Lactosylceramide and non-Acidic
Oligoglycosylceramides

Non-acidic (neutral) di- and oligoglycosphingolipids, i.e., with two to many more carbohydrate moieties attached to a ceramide unit, are vital components of cellular membranes of most eukaryotic organisms and some bacteria. Of these, lactosylceramide (diglycosyl) is the key intermediate in the biosynthesis of more highly glycosylated sphingolipids in animals but has crucial functions of its own. Oligoglycosphingolipids possess a high degree of diversity in their structures and are amphipathic molecules because of the presence of the variable hydrophilic carbohydrate-rich head group with a hydrophobic ceramide tail. Although their abundance relative to other lipids is usually low other than in epithelial and neuronal cells, with the nature and proportions of the different glycolipid classes varying with the tissue and stage of growth, they are required for the viability of cells.

1. Lactosylceramide and Other Diosylceramides in Animals

The most important and abundant of the diosylceramides is β-D-galactosyl-(1-4)-β-D-glucosyl-(1-1')-ceramide (Galβ1-4Glcβ1Cer), more conveniently termed lactosylceramide (LacCer), using the trivial name of the disaccharide. In the early literature, it was termed 'cytolipin H'.

Structural formula of lactosylceramide

It is found in small amounts only in most animal tissues, but it has a number of significant functions, and it is the biosynthetic precursor of most of the neutral oligoglycosylceramides and the acidic sulfatides and gangliosides, discussed in separate web pages.

In animal tissues, the precursor of lactosylceramide is glucosylceramide, which is transported by the sphingolipid transport protein FAPP2 from the site of synthesis in the cis-Golgi to the trans-Golgi, where it must first cross from the cytosolic side of the membrane possibly via the action of a flippase. Biosynthesis of lactosylceramide then involves addition of the second monosaccharides unit as its activated nucleotide derivative (UDP-galactose) to monoglucosylceramide on the lumenal side of the Golgi apparatus in a reaction catalysed by β‑1,4‑galactosyltransferases of which two are known. The lactosylceramide produced can be further glycosylated (see below), or it can be transferred to the plasma membrane mainly by a non-vesicular mechanism that is poorly understood, but it cannot be translocated back to the cytosolic leaflet. Deletion of the lactosylceramide synthase by gene targeting is embryonically lethal.

Figure 1. Lactosylceramide as a glycosphingolipid intermediate.

Lactosylceramide may assist in stabilizing the plasma membrane and activating receptor molecules in the micro-domains known as rafts, as with the oligoglycosylceramides (see below) with very-long chain fatty acid constituents (24:0 and 24:1) necessary for this purpose, although it also participates in glycan-glycan interactions with gangliosides such as GM3. In this membrane environment, its interaction with growth factor receptors regulates their physiological properties, for example.

Scottish thistle Lactosylceramide forms such micro-domains on the plasma membrane of neutrophils (where it amounts to 70% of the glycosphingolipids) and macrophages, which recognize, engulf and eliminate pathogens. In these domains, the lipid has its own role in the innate immune system in that it is known to bind to many species of pathogenic bacteria and fungi by reacting with fungal-derived β-glucans and Mycobacteria-derived lipoarabinomannans via carbohydrate-carbohydrate interactions (like the oligoglycosylceramides below), and it is a receptor for lectins from such pathogenic fungi as Candida and Cryptococcus sp.. Mechanistically, it acts as a pattern recognition receptor, as it can detect pathogen-associated molecular patterns and then promote phagocytes for chemotaxis, phagocytosis and superoxide generation, by binding to specific membrane enzymes (e.g., Src family kinase Lyn) and mediating the immune and inflammatory responses by stimulating the formation of microbiocidal molecules.

It is believed that several pro-inflammatory factors induce its synthase to generate lactosylceramide, which in turn induces "oxygen-sensitive" signalling pathways that affect such cellular processes as proliferation, adhesion, migration and angiogenesis, especially in aortic smooth muscle cells. Disruption of these pathways can affect cancer and several diseases of the cardiovascular system, while inflammatory states involving neural damage, such as multiple sclerosis, are also affected. Lactosylceramide is reported to enhance the migration of monocytes with effects upon atherothrombosis, so the metabolism of this lipid is a potential target for new therapeutic treatments.

Lactosylsphingosine, i.e., the deacylated or lyso form, has been detected at low levels in brain where it may have some unspecified function.

Other diglycosylceramides: Galabiosylceramide (Galα1→Galβ1-1'Cer) has been found in small amounts in kidney, pancreas and cerebrospinal fluid, and it is one of the lipids that accumulates in excessive amounts in Fabry's disease (see below). It is the precursor of the minor 'gala' series of oligoglycosylceramides. Other diosylceramides containing mannose units may be present in some primitive animal species and in a few plants, while several insect species contain mannose linked to glucosylceramide (βMan(1→4)Glc), and a starfish has been found to contain gentiobiosyl- and cellobiosylceramide as well as lactosylceramide; other marine invertebrates contain melibiosylceramides.

2. Non-Acidic Oligoglycosylceramides of Animal Tissues

Neutral oligoglycosylceramides with from three to more than twenty monosaccharide units in the head group have been detected in many animal tissues ('megaloglycolipids' with up to 50 carbohydrate groups occur in erythrocytes). Of these, tri- to pentaglycosylceramides are often the most abundant or at least are the most intensively studied. Invertebrates such as those of marine origin tend to have very different sphingolipid compositions from those in higher animals. A recent review (2022) reports that more than 500 oligoglycosylceramides with variations in the carbohydrate moiety have been characterized if gangliosides and other acidic forms are included.

Seven main and several minor series of oligoglycosylceramides are recognized from structural and biosynthetic relationships, and the recommended root names and structures are listed in Table 1. In most, the primary unit linked to ceramide is glucose (Glc), with galactose (Gal), N-acetylglucosamine (GlcNAc), N‑acetylgalactosamine (GalNAc) or mannose (Man) as the other monosaccharides units. Only in the minor gala series is galactose linked to ceramide in vertebrates, although marine organisms can produce other oddities. The same main series occur in gangliosides and sulfatides as is discussed in other web pages here.

Table 1. Root names and structures of the main series of oligoglycolipids
Root^a	Symbol	Root structure^b	Trivial name

ganglio	Gg	Galβ1→3GalNAcβ1→4Galβ1→4Glc‑Cer	gangliotetraosylceramide
lacto^c	Lc	Galβ1→3GlcNAcβ1→3Galβ1→4Glc‑Cer	lactotetraosylceramide
neolacto^d	nLc	Galβ1→4GlcNAcβ1→3Galβ1→4Glc‑Cer	neolactotetraosylceramide
globo	Gb	GalNAcβ1→3Galα1→4Galβ1→4Glc‑Cer	globotetraosylceramide
isoglobo^e	iGb	GalNAcβ1→3Galα1→3Galβ1→4Glc‑Cer	isoglobotetraosylceramide
mollu	Mu	2Manα1→3Manβ1→4Glc‑Cer	mollutetraosylceramide
arthro	At	GalNAcβ1→4GlcNAcβ1→3Manβ1→4Glc‑Cer	arthrotetraosylceramide
muco	Mc	Galβ1→4Galβ1→4Galβ1→4Glc‑Cer	mucotetraosylceramide
gala	Ga	Galα1→4Galα1→4Gal‑Cer	galatriaosylceramide
^a There are also minor isoganglio-, lactoganglio-, neogala-, schisto- and spirometo-series. ^b The link from carbohydrate to ceramide in all is β1-1'. All sugars are in pyranose form. ^c In this instance, 'lacto' only refers to lactose tangentially. ^d The prefix 'neo' is used here to denote a (1→4) vs (1→3) difference in the linkage position between the monosaccharide units IV and III. ^e The prefix 'iso' is used here to denote a (1→3) vs (1→4) difference in the linkage position between the monosaccharide units III and II.

The name of a given glycosphingolipid is composed of (root name)(root size)osylceramide. Thus, lactotetraosylceramide or LcOse₄Cer designates the second structure listed in Table 1 linked to a ceramide. When it is necessary to refer to specific glycose residues, Roman numerals are sometimes used, counting from that nearest the ceramide (i.e., I to IV in this example). As further monosaccharides units are added or are substituted for these basic structures, or when branches occur, the nomenclature becomes increasingly complex and cumbersome, and further discussion would take us into the realm of the specialist; the IUPAC-IUB recommendations cited below should then be consulted. Semi-systematic names are usually recommended for ease of discussion in which trivial names for root structures are used as a prefix. For the most studied oligoglycosylceramides, such as those of the globo-series, the short-hand terms Gb3, Gb4 and Gb5 can be used to refer to those molecules containing a tri-, tetra- and pentasaccharide glycan, respectively, although many of the less common series do not have simple names that are broadly accepted.

Many different neutral oligoglycosphingolipids are present in various organs and species of animals with more complexity being added by the nature of the ceramide unit. In any given cell type, the number of different glycosylceramides may be relatively small, but their nature and compositions may be characteristic and related in some way to their function in the cell with this composition tending to change, often substantially, as an animal develops. Comprehensive descriptions of all the innumerable complex oligoglycosylceramides are not possible here, and only a few representatives can be discussed below.

Often with these sphingoglycolipids, it has been the nature of the carbohydrate moiety that has received most study, as this is usually presumed to carry the primary biochemical information. That said, it is apparent that the composition of the ceramide component of oligoglycosylceramides is necessary for their optimum performance in membranes. Data on the nature of the fatty acid and long-chain base constituents are relatively sparse, but in general, it appears that these tend to reflect their biosynthetic origins and resemble those of the precursor glucosylceramides. In general, as might be expected, the fatty acids are long-chain saturated and monoenoic in nature, but an exception is in the testicular lipids where there are distinctive fucolipids containing polyunsaturated fatty acids; these lipids appear to be essential for spermatogenesis and fertility in male mice (fucolipids are oligoglycolipids in any of the above series in which a fucose (Fuc) residue substitutes for one or more of the usual carbohydrate residues, but most often the terminal one).

3. Biosynthesis and Function of Oligoglycosphingolipids in Animals

The pathways for the biosynthesis of neutral oligoglycosphingolipids are illustrated. In the cytoplasm, nucleotide sugars, e.g., UDP-Glc, UDP-Gal, etc., are synthesised as donor substrates for the transfer of sugar residues, and glucosylceramide is synthesised in the cis-Golgi membranes for transfer to the luminal side of the trans-Golgi by the sphingolipid transfer protein FAPP2 where synthesis of lactosylceramide and the oligoglycosylceramides occurs. Further glycosylation is carried out by glycosyltransferases with at least one predicted nucleotide binding domain and differing substrate specificities; some appear to recognize only the carbohydrate portion of the molecule while others respond to the nature of the ceramide backbone. During the reaction, they invert the anomeric configuration of the nucleotide donor substrate in the product, while acting as part of a complex but highly structured organization of enzymes in the Golgi that includes membrane-bound sugar transporters and donor sugar nucleotides. This enzyme complex directs the synthetic steps that determine which series of glycolipids are formed by adding the relevant monosaccharides sequentially to the non-reducing end of the growing carbohydrate chain.

In the main pathway, the first step is the synthesis of lactosylceramide catalysed by galactosyltransferase I (discussed above), and this is the main metabolic branch point for the formation of the different classes of complex glycosphingolipids, both neutral and acidic, in mammals. Eventually the oligoglycosylceramide products are transported to the plasma membrane, where the carbohydrate head group of glycosphingolipids can undergo some further modification by glycohydrolases and transferases.

Figure 2. Biosynthesis of the neutral oligoglycosphingolipids

Thus, with lactosylceramide as the precursor, a β-1,4-N-acetylgalactosylaminyltransferase yields GA2, the α-2,3-sialyltransferase yields ganglioside GM3, the α‑1,4‑galactosyltransferase yields Gb3, and the β-1,3-N-acetylglucosaminyltransferase yields Lc3. In turn, GA2, GM3, Gb3 and Lc3 are the precursors for the biosynthesis of the asialo, ganglio (a-series gangliosides), globo/isoglobo and lacto/neolacto series of oligoglycosphingolipids, respectively, by addition of the appropriate monosaccharides. As well as formation of the minor gala series oligoglycosphingolipids, galactosylceramide can be sialylated to produce GM4 ganglioside or sulfated to produce sulfogalactolipids. The nature of the products is dependent upon the organizational topology of the multi-enzyme complexes and the availability of the required precursors, but further regulatory factors are certainly involved.

Biosynthesis of complex oligoglycosylceramides

Figure 3. Biosynthesis of the more complex oligoglycosylceramides.

The structure of the carbohydrate component is critical for most of the functions of oligoglycosphingolipids, 80-90% of which occur on the plasma membrane exclusively facing into the extra-cellular space to provide it with a protective carbohydrate coating in processes that are energetically inexpensive. By interacting with components of the signal transduction machinery, such as hormones and receptors, glycosphingolipids regulate cellular signalling pathways by various means including controlling protein conformation, regulation of protein multimerization and protein segregation to membrane domains that may involve the placement of signalling molecules next to their effectors, i.e., in the subdomains of membranes known as 'rafts'. For example, trafficking of the main apical glycosphingolipid in many animal species, the Forssman glycolipid, from the Golgi apparatus to the apical surface of epithelial cells requires its interaction with the apically secreted protein galectin-9 to facilitate the delivery of both lipid and proteins into membrane raft domains.

On the other hand, there is an increasing body of evidence to suggest that the composition of the ceramide unit as well as that of the carbohydrate is important. At the very least, the composition of the ceramide unit ensures that the lipid takes its correct place in the plasma membrane, with the hydroxy fatty acid constituents providing an additional and apparently essential hydrogen bonding capacity, while binding of various bacteria, viruses and antibodies to glycolipids seems to require specific ceramide compositions, often requiring the presence of hydroxy fatty acids. Once more, those species containing very-long chain fatty acid components (24:0 and 24:1) facilitate raft formation.

The globo and extended globo series of glycosphingolipids are the first to appear in developing mouse embryos, followed by those of the neolacto and lacto series. In humans, glycans containing the Galα4Gal structure have been found only in glycosphingolipids and not in glycoproteins, suggesting a special position of the globo series glycosphingolipids in human physiology.

The triose Gb3 or GbOse₃Cer is present on the cell membrane of a wide variety of mammalian tissue cells, including human erythrocytes, and this structure is the inner core of all globo-series oligoglycosylceramides (Table 2). It binds specifically to the verotoxins of Escherichia coli and to Shiga-like verotoxins; indeed, it aids the progression of disease by facilitating entry into cells. While deletion of the Gb3 synthase by gene targeting in animal models does not produce any obvious phenotype, it does protect against the effects of these toxins. Studies with the bacterium Pseudomonas aeruginosa have shown that Gb3 in the plasma membrane interacts with lectins on the bacterial surface to trigger bending of the plasma membrane and complete engulfment of the bacterium. This globotriaosylceramide has been implicated with other glycosphingolipids in host cell interactions with the human immunodeficiency virus (HIV), where it appears to be protective, and in influenza infection, elevated Gb3 promotes reactive antibody responses and cross-protection when expressed on germinal center B cells. In contrast, globotetraosylceramide (Gb4) facilitates the entry of the parvovirus B19 virus into cells. Together with certain gangliosides, the concentration of Gb3 is elevated in several cancers, and it is associated with an increase of multidrug resistance.

Table 2. Structures of some oligoglycosylceramides of the globo-series
Galα1→4Galβ1→4Glc‑Cer		Gb3
Galα1→3Galβ1→4Glc‑Cer		Iso-Gb3
GalNAcβ1→3Galα1→4Galβ1→4Glc‑Cer		Gb4
GalNAcβ1→3Galα1→3Galβ1→4Glc‑Cer		Iso-Gb4
GalNAcα1→3GalNAcβ1→3Galα1→4Galβ1→4Glc‑Cer		Forssman
Galβ1→3GalNAcβ1→3Galα1→4Galβ1→4Glc‑Cer		Gb5 (SSEA-3a)
Fucα1→2Galβ1→3GalNAcβ1→3Galα1→4Galβ1→4Glc‑Cer		globo-H (SSEA-3b)

Further, Gb3 globoside is a factor in apoptosis by interaction with the Fas (CD95) receptor, sometimes termed the 'death receptor', which bears a characteristic domain that interacts specifically with Gb3 and lactosylceramide but not with Gb4 or gangliosides. This defines its internalization route, as well as the signalling outcome, because when paired to Gb3, this receptor is internalized by clathrin-dependent endocytosis and results in the transduction of a cell death signal derived from stimulation of the caspase-8 cascade.

Isoglobotriaosylceramide (iso-Gb3 or iGb3), differing in the terminal glycosidic linkage, is a stimulatory antigen to Natural Killer T cells in vitro, suggesting that it is implicated in controlling the responses of these cells to infections, malignancy and autoimmunity in mice at least, although there are doubts as to whether there is the same requirement in humans in vivo. The first triose in the lacto category, Lc3 (GlcNAcβ1-3Galβ1-4Glcβ1Cer), is associated with embryonic development in the brain.

Among many vital functions, glycosphingolipids are components of the body's immune defence system, either in haptenic reactivity or in antibody-producing potency, i.e., as cellular immunogens or antigens. Certain glycolipids are involved in the antigenicity of blood group determinants, while others bind to specific toxins or bacteria. For example, Propionibacterium, which causes a disease of the skin, binds to the lactosyl moiety of glycosphingolipids as does the organism responsible for gonorrhoea. Some glycosphingolipids are receptors for cellular recognition, they can be specific for particular tissues or tumours, and they can modify the activity of membrane receptors, such as those for insulin, and epidermal and nerve growth factors. Antibodies to specific glycolipids have been implicated in certain diseases of the autoimmune system. The main glycosphingolipid found in human erythrocytes is a tetrahexoside (GalNAcβ1-3Galα1-4Galβ1-4Glcβ-Cer), often abbreviated to Gb4 or GbOse₄Cer, which was termed ‘globoside’ (as it was first obtained as a globular, birefringent precipitate), and it gives the root name to the globo-series.

Formula of a globotetraosylceramide

Gb5 is a pentaosyl-ceramide of the globo-series and is a cell-surface marker used to define human embryonic stem cells, but the related hexasaccharide globoside designated Globo-H, first observed in a breast cancer cell line, has been associated with several other cancers, including those of the prostate, lung, colon, liver, ovary and uterus. Antibodies to Globo-H have been produced and have reached the phase 3 stage in clinical trials against triple negative breast cancer.

The oligoglycosylceramides of erythrocytes have attracted special interest, as they are required for cellular interactions and include blood-type determinants, which are factors in deciding the suitability of blood and other tissues for transplantation. Gb3 and Gb4 constitute the basis of the blood group system, and the blood group O antigen is derived from Gb4 by addition of a fucose unit to the terminal galactose in α1,2‑linkage; the A antigen has a further N-acetylgalactosamine added, while in the B antigen the new terminal unit is galactose as illustrated.

Biosynthesis of the blood group antigens

Figure 4. Biosynthesis of the blood group group antigens.

The Forssman antigen, i.e., GbOse₅Cer with an additional terminal α-N-acetylgalactosamine, is species specific; some animals (e.g., horse, cat, dog) are Forssman-positive and others (e.g., rat, pig) are Forssman-negative and lack this glycolipid because of a mutated α1-3 N‑acetylgalactosaminyltransferase that cannot transfer N‑acetylgalactosamine to the precursor Gb4Cer. While most humans belong to the latter group, a few populations have been found with the Forssman antigen in the intestinal mucosa. There are further globo-series glycolipids with alternative carbohydrate moieties attached to the terminal GalNAc of Gb4, which have been isolated from human embryonic carcinoma and are believed to influence the development of stem cells. Cats, dogs and horses contain a glycolipid similar to globosides but with sialic acid replacing the galactosamine and has been termed ‘hematoside’ (other gangliosides based on the globo- and lacto-root structure are discussed elsewhere on this site). Many glycolipid blood group antigens act as receptors or co-receptors for pathogens or toxins, and the A and B blood group antigens bind to the cholera toxin to in effect act as decoys away from similarly fucosylated glycoproteins to prevent cellular entry of the pathogen.

Oligoglycosylceramides of the neolacto-series contain alternating residues of 1,3-substituted galactose and 1,4-substituted N‑acetylglucosamine, i.e., N‑acetyllactosamine units. At least 15 types are known with both linear and branched structures that can be heavily substituted with fucose moieties and are important in that they are associated with blood groups A, B and H; there are some less well-characterized forms that contain up to fifty carbohydrate groups. Most of these lipids are antigens, and some are associated with specific carcinomas such as human gastric adenocarcinoma where forms with 5, 7 and 10 carbohydrate moieties (the last branched) have been detected. Some have distinctive ceramide structures including phytosphingosine and hydroxy acids.

Scottish thistle Mucolipids, i.e., oligoglycosylceramides with di- to hexagalactosyl glycosides (some with branch structures) linked to glucosylceramide, are found predominantly in the gastrointestinal mucosa, and they are protective components of the membranes of the brush border of intestinal cells where much of the digestion and absorption of nutrients occurs. Many of the enzymes for these processes are located in membrane rafts with little cholesterol but high concentrations of glucosylceramide and oligoglycosylceramides, such as lactosylceramide, globotriaosylceramide and ganglioside GM3, which are resistant to pancreatic enzymes, bile salts and the products of digestion. Once more, phytosphingosine accounts for 70% of the sphingoid bases in these lipids, and a high proportion of the fatty acids are hydroxylated. Pathogenic microorganisms, such as Helicobacter pylori, may be present in the digestive tract with the potential to enter the host system if the protective barrier of mucin and glycosphingolipids is damaged. Within the host, organisms such as Streptococcus pneumonia have the capacity to hydrolyse membrane glycolipids and so increase the virulence of an infection.

Mannosylglucosylceramide (Manβ1-4Glcβ1-cer) is the precursor diosylceramide for the complex glycosphingolipids in Drosophila, widely studied as an insect model. N‑Acetylgalactosamine residues can be added to this sequentially, the first via a β1-3 linkage and subsequently via β1-4 linkages (up to eight carbohydrate residues, some linked to phosphoethanolamine units). The sphingoid bases tend to be C₁₄ and C₁₆ in chain-length.

Parasitic nematodes contain zwitterionic oligoglycosphingolipids of the arthro series with phosphocholine moieties attached as highly conserved, antigenic glycolipid markers. The glycosphingolipids of the pig parasitic nematode, Ascaris suum, have been most studied, and they are characterized by a phosphodiester-bound phosphocholine substituent linked to C6 of a central N‑acetylglucosamine residue, while some carry phosphorylethanolamine linked to C6 of an adjacent mannose residue. In the ceramide moiety, 2R‑hydroxytetracosanoic acid is the main fatty acid component with C₁₇ iso-branched sphingosine and sphinganine bases.

4. Oligoglycosylceramides of Plants and Microorganisms

In plants and fungi, elongation of glucosylceramide, not galactosylceramide, occurs to form two series of oligoglycosylceramides with either mannosyl or galactosyl units by mechanisms that are presumably related to those in animals. In the mannosyl series in higher plants, up to four mannosyl units may be added via β1→4 linkages, while in a second series, addition of a 1-4 linked β‑D‑glucopyranosyl unit terminates the elongation step after 1, 2 or 3 mannose units have been coupled together This results in a series of di-, tri-, tetra- and pentaglycosylceramides, terminating in either a glucose or mannose unit. In barley, oligohexosyl ceramides consisting of two to five glucose units in linear β1→4 linkages have been identified. These complex glycolipids are found mainly as constituents of the endoplasmic reticulum, Golgi, tonoplasts and plasma membrane, but little is known of why they are required by plants.

Figure 5. Biosynthesis of oligoglycosylsphingolipids in higher plants

A series of oligoglycolipids found in algae and fungi involves linkage of up to three D-galactopyranosyl residues to the primary glucosylceramide unit, although not all members of the series have been characterized in a single organism. In the primitive picoalga Ostreococcus tauri, species with five hexose units (some acidic) linked to ceramide have been detected.

The fatty acid and sphingoid base compositions tend to reflect their biosynthetic origins and resemble those of the precursor monoglycosylceramides. Although some species differences are found, the fatty acids are mainly saturated 2R-hydroxy acids with 14 to 26 carbon atoms. Phytosphingosines (including 8-cis/trans isomers) and various 4t,8c/t-sphingadienes are the main long-chain bases. In the oligoglycolipids of barley, trihydroxy bases are associated with hydroxylated fatty acids.

A tetraglycosylceramide has been isolated from the mould Neurospora crassa and from the bacterial genus Sphingomonas, which contains a cerebroside analogue and α‑D‑Manp(l→2)-α-D-Galp-(1→6)-α-D-GlcpN-(1→4)-α-D-Glcpα(1→1)Cer, i.e., with an α- rather than a β-linkage to the ceramide unit; other bacterial species produce similar lipids during phosphate starvation. While monoglycosylceramides are frequently reported from fungi and yeasts, complex lipopolysaccharides and glycosylinositol phosphoceramides, the main sphingolipids in plants, appear to take the place of neutral oligoglycosylceramides. In the model marine diatom, Thalassiosira pseudonana, a diglycosylceramide (not fully characterized) replaces phosphatidylcholine in part when phosphate is limiting.

5. Catabolism of Oligoglycosylceramides in Animals and Genetic Disease

As discussed in the web page dealing with monoglycosylceramides, oligoglycosphingolipids are degraded in lysosomal compartments within the cell by water-soluble hydrolases, aided by various activator proteins termed saposins and by anionic lipids such as bis(monoacylglycero)phosphate, which is enriched in lysosomal membranes. In brief, sections of plasma membrane containing glycosphingolipids intended for degradation are endocytosed and transported to the lysosomes where exohydrolases cleave sugar residues sequentially from the non-reducing end. The cleaved fragments, i.e., the sugar residues and ceramides, with the latter often hydrolysed subsequently to fatty acids and sphingoid bases, can then leave the lysosomes and be degraded further or re-enter the biosynthetic pathways.

Figure 6. Catabolism of oligoglycolipids in animals.

An alternative catabolic pathway in certain bacteria and leeches involves the action of a ceramide glycanase to cleave the β-glucosidic linkage between glucose and ceramide, which results in the formation of ceramide and an oligosaccharide. This enzyme has proved of value in the structural analysis of glycosphingolipids.

Fabry's disease: If any of the ten different hydrolase enzymes is deficient in animal tissues, the corresponding lipid substrate accumulates and is stored in the lysosomal compartment, sometimes to a considerable extent as in the inherited sphingolipid storage pathologies such as Fabry's disease, a genetic disorder (X‑linked) arising from the absence of the lysosomal enzyme α-galactosidase A, which catalyses the hydrolytic cleavage of the terminal galactose unit. This leads to deposition of excessive amounts of glycosphingolipids with terminal galactose units, such as globotriaosylceramide (Gb3), galabiosylceramide and lactosylceramide in the kidney, heart and brain, and the presence of Gb3 in urine or its lyso (non-acylated) form in serum aids diagnosis. Globotriaosylsphingosine (lyso-Gb3) can accumulate to high levels in vasoendothelial cells and has a role in nephropathy and secondary inflammatory events. Enzyme replacement therapy is now making a significant contribution to ameliorating the symptoms of the disease.

6. Analysis

High-performance thin-layer chromatography and high-performance liquid chromatography (sometimes after benzoylation) are widely used for separation of various types of oligoglycolipids, based on the number and to some extent the type of monosaccharide units. Mass spectrometry is the main method for identifying and sequencing the carbohydrate chains, with invaluable assistance from nuclear magnetic resonance spectroscopy. Indeed, modern mass spectrometric methods with electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) appear to have greatly eased the technical problems of structural analysis.

	© Author: William W. Christie
	Contact/credits/disclaimer	Updated: October 23^rd, 2024

Lactosylceramide and non-Acidic Oligoglycosylceramides