Enzyme-linked immunosorbent assay for the ganglioside GM2-activator protein. Screening of normal human tissues and body fluids, of tissues of GM2 gangliosidosis, and for its subcellular localization

Are there useful biochemical markers of disease activity in lysosomal storage diseases?

The primary biochemical consequence of a defect in a gene encoding a functional component of the lysosomal system is disruption of the catabolism or processing of macromolecules in the lumen of the lysosome. Transport of the resulting digestion products through the lysosomal membrane may also be affected. This leads to the accumulation of specific metabolites within the lysosomes of affected cells. The nature of these storage products depends upon the functional protein affected and the cell type. The accumulation of storage products is progressive and leads to hypertrophy of the lysosomal system, the hallmark of lysosomal storage diseases (LSDs). Subsequent cell necrosis or, possibly, exocytosis results in the appearance in body fluids of the storage products and components of the lysosomes at much higher concentrations than seen in normal unaffected individuals. Measurement of these increased levels of metabolites and proteins provides disease-specific and generic biochemical markers for LSDs. Secondary changes in metabolism and cellular function may also produce characteristic changes in the levels of metabolites or proteins, which can also be used as markers of the disease process. Although the rate of appearance of these biochemical markers in an individual will depend upon the underlying mutation in the gene and on other genetic and environmental factors, it provides a good indicator of the progression of the disease. As the novel forms of treatment being developed may reverse the hypertrophy of the lysosomal system, biochemical markers could also be used to monitor the reversal of pathology and the efficacy of treatment.

Quantification of mRNAs encoding proteins of the glycosphingolipid catabolism in mouse models of GM2 gangliosidoses and sphingolipid activator protein precursor (prosaposin) deficiency

We have investigated the mRNA amounts of six lysosomal proteins (beta-hexosaminidase alpha- and beta-subunit, sphingolipid activator protein precursor, GM2 activator protein, lysosomal sialidase, beta-glucocerebrosidase) involved in the degradation of glycosphingolipids. We analyzed extracts from brain tissues of mouse models for lysosomal storage diseases, i.e., the GM2 gangliosidoses and the deficiency of the sphingolipid activator protein precursor (prosaposin). The mRNA levels were quantified by real-time reverse transcription-polymerase chain reaction. Although storage of the respective lysosomal proteins has been reported in human and mice, no increase of their mRNA amounts could be detected here. Our results indicate that there is no transcriptional upregulation of lysosomal proteins in the examined neuronal storage disorders.

In mouse alpha -methylacyl-CoA racemase, the same gene product is simultaneously located in mitochondria and peroxisomes

alpha-Methylacyl-CoA racemase, an enzyme of the bile acid biosynthesis and branched chain fatty acid degradation pathway, was studied at the protein, cDNA, and genomic levels in mouse liver. Immunoelectron microscopy and subcellular fractionation located racemase to mitochondria and peroxisomes. The enzymes were purified from both organelles with immunoaffinity chromatography. The isolated proteins were of the same size, with identical N-terminal amino acid sequences, and the existence of additional proteins with alpha-methylacyl-CoA racemase activity was excluded. A racemase gene of about 15 kilobases was isolated. Southern blot analysis and chromosomal localization showed that only one racemase gene is present, on chromosome 15, region 15B1. The putative initial ATG in the racemase gene was preceded by a functional promotor as shown with the luciferase reporter gene assay. The corresponding cDNAs were isolated from rat and mouse liver. The recombinant rat protein was overexpressed in active form in Pichia pastoris. The presented data suggest that the polypeptide encoded by the racemase gene can alternatively be targeted to peroxisomes or mitochondria without modifications. It is concluded that the noncleavable N-terminal sequence of the polypeptide acts as a weak mitochondrial and that the C-terminal sequence acts as a peroxisomal targeting signal.

Characterization of regulatory elements in the 5'-flanking region of the GM2 activator gene

Lysosomal degradation of the ganglioside GM2 by human beta-hexosaminidase A requires the presence of the GM2 activator protein as an essential cofactor. Here we demonstrate that GM2 activator mRNA is differentially expressed and mainly localized to the apical part of the epithelial cells of distal renal tubules and the collecting duct. In order to understand the mechanism underlying the regulation of the GM2 activator gene, we analyzed the genomic organization upstream exon 2 as well as the 5'-flanking region. The GM2 activator gene spans about 16.8 kb with a first intron of 6.5 kb, and the transcription start is located at position -96 upstream from the ATG. DNA elements responsible for GM2 activator expression were identified in a PCR-based method of long-distance DNA walking. Sequence analysis revealed a 2.9 kb region upstream of the ATG that contained regulatory elements like CAAT boxes, Sp1 binding sites as well as AP1, and AP2 sites. Transfection experiments in COS-1 cells with a series of chimeras of 5'-stepwise deletion mutants of the GM2 activator gene 5'-flanking region and the secretory alkaline phosphatase (SEAP)-reporter gene indicated that a genomic fragment encompassing -323 to +1 bp had significant promoter activity. EMSA experiments showed that Sp1 and other transcription factors like AP1, AP2 and CCAAT-Box binding proteins are involved in GM2 activator gene regulation.

Tay-Sachs disease carrier screening: a model for prevention of genetic disease

Tay-Sachs disease (TSD) is an autosomal-recessive, progressive, and ultimately fatal neurodegenerative disorder. Within the last 30 years, the discovery of the enzymatic basis of the disease, namely deficiency of the enzyme hexosaminidase A, made possible both enzymatic diagnosis of TSD and heterozygote identification. In the last decade, the cloning of the HEXA gene and the identification of more than 80 associated TSD-causing mutations has permitted molecular diagnosis in many instances. TSD was the first genetic condition for which community-based screening for carrier detection was implemented. As such, the TSD experience can be viewed as a prototypic effort for public education, carrier testing, and reproductive counseling for avoiding fatal childhood disease. More importantly, the outcome of TSD screening over the last 28 years offers convincing evidence that such an effort can dramatically reduce incidence of the disease.

Location of ganglioside GM2 activator protein gene expression in sheep

1. The present study was aimed at characterizing and establishing the site of production of a 'novel' protein isolated in 1988 during the course of studies on sheep renal morphology. This protein has subsequently been identified as the GM2 activator protein (GM2AP). 2. The 'novel' protein, with an apparent molecular weight of 18-22 kDa and a pI between 4.7 and 4.9, was isolated from enriched granular fractions of sheep kidney cortex using two-dimensional (2-D) polyacrylamide gel electrophoresis. Following electroelution, the N-terminal amino acid sequence was determined and, applying the preferred codon usage formula, an oligodeoxyribonucleotide probe was constructed for examination of sites of expression of this novel protein using northern analyses and hybridization histochemistry. 3. Western blots of the 2-D gels onto nitrocellulose membranes permitted us to select the appropriate spots for injection into rabbits for production of polyclonal antibodies. The antibodies were used to confirm the sites of protein production using immunohistochemistry. 4. Northern analyses revealed that GM2AP mRNA has a widespread distribution in ovine tissues. In the kidney, GM2 was expressed in all major renal arteries and arterioles. In the liver, the expression of the gene was prominent in the hepatic vein and ducts. Antibodies raised against the GM2AP confirmed that the protein was present at the same sites as the mRNA. 5. These are the first studies showing the location of GM2 activator gene expression in normal mammalian tissues. The arterial site of production has implications for local action or an important role in membrane integrity throughout the kidney.

The GM2 activator protein, its roles as a co-factor in GM2 hydrolysis and as a general glycolipid transport protein

Although there is only one documented function carried out by the GM2 activator protein in the lysosome, new information suggests that other less obvious roles may also be played by this protein in vivo. This information includes data demonstrating that the GM2 activator is a secretory, as well as a lysosomal protein, and that cells possess a carbohydrate-independent mechanism to re-capture the activator, with or without bound lipid, from the extracellular fluid. Additionally the GM2 activator has been shown to bind, solubilize and transport a broad spectrum of lipid molecules, such as glycolipids, gangliosides and at least one phosphoacylglycerol, between liposomes. At pH 7 the GM2 activator's rate of lipid transport is reduced by only 50% from its maximum rate which is achieved at approx. pH 5, suggesting that the GM2 activator may serve as a general intra- and/or inter-cellular lipid transport protein in vivo. Since the late 1970s the lysosomal form of the GM2 activator has been known to act as a substrate-specific co-factor for the hydrolysis of GM2 ganglioside by beta-hexosaminidase A. Gangliosides are a class of negatively charged glycolipids particularly abundant in neuronal cells which have been linked to numerous in vivo functions, such as memory formation and signal transduction events. Deficiency of the GM2 activator protein results in the storage of GM2 ganglioside and severe neurological disease, the AB-variant form of GM2 gangliosidosis, usually culminating in death before the age of 4 years. The exact mode-of-action of the GM2 activator in its role as a co-factor, and its specificity for various glycolipids are currently matters of debate in the literature.

Stereochemistry of peroxisomal and mitochondrial beta-oxidation of alpha-methylacyl-CoAs

The stereochemistry of beta-oxidation of alpha-methyl-branched fatty acids was analyzed, in rat liver and in human cells, with (2R)- and (2S)-2-methyltetradecanoic acid as model substrates. In rat liver, formation of the alpha,beta-unsaturated compound was found to be concentrated in mitochondria while in human cells, this activity co-distributed mainly with peroxisomal marker enzymes. In both cases, the dehydrogenating enzymes were absolutely specific for the (2S)-enantiomer. In human liver, activation was some three times faster with the (2R)- than with the (2S)-isomer while in rat liver both were activated at about the same rate.

Purification and characterization of an alpha-methylacyl-CoA racemase from human liver

A specific racemase for alpha-methylacyl-CoAs, which had previously been studied in rat liver [W. Schmitz, R. Fingerhut, E. Conzelmann (1994) Eur. J. Biochem. 222, 313-323], has now been demonstrated also in human tissues. The human enzyme cross-reacts with a polyclonal antiserum against the rat liver racemase. The racemase was purified from human liver some 3600-fold. It is a monomer of 47 kDa with an isolectric point of pH 6.1 and is optimally active between pH 7-8. It acts only on coenzyme A thioesters, not on free fatty acids, and accepts as substrates a wide range of alpha-methylacyl-CoAs, including pristanoyl-CoA and trihydroxycoprostanoyl-CoA (an intermediate in bile acid synthesis), but neither 3-methyl-branched nor linear-chain acyl-CoAs. A clear difference in subcellular localization of the enzyme was found between humans and rats: the rat enzyme co-distributed exclusively with mitochondrial marker enzymes whereas in human cells, only 10-30% of the activity was found in mitochondria, the bulk activity was located in peroxisomes. Cells from patients with general deficiency of peroxisome assembly (Zellweger syndrome) showed strongly reduced racemase activity, with only the mitochondrial share being present while the peroxisomal form was absent.

Identification of a lysosomal protein causing lipid transfer, using a fluorescence assay designed to monitor membrane fusion between rat liver endosomes and lysosomes

In the present and previous studies [Mullock, Perez, Kuwana, Gray and Luzio (1994) J. Cell Biol. 126, 1173-1182], we have attempted to investigate endosome-lysosome fusion using an assay based on the dilution of the self-quenching fluorescent lipid probe octadecylrhodamine. Although some characteristics of fluorescence dequenching were consistent with those observed in other cell-free assays, we have now demonstrated that increased fluorescence was due to leakage of an intralysosomal lipid-transfer protein. This protein was purified and found to be a 22 kDa molecule with sequence, immunological and functional characteristics strongly suggesting that it is the rat homologue of human GM2-activator protein. Both the 22 kDa protein and recombinant human GM2-activator protein caused fluorescence dequenching either when mixed with octadecylrhodamine-loaded endosomes and lysosomal membranes or in a liposome system. The data were consistent with GM2-activator protein acting as an octadecylrhodamine-transfer protein. Antibodies to the 22 kDa protein added to cell-free endosome-lysosome content-mixing assays had no effect, although they could inhibit fluorescence dequenching caused by the protein. Thus this protein is not required in any fusion event involved in delivery of ligands from endosomes to lysosomes. The existence within an intracellular organelle of a protein capable of acting as an octadecylrhodamine-transfer protein suggests the need for caution in the interpretation of fluorescence-dequenching assays using mammalian subcellular fractions.

GM2 ganglioside activator occurs in multiple forms

The protein which activates the hydrolysis of GM2 ganglioside by hexosaminidase A was purified from human kidney. The GM2 activator had a molecular mass of 28 kDa by gel filtration and was resolved into three major bands using polyacrylamide gel electrophoresis in the presence of SDS with molecular masses of 23, 22 and 21 kDa. These three bands corresponded respectively to strongly binding, weakly binding and non-binding fractions of GM2 activator chromatographed through concanavalin A-Sepharose, indicating that GM2 activator exists in multiple glycosylated forms.

Prenatal diagnosis of GM2-gangliosidosis. Immunofluorescence analysis of ganglioside GM2 in cultured amniocytes by confocal laser scanning microscopy

A confocal laser scanning microscopic system was used to detect the storage of ganglioside GM2 in Tay-Sachs fibroblasts and amniocytes. The diagnosis of the disease was confirmed by counting immunoreactive cells or by digital imaging analysis. This novel system facilitates the prenatal diagnosis of GM2-gangliosidosis.

Degradation of gangliosides by the lysosomal sialidase requires an activator protein

Lysosomal sialidase, which was formerly believed to degrade only water-soluble substrates but not glycolipids, cleaves ganglioside substrates II3NeuNAc-LacCer, IV3NeuNAc, II3NeuNAc-GgOse4Cer, IV3 NeuNAc, II3(NeuNAc)2-GgOse4Cer when these are dispersed either with an appropriate detergent (taurodeoxycholate) or with the sulfatide activator protein, a physiologic lipid solubilizer required for the lysosomal hydrolysis of other glycolipids by water-soluble hydrolases. In the presence of the activator protein, time and protein dependence were linear within wide limits, while the detergent rapidly inactivated the enzyme. The disialo group of the b-series gangliosides was only poorly attacked by the enzyme when the lipids were dispersed with the activator protein, whereas in the presence of the detergent, they were hydrolyzed as fast as terminal sialic acid residues. With the appropriate assay method, significant ganglioside sialidase activity could be demonstrated in the secondary lysosome fraction of normal skin fibroblasts but not of sialidosis fibroblasts. Our results support the notion that there is only one lysosomal sialidase, which degrades both the water-soluble and the membrane-bound sialyl glycoconjugates.

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.

Quantitative correlation between the residual activity of β-hexosaminidase A and arylsulfatase A and the severity of the resulting lysosomal storage disease

A previously suggested model for the correlation between residual activity of a lysosomal enzyme and the turnover rate of its substrate(s) has been extended to a discussion of substrate accumulation rates in individual cells and whole organs. With these considerations, much of the observed variability in age of onset and clinical phenotype, as well as the phenomenon of pseudo-deficiency, can be understood as the consequences of small differences in the residual activity of the affected enzyme. In order to experimentally verify the basic assumptions on which this model rests, studies were performed in cell culture. The radiolabeled substrates ganglioside GM2 and sulfatide were added to cultures of skin fibroblasts with different activities of beta-hexosaminidase A or arylsulfatase A, respectively, and their uptake and turnover measured. In both series of experiments, the correlation between residual enzyme activity and the turnover rate of the substrate was essentially as predicted: degradation increased steeply with residual activity, to reach the control level at a residual activity of approximately 10-15% of normal. All cells with an activity above this critical threshold had a normal turnover. Comparison of the results of these feeding studies with the clinical status of the donor of each cell line basically confirmed our notions but also revealed the limitations of the cell culture approach.

Isolation and expression of a full-length cDNA encoding the human GM2 activator protein

We report the construction of a cDNA clone encoding a functional GM2-activator protein. The sequence of the complete 5' end of the coding region was determined by direct nucleotide sequencing of a fragment generated by multiple RACE PCR procedures from Hela cell cDNA. Specific oligonucleotides were synthesized from these data which allowed us to produce a PCR fragment that contained the complete coding sequence of the protein. This was then cloned into a mammalian expression vector. The ability of purified hexosaminidase A (beta-N-acetylhexosaminidase, EC 3.2.1.52) to hydrolyse labeled GM2 ganglioside was enhanced 10-fold more by the addition in the assay mix of lysate from transfected COS-1 cells than by the addition of identical amounts of lysate from mock transfected cells. Direct sequencing of PCR fragments from two sources also identified three polymorphisms.

[Sphingolipid storage diseases of the central nervous system: bases of biochemical and clinical heterogeneity]

Lysosomal storage disorders are progredient and often fatal diseases most of which result from a pronounced enzyme deficiency. In the case of sphingolipidoses, usually enzymes of sphingolipid catabolism are missing, or only a few percent of normal activity are detectable. For many sphingolipidoses, damage of the central nervous system is characteristic, but neurological and other symptoms can vary greatly, especially in adult variants. This variability is mainly caused by different allelic mutations of the structural genes, resulting in different levels of residual enzyme activity.

Identity of the activator proteins for the enzymatic hydrolysis of sulfatide, ganglioside GM1, and globotriaosylceramide

The activator protein for the enzymatic hydrolysis of sulfatide, ganglioside GM1, and globotriaosylceramide was purified from human kidney, brain, and urine. As far as they could be assayed, these three activities cochromatographed during all steps, indicating that they are due to the same protein. This result was corroborated by immunochemical comparison of individually purified activator preparations. In contrast, the activator for ganglioside GM2 hydrolysis could clearly be separated from the other activities. Kinetic data were determined for the interaction of the sulfatide activator with the different glycolipids and hydrolases.

Metabolism of ganglioside-amides in cultured human fibroblasts

Metabolism of [3H]ganglioside derivatives GM3-amide and GM2-amide has been investigated in normal human skin fibroblasts. In a cell-free system the ganglioside analogues have been shown to enter biosynthetic pathways, their degradation, however, was curtailed at an early stage, as GM3-amide could not be hydrolysed by sialidase action. GM2-amide was susceptible to beta-hexosaminidase degradation yielding GM3-amide. When incorporated into fibroblasts [3H]GM2-amide was degraded to [3H]GM3-amide presumably in the lysosomes, and at the same time glycosylation to [3H]GD1a-monoamide took place most likely in the Golgi apparatus. [3H]GM3-amide, however, did not seem to reach the glycosylation sites in the Golgi apparatus. It could be detected in the lysosomes, where it was not degraded due to its sialidase resistance. From these results we conclude that in cells exogenously administered [3H]GM3-amide and [3H]GM2-amide both are directed to the lysosomes and that [3H]GM2-amide also reaches the Golgi apparatus. The synthesis of higher [3H]ganglioside-amides from incorporated [3H]GM2-amide can occur by direct glycosylation. [3H]GM3-amide, however, even if it reaches the Golgi compartment, does not enter the biosynthetic pathway.

Isocitrate lyase of the alga Chlorogonium elongatum: Immunological quantification in relation to growth conditions

Isocitrate lyase (EC 4.1.3.1) from the acetate flagellate Chlorogonium elongatum was studied in relation to growth conditions. In cells growing heterotrophically or photoheterotrophically with acetate (2 g · l(-1)) specific activities of 0.453 and 0.335 μmol glyoxylate · min(-1) · mg(-1) protein, respectively, were found in the cell-free extracts. The malate synthase activity of heterotrophically grown Chlorogonium cells was found to be 0.572 nmol · min(-1) · mg(-1) protein. In the presence of inhibitors acting on transcription and translation there was no increase in isocitrate lyase activity; this would indicate that the enzyme was synthesized de novo in response to changing conditions in the surroundings. Using an enzyme immunoassay it was shown that the activity and amount of enzyme rose concomitantly when the cells were supplied with acetate, irrespective of whether they had been cultivated photoheterotrophically or heterotrophically on acetate.

A radiometric assay for ganglioside sialidase applied to the determination of the enzyme subcellular location in cultured human fibroblasts

A radiometric method for the assay of ganglioside sialidase in cultured human fibroblasts was set up. As substrate, highly radioactive (1.28 Ci/mmol) ganglioside GDla isotopically tritium-labeled at carbon C-3 of the long chain base was employed; the liberated, and TLC separated [3H]GM1 was determined by computer-assisted radiochromatoscanning. Under experimental conditions that provided a low and quite acceptable (4-5%) coefficient of variation, the detection limit of the method was 0.1 nmol of liberated GM1, using as low as 10 micrograms of fibroblast homogenate as protein. The detection limit could be lowered to 0.02-0.03 nmol, adopting conditions that, however, carried a higher analytical error (coefficient of variation over 10%). The content of ganglioside sialidase in human fibroblasts cultured in 75-cm2 plastic flasks was 5.8 +/- 2.5 (SD) nmol liberated GM1 h-1 mg protein-1. Subfractionation studies performed on fibroblast homogenate showed that the ganglioside sialidase was mainly associated with the light membrane subfraction that was rich in plasma and intracellular membranes. This subfraction displayed almost no sialidase activity on the artificial substrate 4-methylumbelliferyl-D-N-acetylneuraminic acid. A small but measurable ganglioside sialidase activity was also present in the lysosome-enriched subfraction, which contained a very high sialidase activity on the above artificial substrate. All this supports the hypothesis that human fibroblasts contain sialidases with different subcellular location and substrate specificity. Particularly, the sialidase acting on gangliosides seems to have two sites of subcellular location, a major one at the level of plasma membranes and/or intracellular organelles functionally related with the plasma membranes and a minor one in the lysosomes.

Molecular forms of GM2-activator protein. A study on its biosynthesis in human skin fibroblasts

The biosynthesis and secretion of lysosomal GM2-activator was studied in fibroblasts from controls and patients of GM2 gangliosidosis metabolically labelled with [3H]-leucine. Immunoprecipitation was performed with affinity-purified antibodies to human kidney GM2-activator protein. Normal fibroblasts and fibroblasts of variant B and O of GM2 gangliosidosis secrete GM2-activator protein as a 24-kDa polypeptide, which is able to stimulate degradation of ganglioside GM2 by beta-hexosaminidase A in the in vitro assay. In the presence of 10mM NH4Cl the rate of secretion is twice as high as in normal fibroblasts. Intracellularly, GM2-activator protein is represented in these cell lines by polypeptides with apparent molecular masses ranging from 21 kDa-22.5 kDa. Under the same labelling conditions, in two cell lines of patients with variant AB of infantile GM2 gangliosidosis intracellularly only traces of GM2-activator were detectable, whereas significant amounts of polypeptides with molecular masses between 25 and 26.5 kDa could be precipitated from the media of these fibroblasts.

Glycolipid-binding proteins

Proteins which bind glycolipids with high specificity are tentatively divided into two groups. One group consists of activator proteins involved in the catabolism of glycolipids by acid lysosomal hydrolases. Two activator proteins, GM2-activator and sphingolipid activator protein-1, are critically appraised on their glycolipid-binding properties and on their activity to facilitate the transfer of glycolipids. These proteins are glycoproteins localized in the lysosomes. Their molecular weights are in a range of 21 000-27 000, and isoelectric points are 4-5. Glycolipid transfer protein (GLTP) is included in the other group. GLTP purified from pig brain has a molecular weight of about 20 000 and an isoelectric point of 8.3. GLTP facilitates the transfer of various glycosphingolipids and glyceroglycolipids between membranes. The protein does not facilitate the transfer of phospholipids or cholesterol. GLTP binds galactosylceramide. The galactosylceramide-GLTP complex participates in the transfer reaction as the intermediate. Each protein in both groups binds glycolipids with a characteristic specificity to the sugar moiety. A stoichiometry of 1 mol of lipid per mol of protein has been found in all three proteins. Proteins in both groups seem to have a hydrophobic region on their surface, since all three proteins have been efficiently purified by hydrophobic chromatography.

Incorporation and metabolism of ganglioside GM2 in skin fibroblasts from normal and GM2 gangliosidosis subjects

Ganglioside GM2, 3H-labeled in the sphingoid base, was added to the culture medium of normal and GM2 gangliosidosis fibroblasts. Ganglioside was found to adsorb rapidly to the cell surface, most of it could however be removed by trypsination. The trypsin-resistant incorporation was about 10 nmol/mg cell protein, after 48 h. The rates of adsorption and incorporation depended strongly on the concentration of fetal calf serum in the medium, higher serum concentrations being inhibitory. After various incubation times, the lipids were extracted, separated by thin-layer chromatography and visualized by fluorography. In normal cells a variety of degradation products as well as sphingomyelin was found whereas in GM2 gangliosidosis cells, only trace amounts of such products (mainly GA2) were found. In contrast, the higher gangliosides GM1 and GD1a were formed in comparable amounts (2.2-3.6% of total radioactivity after 92 h) in normal and pathologic cell lines. Supplementation of cells from GM2 gangliosidosis, variant AB, with purified GM2-activator protein restored ganglioside GM2 degradation to almost normal rates but had no effect on its glycosylation to gangliosides GM1 and GD1a. From these results we conclude that the synthesis of higher gangliosides from incorporated GM2 can occur by direct glycosylation and not only via lysosomal degradation and resynthesis from [3H]sphinganine-containing degradation products. Preliminary studies with subcellular fractionation after various times of [3H]ganglioside incorporation indicated biphasic kinetics for the net transport of membrane-inserted ganglioside to lysosomes, compatible with the notion that a portion of the glycolipids can also escape from secondary lysosomes and migrate to Golgi compartment or cell surface.

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.

Evidence for the presence of a ganglioside transfer protein in brain

An extract from rat brain has been shown to catalyze the transfer of ganglioside GM1 from sonicated vesicles to erythrocyte ghosts. It also enhanced the transfer of GM1 to a crude neuronal membrane preparation, whereas myelin took up only a very limited amount. The transfer activity was heat-labile. Similar transfer activities were found in extracts from bovine gray and white matter, that of the former being comparable to rat brain whereas the latter was greater per milligram protein.

Mapping of the gene coding for the human GM2 activator protein to chromosome 5

The gene coding for the GM2 activator protein has been mapped to human chromosome 5, using an enzyme-linked immunoadsorbent assay (ELISA) to identify the human protein in human-mouse somatic cell hybrids.