Juvenile GM2 gangliosidosis (AMB variant): inability to activate hexosaminidase A by activator protein.

Two sibling from a consanguineous Puerto Rican marriage were found to have a juvenile-onset type of lipidosis first noted at age 2 1/2 by expressing difficulties with motor function and developmental delay. They continued to deteriorate, showing muscle atrophy, spasticity, and loss of speech, and death occurred at ages 7 and 8. Examination of the brains from these patients revealed that the concentration of GM2 ganglioside was about 56% of the total gangliosides. Hexosaminidase and percent hexosaminidase A (HEX A) and other lysosomal enzymes were normal in cultured skin fibroblasts, liver, and brain. The concentration of the activator protein required for the enzymatic hydrolysis of GM2 ganglioside was in high normal levels in the brain of the patient available. However, the HEX A from the patient's brain and liver as well as from skin fibroblast lysates could not be activated to hydrolyze GM2 ganglioside by the activator protein from a control or himself. The HEX A from a control could be activated by the activator protein from controls or this patient. These patients appear to have a defect in HEX A, which does not affect it heat stability, electrophoretic migration, and activity toward fluorogenic substrates, but may affect the binding of the activator protein required for GM2 ganglioside hydrolysis. We propose to call these patients the AMB variant of GM2 gangliosidosis to denote the mutation in HEX A but with normal levels of HEX A and B with synthetic substrates. This is to distinguish these patients from those missing the activator protein and normal HEX A and B levels.     

The Protein Activator Specific for the Enzymic Hydrolysis of GM2 Ganglioside in Normal Human Brain and Brains of Three Types of GM2 Gangliosidosis

Abstract: In order to understand the etiology of Type AB GM2 gangliosidosis, we have purified and characterized the activator protein (GM2 activator) specific for the enzymic hydrolysis of GM₂ ganglioside from normal human brain. The purified activator from human brain moved as one major protein band in various electrophoretic systems. We have also prepared the antiserum against this activator. The levels and the nature of GM₂ activator and β-hexosaminidase A were examined in the brains of five cases of GM₂. gangliosidosis—one Type B, two Type O, and two Type AB. We found that the levels of GM₂ activator in both Type B and Type O cases were markedly elevated, and that the two Type AB cases were the results of different causes. One case had a defective β hexosaminidase A and an elevated level of GM₂ activator. Although this defective β-hexosaminidase A could hydrolyze synthetic substrates, it was inactive in the cleavage of natural glycosphingolipids in the presence of the GM₂ activator. It could, however, hydrolyze asialo-GM₂ and GbOse₄Cer in the presence of sodium taurodeoxycholate. The other case had normal β-hexosaminidase A, but had a very low level of GM₂ activator when analyzed by in vitro assay, suggesting the deficiency of this activator. By immunoelectrophoresis, this case was found to be completely devoid of the protein that cross-reacts with the antiserum against the GM₂ activator.     

Properties of a protein activator of glycosphingolipid hydrolysis isolated from the liver of a patient with GM1 gangliosidosis,Type 1

The heat stable protein activator of GM1 ganglioside hydrolysis was isolated from the liver of a patient with GM1 gangliosidosis, Type 1. It was found to be present at a level about 35 times that found in a liver sample from an age matched control. This activator protein was demonstrated to stimulate the hydrolysis of GM1 ganglioside and GA1 (asialo-GM1 ganglioside) in the presence of purified GM1 ganglioside β-galactosidase without the need for bile salt detergents. It could not stimulate the hydrolysis of two other galactosphingolipids, galactosylceramide and lactosylceramide, in the presence of the same enzyme. Lactosylceramide was a good substrate for this enzyme when sodium glycodeoxycholate was included in the assay. This activator protein had two isoelectric points pH 4.1 and 4.6, and it had an apparent molecular weight of 27,000 by gel filtration.     

Biochemical Basis of Type AB GM2 Gangliosidosis in a Japanese Spaniel

The biochemical basis of a case of GM2 gangliosidosis in a Japanese Spaniel was studied. This dog had a massive accumulation of GM2 ganglioside in the brain. The beta-hexosaminidase activity in this affected dog brain was approximately 12 times higher than that of normal brain. However, the activity toward p-nitrophenyl-6-sulfo-2-acetamido-2-deoxyglucopyranoside was only four times higher in the affected brain than in normal brain. The GM2 activator preparation obtained from the normal dog brain could stimulate the hydrolysis of GM2 ganglioside by beta-hexosaminidase isolated from the affected dog. However, the corresponding activator fraction from the affected dog could not stimulate such a reaction. It was concluded that the biochemical basis of the GM2 gangliosidosis in this Japanese Spaniel was due to the attenuation in the stimulatory activity of GM2 activator. This case represents the first animal form similar to the activator deficiency (or defect) of Type AB GM2 gangliosidosis in humans.     

Activator protein required for the enzymatic hydrolysis of cerebroside sulfate. Deficiency in urine of patients affected with cerebroside sulfatase activator deficiency and identity with activators for the enzymatic hydrolysis of GM1 ganglioside and globotriaosylceramide

Urine specimens from two sibs affected with cerebroside sulfatase activator deficiency were examined to ascertain whether the deficiency of the supplementary activator protein required for the enzymatic hydrolysis of cerebroside sulfate was also evident in urine. Material from chromatographic fractionations was examined for the activator activity to avoid ambiguities resulting from protein inhibition. There were substantial deficits in all chromatographic fractions corresponding to activator-containing fractions of control urines. Since patient urines contained elevated amounts of lactosylceramide, digalactosylceramide, and globotriaosylceramide and since similarities between activators for cerebroside sulfate and GM1 ganglioside hydrolyses had been noted previously, the chromatographic fractions were also examined for activators in other glycosphingolipid hydrolase systems. There was coincidence of activators for the GM1 ganglioside/beta-galactosidase and the globotriaosylceramide/alpha-galactosidase A reactions with the cerebroside sulfatase activator in control urine fractions, and the patients' urines were deficient in activator activities for the three reactions. Identity of the three activators was suggested and antiserum to purified GM1 ganglioside activator was used to test this possibility. There were depressed levels of cross-reacting material in fractions of patient urines by Ouchterlony double diffusion and in unfractionated urine by enzyme-linked immunosorbent assay. Purified activators for the cerebroside sulfate and GM1 ganglioside systems showed lines of identity with no spurring on Ouchterlony double diffusion, identical mobility on immunoelectrophoresis, and similar stimulatory activities toward hydrolysis of the three glycosphingolipid species by their respective enzymes. Finally, the three activator activities were retained by anti-GM1-activator IgG coupled to Sepharose 4B. The results suggest strongly that the same protein entity serves as activator for the enzymatic hydrolysis of cerebroside sulfate, GM1 ganglioside, and globotriaosylceramide.     

The human GM2 activator protein. A substrate specific cofactor of beta-hexosaminidase A.

Ganglioside GD1a-GalNAc was isolated from Tay-Sachs brain, tritium-labeled in its sphingosine moiety, and its enzymic degradation studied in vitro and in cultured fibroblasts. When offered as micelles, GD1a-GalNAc was almost not hydrolyzed by Hex A or Hex B, while after incorporation of the ganglioside into the outer leaflet of liposomes, the terminal GalNAc residue was rapidly split off by Hex a. In striking contrast to ganglioside GM2, the major glycolipid substrate of Hex A, the enzymic hydrolysis of GD1a-GalNAc was not promoted by the GM2 activator protein, although the activator protein did bind GD1a-GalNAc to form a water-soluble complex. Pathobiochemical studies corroborate these results. After incorporation of [3H]GD1a-GalNAc into cultured skin fibroblasts from healthy subjects and from patients with different variants of GM2 gangliosidosis, its degradation was found to be strongly attenuated in mutant cells with Hex A deficiencies such as variant B (Tay-Sachs disease), variant B1 and variant 0 (Sandhoff disease), while in cells with variant AB (GM2 activator deficiency), its catabolism was blocked only at the level of GM2. In line with these metabolic studies, a normal content of GD1a-GalNAc was found in brains of patients who had succumbed to variant AB of GM2 gangliosidosis whereas in brains from variants B, B1, and 0, its concentration was considerably elevated (up to 19-fold). Together with studies on the enzymic degradation of GM2 derivatives with modifications in the ceramide portion, these results indicate that mainly steric hindrance by adjacent lipid molecules impedes the access of Hex A to membrane-bound GM2 (whose degradation therefore depends on solubilization by the GM2 activator) and in addition that the interaction between the GM2. GM2 activator complex and the enzyme must be highly specific.     

Occurrence of an anomalous endocytic compartment in fibroblasts from Sandhoff disease patients

Sandhoff disease (SD) is a lysosomal storage disorder due to mutations in the gene encoding for the β-subunit of β-hexosaminidase, that result in β-hexosaminidase A (αβ) and β-hexosaminidase B (ββ) deficiency. This leads to the storage of GM2 ganglioside in endosomes and lysosomes, which ends in a progressive neurodegeneration. Currently, very little is known about the biochemical pathways leading from GM2 ganglioside accumulation to pathogenesis. Defects in transport and sorting by the endosomal–lysosomal system have been described for several lysosomal storage disorders. Here, we have investigated the endosomal–lysosomal compartment in fibroblasts from SD patients and observed that both late endosomes and lysosomes, but not early endosomes, have a higher density in comparison with normal fibroblasts. Moreover, Sandhoff fibroblasts have an intracellular distribution of terminal endocytic organelles that differs from the characteristic perinuclear punctate pattern observed in normal fibroblasts and endocytic vesicles also appear larger. These findings reveal the occurrence of an alteration in the terminal endocytic organelles of Sandhoff fibroblasts, suggesting an involvement of this compartment in the disruption of cell metabolic and signalling pathways and in the onset of the pathological state.     

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.     

Evidence for two different active sites on human beta-hexosaminidase A. Interaction of GM2 activator protein with beta-hexosaminidase A

Competition experiments were carried out on the hydrolysis of different substrates by beta-hexosaminidase A isolated from human liver. The results show that ganglioside GM2 in the presence of the GM2 activator protein and a new synthetic substrate, 4-methylumbelliferyl-beta-N-acetylglucosaminide 6-sulfate, are hydrolyzed at the same active site on the alpha subunit of beta-hexosaminidase A, whereas 4-methylumbelliferyl-beta-N-acetylglucosaminide is degraded predominantly by a different active site on the beta-subunit. This finding provides for the first time a possible molecular basis for the observation that, in variant B1 of the GM2 gangliosidoses, beta-hexosaminidase A has lost its activity toward GM2 ganglioside and the sulfated artificial substrate while being still able to hydrolyze the unsulfated artificial substrate at a normal rate. Furthermore, the finding that the GM2 activator protein inhibits the degradation of the sulfated substrate by beta-hexosaminidases A and S indicates that the alpha subunit common to both isoenzymes might provide a binding site for the activator protein.     

Variant of GM2-gangliosidosis with hexosaminidase A having a severely changed substrate specificity

The levels of hexosaminidase A activity in cultivated fibroblasts of two patients with GM2-gangliosidosis were close to the normal range with 4-methylumbelliferyl-beta-D-2-acetamido-2-deoxyglucopyranoside and 4-methylumbelliferyl-beta-D-2-acetamido-2-deoxygalactopyranoside as substrates, and the enzymes were normal in most parameters analyzed. However, the enzymes of both patients were almost completely inactive against two specific substrates for hexosaminidase A, rho-nitrophenyl-6-sulfo-2-acetamido-2-deoxy-beta-D-glucopyranoside, and ganglioside GM2 in the presence of GM2-activator. Fibroblast extracts of both patients showed normal hexosaminidase B and GM2-activator activity, the latter was strongly decreased in two cases with variant AB. It is suggested that human hexosaminidase A may contain two different active sites which might be inactivated separately by different mutations.     

Combined replacement effects of human modified β-hexosaminidase B and GM2 activator protein on GM2 gangliosidoses fibroblasts

GM2 gangliosidoses are autosomal recessive lysosomal storage diseases (LSDs) caused by mutations in the HEXA, HEXB and GM2A genes, which encode the human lysosomal β-hexosaminidase (Hex) α- and β-subunits, and GM2 activator protein (GM2A), respectively. These diseases are associated with excessive accumulation of GM2 ganglioside (GM2) in the brains of patients with neurological symptoms. Here we established a CHO cell line overexpressing human GM2A, and purified GM2A from the conditioned medium, which was taken up by fibroblasts derived from a patient with GM2A deficiency, and had the therapeutic effects of reducing the GM2 accumulated in fibroblasts when added to the culture medium. We also demonstrated for the first time that recombinant GM2A could enhance the replacement effect of human modified HexB (modB) with GM2-degrading activity, which is composed of homodimeric altered β-subunits containing a partial amino acid sequence of the α-subunit, including the GSEP loop necessary for binding to GM2A, on reduction of the GM2 accumulated in fibroblasts derived from a patient with Tay-Sachs disease, a HexA (αβ heterodimer) deficiency, caused by HEXA mutations. We predicted the same manner of binding of GM2A to the GSEP loop located in the modified HexB β-subunit to that in the native HexA α-subunit on the basis of the x-ray crystal structures. These findings suggest the effectiveness of combinational replacement therapy involving the human modified HexB and GM2A for GM2 gangliosidoses.     

Activating proteins for ganglioside GM2 degradation by beta-hexosaminidase isoenzymes in tissue extracts from different species

The existence of activator proteins that stimulate hydrolysis of ganglioside GM2 by beta-hexosaminidase was demonstrated in kidney extracts from four species (rat, mouse, cattle and pig). The extent to which these preparations, as well as their human counterpart, promote ganglioside GM2 catabolism by autologous and heterologous hexosaminidase isoenzymes was compared. It was found that these activators can replace each other functionally, although the animal activator proteins do not cross-react immunochemically with an antiserum against the human protein. All preparations examined catalysed the transfer of ganglioside GM2 between liposomal membranes, indicating that the animal activator proteins act by a mechanism similar to the human GM2 activator.     

Degradation of membrane-bound ganglioside GM1. Stimulation by bis(monoacylglycero)phosphate and the activator proteins SAP-B and GM2-AP

According to our hypothesis (Fürst, W., and Sandhoff, K. (1992) Biochim. Biophys. Acta 1126, 1-16) glycosphingolipids of the plasma membrane are digested after endocytosis as components of intraendosomal and intralysosomal vesicles and membrane structures. The lysosomal degradation of glycosphingolipids with short oligosaccharide chains by acid exohydrolases requires small, non-enzymatic cofactors, called sphingolipid activator proteins (SAPs). A total of five activator proteins have been identified as follows: namely the saposins SAP-A, -B, -C, and -D, which are derived from the single chain SAP-precursor protein (prosaposin), and the GM2 activator protein. A deficiency of prosaposin results in the storage of ceramide and sphingolipids with short oligosaccharide head groups. The loss of the GM2 activator protein blocks the degradation of the ganglioside GM2. The enzymatic hydrolysis of the ganglioside GM1 is catalyzed by beta-galactosidase, a water-soluble acid exohydrolase. The lack of ganglioside GM1 accumulation in patients suffering from either prosaposin or GM2 activator protein deficiency has led to the hypothesis that SAPs are not needed for the hydrolysis of the ganglioside GM1 in vivo. In this study we demonstrate that an activator protein is required for the enzymatic degradation of membrane-bound ganglioside GM1 and that both SAP-B and the GM2 activator protein significantly enhance the degradation of the ganglioside GM1 by acid beta-galactosidase in a liposomal, detergent-free assay system. These findings offer a possible explanation for the observation that no storage of the ganglioside GM1 has been observed in patients with either isolated prosaposin or isolated GM2 activator deficiency. We also demonstrate that anionic phospholipids such as bis(monoacylglycero)phosphate and phosphatidylinositol, which specifically occur in inner membranes of endosomes and in lysosomes, are essential for the activator-stimulated hydrolysis of the ganglioside GM1. Assays utilizing surface plasmon resonance spectroscopy showed that bis(monoacylglycero)phosphate increases the binding of both beta-galactosidase and activator proteins to substrate-carrying membranes.     

In Cellulo Examination of a Beta-Alpha Hybrid Construct of Beta-Hexosaminidase A Subunits,Reported to Interact with the GM2 Activator Protein and Hydrolyze GM2 Ganglioside

The hydrolysis in lysosomes of GM2 ganglioside to GM3 ganglioside requires the correct synthesis, intracellular assembly and transport of three separate gene products; i.e., the alpha and beta subunits of heterodimeric beta-hexosaminidase A, E.C. # 3.2.1.52 (encoded by the HEXA and HEXB genes, respectively), and the GM2-activator protein (GM2AP, encoded by the GM2A gene). Mutations in any one of these genes can result in one of three neurodegenerative diseases collectively known as GM2 gangliosidosis (HEXA, Tay-Sachs disease, MIM # 272800; HEXB, Sandhoff disease, MIM # 268800; and GM2A, AB-variant form, MIM # 272750). Elements of both of the hexosaminidase A subunits are needed to productively interact with the GM2 ganglioside-GM2AP complex in the lysosome. Some of these elements have been predicted from the crystal structures of hexosaminidase and the activator. Recently a hybrid of the two subunits has been constructed and reported to be capable of forming homodimers that can perform this reaction in vivo, which could greatly simplify vector-mediated gene transfer approaches for Tay-Sachs or Sandhoff diseases. A cDNA encoding a hybrid hexosaminidase subunit capable of dimerizing and hydrolyzing GM2 ganglioside could be incorporated into a single vector, whereas packaging both subunits of hexosaminidase A into vectors, such as adeno-associated virus, would be impractical due to size constraints. In this report we examine the previously published hybrid construct (H1) and a new more extensive hybrid (H2), with our documented in cellulo (live cell- based) assay utilizing a fluorescent GM2 ganglioside derivative. Unfortunately when Tay-Sachs cells were transfected with either the H1 or H2 hybrid construct and then were fed the GM2 derivative, no significant increase in its turnover was detected. In vitro assays with the isolated H1 or H2 homodimers confirmed that neither was capable of human GM2AP-dependent hydrolysis of GM2 ganglioside.     

A mutation in the gene of a glycolipid-binding protein (GM2 activator) that causes GM2-gangliosidosis variant AB

GM2-gangliosidoses are neurological disorders caused by a genetic deficiency of either the β-hexosaminidase A or the GM2 activator, a glycolipid binding protein. In a patient with an immunologically proven GM2 activator protein deficiency, A T⁴¹² → C transition (counted from A of the initiation codon) was found in the coding sequence, which results in the substitution of Arg for the normal Cys¹⁰⁷ in the mature GM2 activator protein. The remainder of the coding sequence remained entirely normal.     

Preparation and enzymatic degradation of monosulfogangliotriaosylceramide

Gangliotriaosylceramide 3'-sulfate (GgOse3Cer-II3-sulfate) contains the sugar sequence similar to that of GM2 ganglioside except that the NeuAc in GM2 is replaced by a sulfate group. Due to this structural similarity, we have studied the in vitro synthesis of GgOse3Cer-II3-sulfate using the system for GM2. Our results showed that GgOse3Cer-II3-sulfate could be synthesized from lactosylceramide 3'-sulfate and UDP-GalNAc catalyzed by N-acetylgalactosaminyltransferase prepared from rat brain (Dicesare, J. L., and Dain, J. A. (1971) Biochim. Biophys. Acta 231, 385-393). As in the case of GM2, the GgOse3Cer-II3-sulfate biosynthesized in vitro or isolated from rat kidney could also be cleaved by human beta-hexosaminidase A in the presence of GM2-activator (Li, S.-C., Hirabayashi, Y., and Li, Y.-T. (1981) J. Biol. Chem. 256, 6234-6240). The fact that the GM2-activator could stimulate beta-hexosaminidase A to hydrolyze both GM2 and Gg-Ose3Cer-II3-sulfate indicates that these two glycolipids may be catabolyzed by the same mechanism.     

Isolation and characterization of ganglioside GM1b from normal human brain

A sialidase-susceptible monosialoganglioside was isolated from normal human brain by DEAE-Sephadex A-25 and Iatrobeads column chromatography. The yield of this ganglioside was about 6 mg per whole brain. Its structure was elucidated by sugar analysis, sialidase digestion, permethylation, and proton NMR studies. This ganglioside had carbohydrate, fatty acid, and long-chain base compositions identical to those of brain GM1a. However, the sialosyl residue was found to be linked (alpha 2-3) to the terminal galactosyl residue of the asialo-GM1a backbone. The complete structure of this ganglioside was therefore identified as GM1b or IV3NeuAcGgOse4Cer.     

Recombinant GM2-Activator Protein Stimulates In Vivo Degradation of GA2 in GM2 Gangliosidosis AB Variant Fibroblasts But Exhibits No Detectable Binding of GA2 in an In Vitro Assay

The interaction between glycosphingolipids and recombinant human GM2-activator was studied in a microwell binding assay. A-series gangliosides like GM3, GM2 and GM1 were strongly bound by the recombinant human GM2 activator. A weak binding was observed to GD1b and sulfatide, while neutral glycolipids were not bound. Optimal binding occurred at pH 4.2 and was inhibited by increasing concentrations of citrate buffer and NaCl. In contrast with these in vitro results the recombinant human GM2-activator is able to restore the degradation of GA2 in fibroblasts from patients with the AB variant of GM2 gangliosidosis in vivo.     

A monoclonal antibody recognizes an O-acylated sialic acid in a human melanoma-associated ganglioside

Monoclonal antibody D1.1 originally prepared against the B49 cell line derived from a rat brain tumor was shown to react with a ganglioside present in fetal rat brain. We have found that this antigen is also present in human malignant melanoma tumors as well as many melanoma cell lines. The ganglioside from human melanoma cell lines migrates between GM1 and GM2 on one-dimensional thin layer chromatography. Analysis by two-dimensional thin layer chromatography with intermediate ammonia treatment suggests that the ganglioside contains one or more base-labile O-acyl esters. Mild base hydrolysis under conditions known to remove O-acyl esters results in complete loss of antigenic reactivity. Thus, the alkali-labile moiety is a critical component of the epitope recognized by the antibody. Analysis of the sialic acids of total gangliosides from [6-3H]glucosamine-labeled melanoma cells showed that approximately 10% of these molecules are O-acylated. Similar analysis of the purified ganglioside showed that greater than 30% of the sialic acids comigrated with authentic 9-O-acetyl-N-acetylneuraminic acid. The antibody did not cross-react with normal human skin melanocytes nor with any of a large number of normal human adult and fetal tissues. The antibody also did not react with numerous other malignant cell lines studied. These findings suggest that the antigenic epitope defined by antibody D1.1 contains an O-acylated sialic acid and may arise from aberrant O-acetylation occurring in human malignant melanoma cells.     

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.