A change in soybean agglutinin binding patterns of bovine milk fat globule membrane glycoproteins during early lactation

The Influences of Soybean Agglutinin and Functional Oligosaccharides on the Intestinal Tract of Monogastric Animals

Soybean agglutinin (SBA) is a non-fiber carbohydrate-related protein and the main anti-nutritional factor that exists in soybean or soybean products. SBA possesses a specific binding affinity for N-glyphthalide-d-galactosamine or galactose and has a covalently linked oligosaccharide chain. SBA mediates negative effects on animal intestinal health by influencing the intestinal structure, barrier function, mucosal immune system, and the balance of the intestinal flora. Functional oligosaccharides are non-digestible dietary oligosaccharides that are commonly applied as prebiotics since the biological effects of the functional oligosaccharides are to increase the host health by improving mucosal structure and function, protecting the integrity of the intestinal structure, modulating immunity, and balancing the gastrointestinal microbiota. The purpose of this review is to describe the structure and anti-nutritional functions of SBA, summarize the influence of SBA and functional oligosaccharides on the intestinal tract of monogastric animals, and emphasize the relationship between SBA and oligosaccharides. This review provides perspectives on applying functional oligosaccharides for alleviating the anti-nutritional effects of SBA on the intestinal tract.

Comparative Glycomics of Fat Globule Membrane Glycoconjugates from Buffalo (Bubalus bubalis) Milk and Colostrum

The health-promoting effects of milk fat globule membrane (MFGM) glycoconjugates has attracted curiosity especially with regard to the challenges encountered to unravel the glycan complexities of MFGM glycoproteins and glycosphingolipids. In this context, we characterized glycans present in buffalo milk and colostrum fat globule membranes by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS analysis by adopting chemoselective glycoblotting technique. Unlike human and bovine MFGM glycoproteins, the variations were obvious with respect to their number, size, heterogeneity, and abundance among the samples analyzed. Among N-linked glycans, mono-, di-, and trisialyl glycans were apparent in colostrum, while MFGM predominantly contained mono- and disialyl glycans, in addition to neutral and high-mannose glycoforms. The structural assignments of major glycans were confirmed by TOF/TOF analysis. Core 1 O-glycans were more common in both samples, and the major glycosphingolipids were GM3 and GD3 irrespective of the samples analyzed. The colostrum N-glycans, being effective antibacterials against human pathogens, established the structure-function relationship of oligosaccharides in early milk in providing innate protection to the newborn.

Expression of LacdiNAc groups on N-glycans among human tumors is complex

Aberrant glycosylation of proteins and lipids is one of the characteristic features of malignantly transformed cells. The GalNAcβ1 → 4GlcNAc (LacdiNAc or LDN) group at the nonreducing termini of both N- and O-glycans is not generally found in mammalian cells. We previously showed that the expression level of the LacdiNAc group in N-glycans decreases dramatically during the progression of human breast cancer. In contrast, the enhanced expression of the LacdiNAc group has been shown to be associated with the progression of human prostate, ovarian, and pancreatic cancers. Therefore, the expression of the disaccharide group appears to be dependent on types of tumors. The mechanism of formation of the LacdiNAc group in human tumors and cancer cells has been studied, and two β4-N-acetylgalacto-saminyltransferases (β4GalNAcTs), β4GalNAcT3 and β4GalNAcT4, have been shown to be involved in the biosynthesis of this disaccharide group in a tissue-dependent manner. Transfection of the β4GalNAcT3 gene brought about significant changes in the malignant phenotypes of human neuroblastoma, indicating that this disaccharide group is important for suppressing the tumor growth.

Structural and functional characteristics of bovine milk protein glycosylation

Most secreted and cell membrane proteins in mammals are glycosylated. Many of these glycoproteins are also prevalent in milk and play key roles in the biomodulatory properties of milk and ultimately in determining milk's nutritional quality. Although a significant amount of information exists on the types and roles of free oligosaccharides in milk, very little is known about the glycans associated with milk glycoproteins, in particular, the biological properties that are linked to their presence. The main glycoproteins found in bovine milk are lactoferrin, the immunoglobulins, glycomacropeptide, a glycopeptide derived from κ-casein, and the glycoproteins of the milk fat globule membrane. Here, we review the glycoproteins present in bovine milk, the information currently available on their glycosylation and the biological significance of their oligosaccharide chains.

Prognostic significance of reduced expression of beta-N-acetylgalactosaminylated N-linked oligosaccharides in human breast cancer

Our previous studies showed that expression of the GalNAcbeta1-->4GlcNAc group on N-linked oligosaccharides is associated with functional differentiation of the bovine mammary gland. In the present study, the occurrence of the GalNAcbeta1-->4GlcNAc group was established in human milk proteins and membrane glycoproteins from a human breast cancer cell line, MRK-nu-1, by structural analysis of oligosaccharides released by hydrazinolysis. Whether the expression level of the disaccharide group is affected upon malignant transformation was examined in human breast cancer specimens using Wistaria floribunda agglutinin (WFA) which interacts with oligosaccharides with N-acetylgalactosamine at their termini. Lectin blot analysis of membrane glycoprotein samples from human breast cancer specimens showed that the number of protein bands reacting with WFA, as well as their intensities, are lower in samples from primary carcinoma lesions compared with samples from surrounding normal tissues. No lectin binding was observed when the blots were treated with jack bean beta-N-acetylhexosaminidase or N-glycanase, indicating that WFA-reactive oligosaccharides are N-linked. A histochemical study of tissue specimens from 92 patients with breast cancer revealed that the reduced WFA staining levels in primary carcinoma lesions correlate with advancing clinical stages and prognostic status (i.e., 58% of patients in a group showing reduced/negative staining died of disease recurrence, whereas more than 90% of those in the positive staining group survived for 5 years after surgery). These results indicate that reduced expression of beta-N-acetylgalactosaminylated N-linked oligosaccharides on primary carcinoma lesions predicts a poor prognosis for patients with breast cancer.

Differentiation-Associated Expression of ß-N-Acetylgalactosaminylated N-Linked Oligosaccharides in Mammary Epithelial Cells

Not only mammalian pituitary glycoprotein hormones but also many glycoproteins from a variety of animal species have been shown to contain N-linked oligosacchardies with the GalNAcbeta1 --> 4GlcNAc structure. Two types of beta-1,4-GalNAcT were found; one transfers N-acetylgalactosamine to acceptor oligosaccharides, which is stimulated by the hormone peptide and the other simply transfers sugar without such activation. In the case of bovine mammary membrane glycoproteins, the expression of beta-N-acetylgalactosaminylated N-linked oligosaccharides was associated with the functional differentiation of the epithelial cells. In contrast, the expression level of such oligosaccharides was much reduced in glycoprotein samples from human breast tumors compared with those from the unaffected regions. These results strongly suggest that the beta-N-acetylgalactosaminylation is one which is regulated under cellular differentiation and dedifferentiation of the mammary gland. Whether or not beta-N-acetylgalactosaminylated N-linked oligosaccharides have unique functions in addition to clearance of the hormone from the circulation remains to be elucidated.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Developmental and hormonal regulation of protein N-glycosylation in the mammary gland

Glycosylation represents the most common conjugation of both membrane-bound and secreted proteins of animal cells. Among the different types of glycosylation, the N-linked attachment of sugars to the polypeptide backbone is by far the most abundant modification. The biosynthesis of the precursor carbohydrate unit of these proteins is initiated by a stepwise assembly of Glc3Man9GlcNAc2P-P-Dol in the dolichol cycle, its transfer en bloc to the nascent polypeptide in the rough endoplasmic reticulum (RER), followed by excision of the glucosyl residues by processing-specific enzymes, glucosidase I and II, also resident in the endoplasmic reticulum. Additional posttranslational modifications of the carbohydrate moiety in the RER, Golgi, and trans-Golgi network, differ for individual glycoproteins for the completion of final products as high mannose, complex or hybrid glycoproteins en route to their final destinations in the secretory pathway. The enzyme GlcNAc-1-P transferase (GPT) catalyzes the first and committed step, i.e., the transfer of GlcNAc-1-P from UDP-GlcNAc to Dol-P to form GlcNAc-P-P-Dol, in the assembly of the oligosaccharide precursor. Glucosidase I triggers the maturation phase by clipping the distal alpha 1,2-linked Glc residue on the incipient glycoprotein. The critical juxtaposition of the two enzymes in the multistep pathway makes them excellent candidates for the overall regulation of protein N-glycosylation. The highly elevated needs of glycosylation during lactation demand regulation of glycosylation in the gland over and above the levels in the quiescent, virgin and postlactating, regressed gland.

Distribution of glycoproteins with beta-N-acetylgalactosaminylated N-linked sugar chains among bovine tissues

Only a small number of glycoproteins has been reported to contain N-linked sugar chains with GalNAcbeta1-->4GlcNAc structure. Our previous studies showed that most glycoproteins from bovine milk fat globule membranes contain beta-N-acetylgalactosaminylated N-linked sugar chains [Sato et al., J. Biochem. 114 (1993) 890-900]. In order to study how widely this glycosylation occurs, lectin blot analysis of membrane glycoproteins from 12 bovine tissues was performed using Wistaria floribunda agglutinin (WFA), which interacts with oligosaccharides terminating with N-acetylgalactosamine. The WFA-positive bands were detected in samples from most tissues except for intestine although the number and reactivity of bands to lectin varied among the tissues. Upon pretreatment of blotted filters with Bacillus beta-N-acetylgalactosaminidase or N-glycanase, no lectin binding was observed. WFA-agarose column chromatography of oligosaccharides released by hydrazinolysis from membrane glycoproteins of bovine tissues except for intestine revealed that a few to 18% of the released oligosaccharides bind and are eluted from the column with 100 mM N-acetylgalactosamine. These results indicate that many glycoproteins from a variety of bovine tissues contain N-linked sugar chains with GalNAcbeta1-->4GlcNAc structure, suggesting a wider occurrence of this glycosylation in bovine tissues.

Species-specific beta-N-acetylgalactosaminylation of serum IgG proteins

Lectin blot analysis of bovine, goat, human, rabbit and mouse serum immunoglobulin G (IgG) samples revealed that Wisteria floribunda agglutinin (WFA) binds to the heavy chains of bovine, goat and human serum IgG proteins but not those of the rabbit and mouse proteins. WFA-positive light chain bands were also detected in bovine, goat and human serum IgG samples only after the filters were treated with Arthrobacter ureafaciens sialidase. The WFA-binding to these IgG proteins was abolished by treatment of the filter with sialidase and then beta-N-acetylhexosaminidase or N-glycanase prior to incubation with the lectin. WFA-agarose column chromatography of the oligosaccharides released by hydrazinolysis from the IgG samples followed by reduction with NaB3H4 revealed that 0.15, 0.09 and 0.07% of the total oligosaccharides from bovine, goat and human serum IgG samples bind to the column, respectively. Partial characterization of WFA-positive bovine IgG oligosaccharides by Bio-Gel P-4 column chromatography suggested that the major oligosaccharide is of non-fucosylated biantennary complex-type. These results indicate that beta-N-acetylgalactosaminylation occurs to N-linked sugar chains of heavy and light chains of IgG proteins in a species-specific manner.

Alpha-lactalbumin induces bovine milk beta 1,4-galactosyltransferase to utilize UDP-GalNAc

We now report that alpha-lactalbumin (alpha-LA) has a novel effect on bovine milk UDP-Gal:GlcNAc-beta 1,4-galactosyltransferase (beta 1,4-GT) and induces the enzyme to efficiently utilize UDP-GalNAc as a donor. In the presence of alpha-LA the enzyme transfers GalNAc to free GlcNAc to produce GalNAc beta 1-4GlcNAc at a rate 55% of that compared to the rate when UDP-Gal is the donor in the absence of alpha-LA. The stimulation by alpha-LA is dependent on the concentrations of alpha-LA, acceptor, and sugar nucleotide. Interestingly, beta 1,4-GT is unable to transfer Gal-NAc to Glc with or without alpha-LA. alpha-LA also stimulates the transfer of GalNAc from UDP-GalNAc to various chitin oligomers, although the degree of stimulation decreases as the acceptor size increases. Thus, bovine milk beta 1,4-GT has an inherent ability to utilize two different sugar nucleotides and the sugar nucleotide preference is regulatable by alpha-LA.

Precursor-product relationship between chicken vitellogenin and the yolk proteins: the 40 kDa yolk plasma glycoprotein is derived from the C-terminal cysteine-rich domain of vitellogenin II

Chicken vitellogenin, a serum lipoprotein specific for laying hens, has been thought to be proteolytically cleaved into the heavy and light chain lipovitellins and phosvitin, the major yolk granule proteins, during or after transportation into oocyte. In this study, another proteolytic product of vitellogenin has newly been isolated from the 'beta-livetin' fraction of yolk plasma. It is a yolk glycoprotein of 40 kDa (YGP40) with asparagine-linked carbohydrate chain(s) recognized by Concanavalin A and castor bean lectin (RCA-I), and it is identified as a C-terminal cysteine-rich fragment of the major vitellogenin (vitellogenin II), the cysteine-rich domain homologous to D2 region of von Willebrand factor. Another yolk plasma glycoprotein of 42 kDa is suggested to be one of the proteolytic products of the minor vitellogenin (vitellogenin I). Both 40 kDa and 42 kDa glycoproteins were shown to be present in growing oocytes but absent in laying hen's serum. Limited proteolysis of vitellogenin II with cathepsin D produced a 40 kDa protein with reactivity to anti-YGP40 antibody. Gel filtration analysis of vitellogenin II digested with cathepsin D showed that YGP40 dissociated from lipovitellin-phosvitin complex after the proteolytic cleavage. These results suggest that after incorporation from serum via a specific receptor vitellogenin II is cleaved in the oocyte into four fragments, heavy and light chain lipovitellins, phosvitin and YGP40, and that YGP40 is released into the yolk plasma before or during compartmentation of lipovitellin-phosvitin complex into the yolk granule.

Immunologically cross-reactive 57 kDa and 53 kDa glycoprotein antigens of bovine milk fat globule membrane: isoforms with different N-linked sugar chains and differential glycosylation at early stages of lactation

Two glycoprotein antigens with molecular masses of 57 kDa (MGP57) and 53 kDa (MGP53) were co-purified from bovine milk fat globule membrane (MFGM) by immunoaffinity chromatography using a monoclonal antibody raised against the MFGM. Their N-terminal sequences of 22 amino acids determined were identical, and the sequence was homologous (about 60% identical) to the deduced amino acid sequence of mouse milk fat globule epidermal growth factor (EGF) factor 8 (MFG-E8) (Ref. [12], Stubbs, J.D. et al., Proc. Natl. Acad. Sci. USA, 87, 8417-8421, 1990). This suggests that MGP57/53 are bovine MFGM components 15/16 (PAS-6 and PAS-7), which have recently been reported to be bovine homologs of MFG-E8. N-Glycanase treatment of these glycoproteins reduced their molecular masses, and consequently the enzymatically deglycosylated MGP57 and MGP53 converged on a single band of 50 kDa as measured by SDS-PAGE, indicating that the polypeptide portions of these two distinct glycoprotein antigens are very similar or identical and that their N-linked sugar chains contributed to minor difference in their molecular masses. Western blot analyses using lectins also revealed that they were differentially glycosylated; MGP57 was stained with concanavalin A (Con A) more strongly than MGP53, whereas MGP 53 was stained well with soybean agglutinin (SBA). Reactivity with SBA remarkably increased during early stages of lactation. Two-dimensional gel electrophoresis showed that MGP57 and MGP53 were electrically heterogeneous; from day 9 after parturition, both glycoproteins fell in almost the same range of isoelectric points between 6.4 and 7.6, also, such glycoproteins from day 1 after parturition were more acidic, probably due to terminal sialylation of their sugar chains.