its purity determined by capillary electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The composition of the synthetic saposin C was determined by amino acid analysis. Its sequence was verified by Edman sequence analysis of overlapping peptide fragments generated by chymotryptic and Staphylococcus aureus V8 digestions. The sequence at the C-terminus was determined by digestion with carboxypeptidase P, followed by phenylthiohydantoin (PTH) derivitization and HPLC analysis of the released amino acid residues. Deglycosylated native saposin C appeared as a lower molecular-weight species than synthetic saposin C on SDS-PAGE. This has been explained by amino acid and C-terminal analysis showing native saposin C to be two amino acids shorter at the C terminus than a deduced sequence (from cDNA) previously published. Synthetic saposin C displayed 85% of full biological activity as determined by its ability to stimulate glucocerebrosidase activity in vitro: Synthetic and native saposin C increased glucocerebrosidase catalyzed hydrolysis of 4-methylumbelliferyl beta-D-glucoside by factors of 6.0 and 7.1, respectively. Furthermore, synthetic and native saposin C share similar K(act) values (0.5 and 1.5 microM respectively) indicating that they bind to glucocerebrosidase with similar affinities.

Effect of saposins on acid sphingomyelinase

The effect of saposins (A, B, C and D) on acid sphingomyelinase activity was determined using a crude human kidney sphingomyelinase preparation and a purified sphingomyelinase preparation from human placenta. Saposin D stimulated the activity of the crude enzyme by increasing its apparent Km and Vmax. values for sphingomyelin hydrolysis. Unlike the crude enzyme, the activity of the purified enzyme was strongly inhibited by saposin D as well as other saposins. Saposin D decreased the apparent Km and Vmax values of purified sphingomyelinase activity. The effects of saposin D on the activity of different sphingomyelinase preparations appear to depend on Triton X-100, which is present in the crude enzyme but not in the purified enzyme. When the detergent was removed from the crude preparation, the effect of saposin D changed from being stimulatory to inhibitory. Conversely, when the detergent is added to the purified enzyme, the effect of saposin D on sphingomyelinase activity changed from being inhibitory to stimulatory. While other saposins were inhibitory or had no effect on sphingomyelinase activity in the above assay system, not only saposin D but also saposins A and C exhibited a stimulatory effect upon purified sphingomyelinase activity when the substrate, sphingomyelin, was added in the form of liposomes without detergent. Saposin B was not only inhibitory in the liposome system, but also reduced the stimulatory effect of saposins A, C and D. These observations indicate that the stimulatory effect of saposins A, C and D on acid sphingomyelinase activity is greatly influenced by the physical environment of the enzyme and suggest that similar effects by saposins may be exerted in lysosomal membranes.

Additional biochemical findings in a patient and fetal sibling with a genetic defect in the sphingolipid activator protein (SAP) precursor, prosaposin. Evidence for a deficiency in SAP-1 and for a normal lysosomal neuraminidase

It has been shown that sphingolipid activator proteins (SAPs) 1 and 2 are encoded on the same gene along with two other putative activator proteins [Fürst, Machleidt & Sandhoff (1988) Biol. Chem. Hoppe-Seyler 369, 317-328 and O'Brien, Kretz, Dewji, Wenger, Esch & Fluharty (1988) Science 241, 1098-1101]. We have undertaken further biochemical investigations on a patient and fetal sibling, who were previously shown to have a unique sphingolipid storage disorder associated with an SAP-2 deficiency [Harzer, Paton, Poulos, Kustermann-Kuhn, Roggendorf, Grisar & Popp (1989) Eur. J. Pediatr. 149, 31-39]. The severity of their disorder suggested that other products of the SAP precursor or prosaposin gene may also be deficient. The turnover of cerebroside sulphate and globotriaosylceramide were investigated and were both impaired in fibroblasts from the patient and fetus. However, the activities of cerebroside sulphate sulphatase and globotriaosylceramide alpha-galactosidase in vitro were normal in cells from the fetus and patient respectively. In addition, there was an increase in cerebroside sulphate concentration in the kidney of the affected fetus. These results indicate that, in addition to the SAP-2 deficiency, there was a defect in SAP-1 function in this disorder. Additional increases in the concentration of monohexosyl- and dihexosyl-ceramide in the fetal kidney probably reflect the deficiency of SAP-2 in the case of monohexosylceramides, and the combined activator deficiency in the case of dihexosylceramides. Lactosylceramide-loading studies confirmed that there was a defect in the turnover of this lipid in fibroblasts from the affected patient and fetus but not from a patient with an isolated SAP-1 deficiency, or from patients with Krabbe disease, GM1 gangliosidosis or galactosialidosis. It has been suggested [Potier, Lamontagne, Michaud & Tranchemontagne (1990) Biochem. Biophys. Res. Commun. 173, 449-456] that the prosaposin gene also codes for lysosomal neuroaminidase. However, we found normal neuraminidase activity in fibroblasts from our patient, using assay conditions which are diagnostic for sialidosis patients. The role of prosaposin gene products in sphingolipid metabolism is discussed in view of our biochemical findings in this genetic disorder.

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.

Evidence for two cDNA clones encoding human GM2-activator protein

Two cDNAs encoding GM2 activator, pGM2A (648 bp) and GAP (1093 bp), were isolated from human placenta lambda gt11 libraries. The DNA sequence of pGM2A from 1 to 302 was almost identical with GAP, but diverged from 303-648. PCR was used to demonstrate the presence of both species of GM2 activator in placental RNA. Both cDNAs hybridized to mRNAs of approximately 2.3 kb and to identical single bands on genomic Southern blots.

The use of glycosides of 6- and 8-acylamino-4-methylumbelliferone in studies of the specificity and properties of human lysosomal glycolipid hydrolases

A series of 6- and 8-acylamino-4-methylumbelliferyl beta-D-galactopyranosides, beta-D-glucopyranosides, and alpha-L-fucopyranosides having various fatty acid residues were synthesized; 6-(9) and 8-hexadecanoylamino-4-methylumbelliferyl beta-D-galactopyranoside (10) were shown to be substrates for human galactocerebrosidase. Analogs of 9 with shorter acyl residues (octanoyl and butanoyl) were substrates for another type of beta-D-galactosidase, i.e., GM1-ganglioside-beta-D-galactosidase. The specificity of various beta-D-galactosidases for synthetic D-galactopyranosides, differing in the length and position of their acylamide residue, tested with enzyme preparations from patients with two types of glycolipidosis, Krabbe's disease (galactocerebrosidase deficiency) and GM1-beta-galactosidase deficiency), suggested that 9 is a specific substrate for galactocerebrosidase in biochemical tests for Krabbe's disease. Fluorogenic 6-octanoyl- and 6-hexadecanoyl-amino-4-methylumbelliferyl beta-D-glucopyranoside were much less readily hydrolyzed by both human and animal glucocerebrosidase than chromogenic 2-hexadecanoylamino-4-nitrophenyl beta-D-glucopyranoside. Comparison of the hydrolysis of 4-methylumbelliferyl alpha-L-fucopyranoside with that of 6-hexadecanoylamino-4-methylumbelliferyl alpha-L-fucopyranoside by multiple forms of human alpha-L-fucosidase showed that the enzyme is capable of hydrolyzing not only hydrophilic but also synthetic, lipid-like substrates.

The mechanism for a 33-nucleotide insertion in mRNA causing sphingolipid activator protein (SAP-1)-deficient metachromatic leukodystrophy

Metachromatic leukodystrophy is a severe autosomal recessive disorder caused by accumulation of sulfatide resulting from deficient lysosomal degradation. While most patients have mutations in the lysosomal enzyme arylsulfatase A, some patients have mutations in a required heat stable sphingolipid activator protein, we call SAP-1. One patient with SAP-1 deficiency was previously demonstrated to have a 33-nucleotide insertion in her mRNA. This resulted in the production of mature SAP-1 with 11 extra amino acids, which was unstable during intracellular processing. In this manuscript we demonstrate that the 33 nucleotides are present near the middle of a 4-kb intron, and that a single base change, c to a, in the second position preceding the 33-nucleotide insertion, coupled with the presence of a string of pyrimidines immediately upstream from this change, creates a new 3' splice junction. The presence of a string of pyrimidines within the 33-nucleotide insertion, which has three cag trinucleotides near the 3' end, leads to alternative splicing in normal people as found in this laboratory and by others. The insertion region is followed by a gt dinucleotide that is spliced to a typical 3' consensus sequence. The single nucleotide change, c to a, was confirmed by identifying normal and mutant sequence in the consanguineous parents and a sister, previously identified as a carrier of this disorder.

Intracellular transport and metabolism of sphingomyelin

SM is unique among the phospholipids because it is restricted to the lumenal aspect of organelles involved in the secretory and endocytic pathways. Given the intracellular sites of SM biosynthesis and hydrolysis, and the interconnections between these sites by vesicle-mediated transport pathways, the basic mechanism for maintaining the intracellular distribution of SM seems clear. It remains to be determined how SM metabolism and transport are coordinated to maintain the SM content of each organelle. For example, the size of the SM pool at the cell surface is maintained by regulation of at least five processes: transport of newly synthesized SM from the Golgi apparatus, plasma membrane lipid recycling, local SM synthesis, local SM hydrolysis, and SM transport from the cell surface to lysosomes. Although SM cannot undergo spontaneous transbilayer movement, SM metabolism generates both DAG, Cer and (indirectly) SPhB which can rapidly 'flip-flop', and thus gain access to the cytoplasmic leaflet of a membrane. It is of particular interest that these lipid species may be involved in the regulation of PK-C, suggesting that SM metabolism could play a role in signal transduction. However, physiological effects of endogenous Cer and SPhB remain elusive, even though the pharmacological effect of SPhB on PK-C is well established. Aside from the direct generation of second messengers, stimulation of SM hydrolysis has also been shown to induce cholesterol movement from the cell surface to intracellular membranes. It is not known whether this reflects the possibility that cholesterol may act as a second messenger. Alternatively, this phenomenon suggests that SM metabolism may cause rapid changes in the physical properties of the cell surface. For example, erythrocytes extensively treated with exogenously-added SMase will undergo endovesiculation It is tempting to speculate that any involvement of SM in the regulation of intracellular processes requires a combination of both the generation of biochemical second messengers and the alteration of membrane biophysical properties that can result from SM metabolism.

Saposins (sphingolipid activator proteins) in the twitcher mutant mouse

The twitcher mutant mouse, the animal model of Krabbe disease (human globoid cell leukodystrophy), is characterized by apparent deficiency of galactosylceramide beta-galactosidase activity. Saposin A and C, the heat-stable small sphingolipid activator glycoproteins, stimulate the activity of galactosylceramide beta-galactosidase as well as glucosylceramide beta-glucoside. The role of these saposins in the twitcher mutation was investigated. Boiled supernatant fractions, which contained saposins, were prepared from homogenates of twitcher brain, liver, kidney, and spleen. These preparations showed an almost identical effect on the activity of purified glucosylceramide beta-glucosidase (measured by hydrolysis of 4-methylumbelliferyl-beta-glucoside) with similar preparations from control tissues. The effect on the activity of galactosylceramide beta-galactosidase as well as 4-methylumbelliferyl-beta-glucoside beta-glucosidase in the twitcher brain and liver homogenates by authentic saposin A and C was similar to that in control tissues. These results suggest that the twitcher mutation does not affect the concentrations of saposin A or C or their interaction with galactosylceramide beta-galactosidase.

An update of the enzymology and regulation of sphingomyelin metabolism

Sphingomyelin is found in plasma membranes and related organelles (such as endocytic vesicles and lysosomes) of all tissues, as well as in lipoproteins. Abnormalities in sphingomyelin metabolism have been associated with atherosclerosis, cancer and genetically transmitted diseases; however, except for Niemann-Pick disease, little is known about the mechanism for these disorders. Sphingomyelin biosynthesis de novo involves ceramide formation from serine and two mol of fatty acyl-CoA followed by addition of the phosphocholine headgroup. The headgroup appears to come from phosphatidylcholine, but other sources have not been ruled out. Factors that influence the rate of sphingomyelin synthesis include the availability of serine and palmitic acid, plus the relative activities of key enzymes of this pathway. Sphingomyelin turnover involves removal of the headgroup and amide-linked fatty acid by sphingomyelinases and ceramidases, respectively, which have been found in both lysosomes (with acidic pH optima) and plasma membranes (with neutral to alkaline pH optima). The enzymes of sphingomyelin turnover release ceramide and free sphingosine from endogenous substrates, which may have implications for the participation of a sphingomyelin/sphingosine cycle as another 'lipid second messenger' system.

Distribution of saposin proteins (sphingolipid activator proteins) in lysosomal storage and other diseases

Saposins (A, B, C, and D) are small glycoproteins required for the hydrolysis of sphingolipids by specific lysosomal hydrolases. Concentrations of these saposins in brain, liver, and spleen from normal humans as well as patients with lysosomal storage disease were determined. A quantitative HPLC method was used for saposin A, C, and D and a stimulation assay was used for saposin B. In normal tissues, saposin D was the most abundant of the four saposins. Massive accumulations of saposins, especially saposin A (about 80-fold increase over normal), were found in brain of patients with Tay-Sachs disease or infantile Sandhoff disease. In spleen of adult patients with Gaucher disease, saposin A and D accumulations (60- and 17-fold, respectively, over normal) were higher than that of saposin C (about 16-fold over normal). Similar massive accumulations of saposins A and D were found in liver of patients with fucosidosis (about 70- and 20-fold, respectively, over normal). Saposin D was the primary saposin stored in the liver of a patient with Niemann-Pick disease (about 30-fold over normal). Moderate increases of saposins B and D were found in a patient with GM1 gangliosidosis. Normal or near normal levels of all saposins were found in patients with Krabbe disease, metachromatic leukodystrophy, Fabry disease, adrenoleukodystrophy, I-cell disease, mucopolysaccharidosis types 2 and 3B, or Jansky-Bielschowsky disease. The implications of the storage of saposins in these diseases are discussed.

Characterization of a mutation in a family with saposin B deficiency: a glycosylation site defect

Saposins are small, heat-stable glycoproteins required for the hydrolysis of sphingolipids by specific lysosomal hydrolases. Saposins A, B, C, and D are derived by proteolytic processing from a single precursor protein named prosaposin. Saposin B, previously known as SAP-1 and sulfatide activator, stimulates the hydrolysis of a wide variety of substrates including cerebroside sulfate, GM1 ganglioside, and globotriaosylceramide by arylsulfatase A, acid beta-galactosidase, and alpha-galactosidase, respectively. Human saposin B deficiency, transmitted as an autosomal recessive trait, results in tissue accumulation of cerebroside sulfate and a clinical picture resembling metachromatic leukodystrophy (activator-deficient metachromatic leukodystrophy). We have examined transformed lymphoblasts from the initially reported saposin B-deficient patient and found normal amounts of saposins A, C, and D. After preparing first-strand cDNA from lymphoblast total RNA, we used the polymerase chain reaction to amplify the prosaposin cDNA. The patient's mRNA differed from the normal sequence by only one C----T transition in the 23rd codon of saposin B, resulting in a threonine to isoleucine amino acid substitution. An affected male sibling has the same mutation as the proband and their heterozygous mother carries both the normal and mutant sequences, providing additional evidence that this base change is the disease-causing mutation. This base change results in the replacement of a polar amino acid (threonine) with a nonpolar amino acid (isoleucine) and, more importantly, eliminates the glycosylation signal in this activator protein. One explanation for the deficiency of saposin B in this disease is that the mutation may increase the degradation of saposin B by exposing a potential proteolytic cleavage site (arginine) two amino acids to the amino-terminal side of the glycosylation site when the carbohydrate side chain is absent.

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.

Molecular cloning of a human co-beta-glucosidase cDNA: evidence that four sphingolipid hydrolase activator proteins are encoded by single genes in humans and rats

Authentic cDNAs encoding the activator protein for acid beta-glucosidase (EC3.2.1.45), co-beta-glucosidase, were cloned from the pCD and lambda gt11 human cDNA libraries. Initial screening with oligonucleotide mixtures encoding amino acid sequences of co-beta-glucosidase identified partial cDNAs which were used to obtain a potentially full-length cDNA from the lambda gt11 library. This clone (2767 bp), EGTISI, contained 5' (38 bp) and 3' (1157 bp) noncoding sequences, a translation initiation site, and an open reading frame encoding 524 amino acids which included a typical hydrophobic signal sequence (16 amino acids). Computer analyses identified three regions of high similarity to co-beta-glucosidase encoded by tandem sequences in EGTISI. Searches revealed that two of these regions encoded peptides of known function; SAP1 (sphingolipid activator protein 1) and protein C (a new sphingolipid activator protein) were encoded by EGTISI sequences 5' and 3', respectively, to those for co-beta-glucosidase. The third region of similarity, encoding a theoretical peptide (undefined function), was located most 5' in the cDNA. EGTISI and its encoded polypeptide had high similarity (77% nucleotide identity and about 80% amino acid similarity) to a rat Sertoli cell cDNA and its encoded sulfated glycoprotein-1. These results indicate that a single highly conserved gene encodes the precursor for four potential sphingolipid activator proteins in rat and man.

Properties of acid ceramidase from human spleen

We have characterised ceramidase activity in extracts of human spleen from control subjects and from patients with Gaucher disease. In Triton X-100 extracts of control spleens, a broad pH optimum of pH 3.5-5.0 was found; no ceramidase activity was detectable at neutral or alkaline pH. About 45-60% of acid ceramidase could be extracted from spleen without detergents, but for complete extraction, Triton X-100 was required. For the radiolabelled substrate oleoylsphingosine, a Km of 0.22 +/- 0.09 mM and a Vmax of 57 +/- 11 nmol/h per mg protein was calculated in spleen from a control subject. Flat-bed isoelectric focussing in the presence of Triton X-100 revealed a pI of 6.0-7.0 for acid ceramidase; similar values were found for sphingomyelinase and glucerebrosidase. HPLC-gel filtration indicated that in the presence of Triton X-100, acid ceramidase has an Mr of about 100 kDa. In the absence of detergents, the enzyme forms high-molecular-weight aggregates. Similar aggregation behaviour was observed for sphingomyelinase, while the elution of beta-hexosaminidase was not affected by detergents. The elution profile of glucocerebrosidase was only slightly altered by Triton X-100. There was no difference in the properties of acid ceramidase present in spleen from control subjects and from patients with type I Gaucher disease.

Saposin A: second cerebrosidase activator protein

Saposin A, a heat-stable 16-kDa glycoprotein, was isolated from Gaucher disease spleen and purified to homogeneity. Chemical sequencing from its amino terminus and of peptides obtained by digestion with protease from Staphylococcus aureus strain V-8 demonstrated that saposin A is derived from proteolytic processing of domain 1 of its precursor protein, prosaposin. Processing of prosaposin (70 kDa) also generates three other previously reported saposin proteins, B, C, and D, from its second, third, and fourth domains. Similar to saposin C, saposin A stimulates the hydrolysis of 4-methylumbelliferyl beta-glucoside and glucocerebroside by beta-glucosylceramidase and of galactocerebroside by beta-galactosylceramidase, mainly by increasing the maximal velocity of both reactions. Saposin A is as active as saposin C in these reactions. Saposin A has no significant effect on other sphingolipid and 4-methylumbelliferyl glycoside hydrolases tested. Saposin A has two potential glycosylation sites that appear to be glycosylated. After deglycosylation, saposin A had a subunit molecular mass of 10 kDa and was as active as native saposin A. However, reduction and alkylation abolished the activation. A three-dimensional model comparing saposins A and C reveals significant sequence homology between them, especially preservation of conserved acidic and basic residues in their middle regions. Each appears to possess a conformationally rigid hydrophobic pocket stabilized by three internal disulfide bridges, with amphipathic helical regions interrupted by helix breakers.

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.

Coding of two sphingolipid activator proteins (SAP-1 and SAP-2) by same genetic locus

Several complementary DNAs (cDNAs) coding for sphingolipid activator protein-2 (SAP-2) were isolated from a lambda gt-11 human hepatoma library by means of polyclonal antibodies. The nucleotide sequence of the largest cDNA was colinear with the derived amino acid sequence of SAP-2 and with the nucleotide sequence of the cDNA coding for the 70-kilodalton precursor of SAP-1 (SAP precursor cDNA). The coding sequence for mature SAP-2 was located 3' to that coding for SAP-1 in the SAP precursor cDNA. Both SAP-1 and SAP-2 appeared to be derived by proteolytic processing from a common precursor that is coded by a genetic locus on human chromosome 10. Two other domains similar to SAP-1 and SAP-2 were also identified in SAP precursor protein. Each of the four domains was approximately 80 amino acid residues long, had nearly identical placement of cysteine residues, potential glycosylation sites, and proline residues. Each domain also contained internal amino acid sequences capable of forming amphipathic helices separated by helix breakers to give a cylindrical hydrophobic domain that is probably stabilized by disulfide bridges. Protein immunoblotting experiments indicated that SAP precursor protein (70 kilodaltons) as well as immunoreactive SAP-like proteins of intermediate sizes (65, 50, and 31 kilodaltons) are present in most human tissues.

Analysis of the multiple forms of Gaucher spleen sphingolipid activator protein 2

Gaucher spleen sphingolipid activator protein 2 was fractionated into concanavalin A binding- and non-binding fractions. These fractions each contained several bands on non-denaturing polyacrylamide gel electrophoresis (PAGE). The two fractions were further fractionated by electroblotting the proteins from preparative gels onto nitrocellulose, staining with Ponceau S to locate the bands of protein and then eluting the protein components from the nitrocellulose. A total of ten fractions, each containing only one or two major components, was collected. All of these subfractions activated beta-glucocerebrosidase and sphingomyelinase and most subfractions also activated beta-galactocerebrosidase. The structural relationship of the bands was investigated using endoglycosidase digestions. The results indicated that the two bands with the fastest mobility on non-denaturing PAGE did not contain any carbohydrate. The remaining bands showed only limited or partial digestion with endoglycosidase H and endoglycosidase D, but were readily hydrolysed with endoglycosidase F. The products of these digestions included bands with similar mobilities to the non-carbohydrate containing bands.

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.

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.

Regional localization of the gene coding for sphingolipid activator protein SAP-1 on human chromosome 10

Sphingolipid activator protein SAP-1 is required for the enzymatic hydrolysis of GMI ganglioside and sulfatide. The gene coding for SAP-1 was previously mapped to human chromosome 10 using monospecific antibodies prepared against SAP-1 in synteny analysis of somatic cell hybrids. In this study, we used a cDNA probe for SAP-1 and in situ hybridization to regionally localize the SAP1 gene to the long arm of chromosome 10, region q21-22. Additional mapping data using cell hybrids containing partial chromosome 10 and skin fibroblasts with trisomy 10p are consistent with the in situ hybridization mapping results.

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.