Chemical and carbon-13 nuclear magnetic resonance studies of the blood group M and N active sialoglycopeptides from human glycophorin A

Isolation of Unusually Composed Sialyl-Compounds from Hemofiltrate

Sialyl compounds are essential components of various biological fluids but relatively little is known about their occurrence in the extracellular fluid of patients with end-stage renal disease. As we have developed a macropreparative method for concentrating and desalting a wide range of fractions from diluted biological fluids we have been able to isolate and identify 5 sialooligosaccharides, 3 sialosugarphosphates, 2 monosialoglycopeptides and 1 disialoglycopeptide. The structures have been elucidated predominantly by one and two-dimensional NMR spectroscopy, enzymatic degradation and FAB mass spectrometry.

The accumulation of these compounds in uremic sera may be of particular interest as they may interact in the molecular biology of diseases typically associated with the uremic state, e.g., immune deficiency, neurological disorders, receptor binding abnormalities, complement system disturbances and cell membrane alterations.

A Comprehensive Review of Our Current Understanding of Red Blood Cell (RBC) Glycoproteins

Human red blood cells (RBC), which are the cells most commonly used in the study of biological membranes, have some glycoproteins in their cell membrane. These membrane proteins are band 3 and glycophorins A-D, and some substoichiometric glycoproteins (e.g., CD44, CD47, Lu, Kell, Duffy). The oligosaccharide that band 3 contains has one N-linked oligosaccharide, and glycophorins possess mostly O-linked oligosaccharides. The end of the O-linked oligosaccharide is linked to sialic acid. In humans, this sialic acid is N-acetylneuraminic acid (NeuAc). Another sialic acid, N-glycolylneuraminic acid (NeuGc) is present in red blood cells of non-human origin. While the biological function of band 3 is well known as an anion exchanger, it has been suggested that the oligosaccharide of band 3 does not affect the anion transport function. Although band 3 has been studied in detail, the physiological functions of glycophorins remain unclear. This review mainly describes the sialo-oligosaccharide structures of band 3 and glycophorins, followed by a discussion of the physiological functions that have been reported in the literature to date. Moreover, other glycoproteins in red blood cell membranes of non-human origin are described, and the physiological function of glycophorin in carp red blood cell membranes is discussed with respect to its bacteriostatic activity.

A nonprotein thermal hysteresis-producing xylomannan antifreeze in the freeze-tolerant Alaskan beetle Upis ceramboides

Thermal hysteresis (TH), a difference between the melting and freezing points of a solution that is indicative of the presence of large-molecular-mass antifreezes (e.g., antifreeze proteins), has been described in animals, plants, bacteria, and fungi. Although all previously described TH-producing biomolecules are proteins, most thermal hysteresis factors (THFs) have not yet been structurally characterized, and none have been characterized from a freeze-tolerant animal. We isolated a highly active THF from the freeze-tolerant beetle, Upis ceramboides, by means of ice affinity. Amino acid chromatographic analysis, polyacrylamide gel electrophoresis, UV-Vis spectrophotometry, and NMR spectroscopy indicated that the THF contained little or no protein, yet it produced 3.7 +/- 0.3 degrees C of TH at 5 mg/ml, comparable to that of the most active insect antifreeze proteins. Compositional and structural analyses indicated that this antifreeze contains a beta-mannopyranosyl-(1-->4) beta-xylopyranose backbone and a fatty acid component, although the lipid may not be covalently linked to the saccharide. Consistent with the proposed structure, treatment with endo-beta-(1-->4)xylanase ablated TH activity. This xylomannan is the first TH-producing antifreeze isolated from a freeze-tolerant animal and the first in a new class of highly active THFs that contain little or no protein.

NMR investigations of the role of the sugar moiety in glycosylated recombinant human granulocyte-colony-stimulating factor

Human granulocyte-colony-stimulating factor (G-CSF) is a hematopoietic growth factor that plays a major role in the stimulation of the proliferation and maturation of granulocyte neutrophil cells. With the recent increased understanding of its biological properties in vivo together with available preparations of recombinant human G-CSF, this growth factor has become an essential agent for clinical applications. The presence of an O-linked carbohydrate chain at position 133 greatly improves the physical stability of the protein. To clarify the molecular basis for the stabilisation effect of saccharide moieties on human G-CSF the whole glycoprotein expressed in CHO cells has been investigated by means of two 1H-NMR-spectroscopy and two 1H-detected-heteronuclear 1H-13C experiments at natural abundance, and compared with the non-glycosylated form. The present NMR study reports assignments of 1H and 13C resonances of the bound saccharidic chain NeuNAc(alpha2-3)Gal(beta1-3)[NeuNAc(alpha2-6)]GalNAc, where NeuNAc represents N-acetylneuraminic acid, and demonstrates the alpha-anomeric configuration of the N-acetylgalactosamine-threonine linkage. It also provides results suggesting that the carbohydrate moiety reduces the local mobility around the glycosylation site, which could be responsible for the stabilising effect observed on the glycoprotein.

O-linked protein glycosylation structure and function

There has been a recent resurgence of interest in the post-translational modification of serine and threonine hydroxyl groups by glycosylation, because the resulting O-linked oligosaccharide chains tend to be clustered over short stretches of peptide and hence they can present multivalent carbohydrate antigenic or functional determinants for antibody recognition, mammalian cell adhesion and microorganism binding. Co-operativity can greatly increase the affinity of interactions with antibodies or carbohydrate binding proteins. Thus, in addition to their known importance in bearing tumour associated antigens in the gastrointestinal and respiratory tracts, glycoproteins with O-linked chains have been implicated as ligands or co-receptors for selectins (mammalian carbohydrate binding proteins). Microorganisms may have adopted similar mechanisms for interactions with mammalian cells in infection, by having relatively low affinity ligands (adhesins) for carbohydrate binding, which may bind with higher affinity due to the multivalency of the host ligand and which are complemented by other virulence factors such as interactions with integrin-type molecules. In addition to specific adhesion signals from O-linked carbohydrate chains, multivalent O-glycosylation is involved in determining protein conformation and forming conjugate oligosaccharide-protein antigenic, and possible functional determinants.

Interaction of immobilized recombinant mouse C-type macrophage lectin with glycopeptides and oligosaccharides

Inflammatory and tumoricidal macrophages express galactose- and N-acetylgalactosamine-specific Ca(2+)-dependent lectins on their surfaces. This lectin is a family member of membrane-bound C-type animal lectins and consists of 304 amino acid residues (molecular weight 34,595). In the present study, expression vectors containing a nucleotide sequence corresponding to the carbohydrate-binding domain of mouse macrophage lectin cDNA have been prepared. The carbohydrate-binding specificity of the recombinant macrophage lectin expressed in Escherichia coli was investigated by comparing elution profiles of various glycopeptides having defined carbohydrate structures on immobilized lectins. When elution profiles of high mannose-type and complex-type Asn-linked carbohydrate chains were compared, the degree of retardation from immobilized macrophage lectin column was in the order tetraantennary complex-type with terminal galactosyl residues > triantennary complex-type with terminal galactosyl residues > biantennary complex-type with terminal galactosyl residues > high mannose-type glycopeptides. N-Terminal octapeptides from human glycophorin A that bore three NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-6)GalNAc serine/threonine-linked tetrasaccharide chains and their sequentially deglycosylated derivatives were also applied to this column. Glycopeptides carrying three constitutive GalNAc-Ser/Thr(Tn-antigen) had the strongest affinity, whereas those with fully sialylated carbohydrate tetrasaccharide chains showed weak interaction. The association kinetics of Asn-linked glycopeptides from bovine asialofetuin to recombinant macrophage lectin was determined by surface plasmon resonance spectroscopy. The results indicate k(assoc) value of 1.63 x 10(4) M-1 s-1. The calculated value for Ka was 6.20 x 10(7) M.

Strong affinity of Maackia amurensis hemagglutinin (MAH) for sialic acid-containing Ser/Thr-linked carbohydrate chains of N-terminal octapeptides from human glycophorin A

The interaction of the Maackia amurensis hemagglutinin (MAH) with various glycopeptides and oligosaccharides was investigated by means of immobilized lectin affinity chromatography. An amino terminal octapeptide obtained from human glycophorin A having three Neu5Ac alpha 2-->3Gal beta 1-->3(Neu5Ac alpha 2-->6)GalNAc tetrasaccharide chains, designated as CB-II, was found to have an extremely strong affinity for MAH. Therefore, it is strongly suggested that hemagglutination by MAH was caused by its interaction with Ser/Thr-linked carbohydrate chains of human glycophorin A on erythrocyte membranes.

Conformational analysis of the amino termini (5 residues) of human glycophorin AM and AN: differentiation of the structural features of the TN and T antigenic determinants in relation to their specificity

The N-terminus of glycophorin A, the main transmembrane erythrocyte glycoprotein responsible for the MN blood-group specificity, has been modelled. As the minimum size of the protein recognised by the antiglycophorin A antibodies is the N-terminal glycopentapeptide, attention was focused on the TN and T antigenic determinants of this size in order to determine wether differences in 3D structure exist and how a specific response with different antibodies is induced.

One- and two-dimensional NMR studies of the N-terminal portion of glycophorin A at 11.7 Tesla

One- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy (at 11.7 Tesla) was used to gain some structural and spectral information about glycophorin AM, glycophorin AM tryptic glycopeptide, a related pentapeptide, and two related monoglycosylated pentapeptides. The protein spectral information suggests that the highly glycosylated N-terminus of glycophorin does not seem to possess a unique tertiary structure. Furthermore, the spectral information provided by the carbohydrate residues also indicates that there is no strong carbohydrate-protein interaction resulting in a unique tertiary structure. This result does not preclude any unique protein-carbohydrate interactions. For the small monoglycosylated pentapeptide containing alpha-D-GalNAc attached to Thr, a unique NOESY cross-peak was observed between the anomeric proton and the beta-proton of Thr. A cross-peak between the beta-proton of Ser and the anomeric proton was not observed for a related monoglycosylated pentapeptide containing alpha-D-GalNAc O-linked to Ser.

Flow cytometric analysis of erythrocyte populations in Tn syndrome blood using monoclonal antibodies to glycophorin A and the Tn antigen

Flow cytometric analysis employing monoclonal antibodies to the Tn antigen and glycophorin A was used to characterize the erythrocyte populations present in blood samples from individuals with Tn syndrome. Four monoclonal antibodies specific for the Tn antigen, Gal-NAc monosaccharide, on human erythrocytes were obtained from a fusion of splenocytes from a Biozzi mouse immunized with red cells from a Tn individual. These monoclonal antibodies specifically recognize GalNAc monosaccharide sites located on the erythrocyte cell surface sialoglycoproteins, glycophorin A and glycophorin B, and do not bind to fixed normal red cells presenting the Neu-NAc alpha 2-3Gal beta 1-3(NeuNAc alpha 2-6)GalNAc alpha 1-O-Ser(Thr) tetrasaccharide or to fixed neuraminidase-digested cells presenting the Gal-GalNAc disaccharide. The percentages of Tn-positive red cells in samples from six unrelated Tn donors ranged from 28 to 99%. Binding of the glycophorin A-specific monoclonal antibodies showed that the erythrocytes composing the Tn-negative fraction presented normal amounts of the M and N epitopes on glycophorin A. The presumed somatic mutational origin of Tn-positive cells was tested in blood samples from five normal donors; three possible Tn cells were observed after analysis of a total of 1.1 x 10(7) erythrocytes, suggesting that the frequency of such cells in normal individuals is less than 1 x 10(-6).

Demonstration of glycosylation variants of human fibrinogen, using the new technique of glycoprotein lectin immunosorbent assay (GLIA)

A new highly sensitive method, incorporation the ELISA technique (enzyme-linked immunosorbent assay), is described for the quantitation of the glycan residues of glycoproteins. With the aid of this "glycoprotein-lectin immunosorbent assay (GLIA)", it is possible to determine the nature of the glycan residues of a single protein in a glycoprotein mixture, without prior purification. The GLIA can be used for the accurate determination of the inhibitor constant for the interaction of any monosaccharide with any lectin. Using the described technique, glycosylation of human fibrinogen from plasma and amniotic fluid were compared. In fibrinogen from amniotic fluid a "fetal" glycosylation type could be demonstrated. In addition, evidence is presented for the first time that plasma fibrinogen possesses (GlcNAc beta 1----4Man beta) residues (bisecting GlcNAc) and O-glycosidically bound carbohydrate units. Preliminary results were published as abstract (E. Köttgen et al. (1988) Fresenius Z. Anal. Chem. 330, 448).

On the utility of 13C-n.m.r. spectroscopy in the identification of the primary structures of manno-oligosaccharides and glycopeptides

The utility of 13C-n.m.r. spectroscopy in the identification of the primary structures of mannose-containing glycans is investigated. Unlike 1H resonances where the chemical shifts reflect multiple short- and long-range effects, the chemical shifts of 13C resonances are dependent largely upon short-range effects classified as glycosylation (linkage) and substitution effects. These effects are parametized for glycans composed of mannose and encoded in a FORTRAN algorithm. Applications of this program to "unknown" sets of experimental chemical shifts for the resonances of anomeric carbons gave the following conclusions. (1) This program can be used to produce a sub-set of possible structures inclusive of the "known" structure. (2) For other than simple oligosaccharides, it is unlikely that a single structure is consistent with the data for anomeric carbons alone, even when the linkage composition of the glycan has been assessed from other spectral data. (3) When used in conjunction with other chemical techniques, this program can provide a powerful tool for primary analysis of the structure of mannose-containing glycans.

Carbohydrate-binding specificities of five lectins that bind to O-Glycosyl-linked carbohydrate chains. Quantitative analysis by frontal-affinity chromatography

The carbohydrate-binding specificities of lectins purified from Agaricus bisporus (ABA-I), Arachis hypogaea (PNA), Bauhinia purpurea (BPA), Glycine max (SBA), and Vicia villosa (VVA-B4) have been studied by affinity chromatography on columns of the immobilized lectins, and quantitatively analyzed by frontal affinity chromatography. These five lectins could be classified into two groups with respect to their reactivities with typical mucin-type glycopeptides, beta-D-Galp-(1----3)-alpha-D-GalpNAc-(1----3)-Ser/Thr (2) and alpha-D-GalpNAc-(1----3)-Ser/Thr (3). One group, which consists of ABA-I, PNA, and BPA, preferentially binds to 2, and the other, which consists of SBA and VVA-B4, shows higher affinity for 3 than for 2. Among the lectins tested, only ABA-I was found to bind to a sialylated glycopeptide, whic which was prepared from human erythrocyte glycophorin A and contains three three tetrasaccharide chains having the structure of alpha-NeuAc-(2----3)-beta-D-GAlp-(1----3)-NeuAC-(2----6)]-alpha-D-Galp NAc-(1----, with an association constant of 15 microM, whereas the association constants of the other four lectins for this sialylated glycopeptide were less than 3.5 mM. On the other hand, removal of the beta-D-galactopyranosyl group from a glycopeptide containing sequence 2 resulted in decreased association constants for the three lectins of the first group, especially ABA-I and PNA. The two lectins of the second group showed a high affinity for 3, but SBA preferentially interacted with oligosaccharides containing the alpha-D-GalpNAc-(1----3)-beta-D-Galp-(1----3)-D-GlapNAc sequence, prepared from a blood group A-active oligosaccharide.

Number and distribution density of ABH and MN antigen sites on young and old human erythrocyte surfaces

There were no differences in the number of A and M antigen sites between young and old human erythrocyte surfaces. No essential differences in the number of A1, N and Vicia graminea N antigen sites could be observed between young and old erythrocytes. The number of B and H antigen sites on cell surface was significantly higher in young erythrocytes than in old ones. The distribution density of A and M antigen sites on young erythrocyte was remarkably higher than that on old ones. Compared with young erythrocytes, significant increases in the distribution density of A1, B, H, N and Vicia graminea N antigen sites were observed in aged erythrocytes. It is suggested from these and other observations that human erythrocyte aging is accompanied by elimination of a small amount of B and H antigens from cell membranes, while A, A1, M, N and Vicia graminea N antigens are not released from cell membranes during in vivo aging.

Measurements of the frequency of human erythrocytes with gene expression loss phenotypes at the glycophorin A locus

An assay method is described for determining the frequency of human erythrocytes having a gene expression loss phenotype at the glycophorin A locus presumably due to in vivo somatic mutational events in erythroid precursor cells. Monoclonal antibodies specific for the M and N glycophorin A alleles are used to identify variant cells that lack the expression of one allele in blood samples from MN heterozygotes. Flow cytometry and sorting are used to enumerate and purify variant cells. Using three different antibody combinations which are sensitive to the loss of either the M or the N allele, we find that variant cells occur at a frequency of 1 X 10(-5) in normal donors. We also detect variant cells with an apparent homozygous phenotype suggesting that events leading to homozygosity may occur at similar frequencies to gene loss events. Significant increases in variant cell frequency are observed in cancer patients after exposure to mutagenic chemotherapy drugs.

Possible role of the carbohydrate residues on the structure of the N-terminus of glycophorin AM

Natural-abundance 13C nuclear magnetic resonance (13C-n.m.r.) was used to study the effect of monoglycosylation on the structure and dynamics of a pentapeptide related to the N-terminus of glycophorin AM. The results of this study indicate that a single point of glycosylation, on the pentapeptide, can significantly affect its structure. Moreover, glycosylation of this pentapeptide also affects its dynamic motion in solution. This study further defines the role that the carbohydrate residue plays in determining the structure about the N-terminus of glycophorin AM.

Evidence for unique homologous peptide sequences around the glycosylated seryl and threonyl residues in polysialoglycoproteins isolated from the unfertilized eggs of the Pacific salmon Oncorhynchus keta

Structures of glycopeptides obtained by exhaustive Pronase digestion of high molecular weight (1.7 X 10(5)) salmon egg polysialoglycoprotein have been elucidated. Six principal glycopeptides isolated by gel chromatography and DEAE-Sephadex A-25 chromatography in the absence or presence of borate ion were analyzed for their carbohydrate and amino acid composition, as well as amino acid sequence, and found to be of two distinct types: glycotripeptides, Thr*-Ser*-Glu, and glycotetrapeptides, Thr*-Gly-Pro-Ser, where an asterisk indicates the amino acid residues to which either the Gal beta 1----3GalNAc or Fuc alpha 1----3GalNAc beta 1----3Gal beta 1----4Gal beta 1----3GalNAc chain is attached. Their final yield corresponds to 64% of the original desialylated glycoprotein. In view of the simple amino acid composition of salmon egg polysialoglycoprotein (molar ratio Asp2Thr2Ser3Glu1Pro1Gly1Ala3) and the result of alkaline beta-elimination indicating three carbohydrate units linked to two of two threonine and one of three serine residues, a unique primary structure comprising repetitive sequences of the above two types of glycopeptides, which are interspersed by short nonglycosylated peptides consisting of alanine and aspartic acid, has been proposed for the core protein. The molecular secondary ion mass spectra of underivatized glycopeptides were used to obtain their structural information. The anomeric configuration of the proximal sugar-peptide linkages was proven to be alpha by proton nuclear magnetic resonance spectroscopy. This is the first systematic reported study of O-glycosidically linked glycopeptides by these instrumental methods.

Purification and characterization of four isolectins of mushroom (Agaricus bisporus)

Four lectins were purified from a mushroom (Agaricus bisporus) by ammonium sulfate fractionation, anion-exchange chromatography, affinity chromatography on bovine submaxillary mucin-Sepharose 4B and preparative isoelectric focusing. They were designated as ABA-I (pI 6.70), II (pI 5.98), III (pI 5.69) and IV (pI 5.53). Polyacrylamide gel electrophoresis of each lectin in the presence of sodium dodecyl sulfate gave a single band with an apparent molecular mass of 16 000 Da. Sedimentation equilibrium analysis suggested that each lectin is a tetramer of subunits. The four lectins were found to have quite similar carbohydrate-binding specificities. The hemagglutination activities of the lectins were effectively inhibited by bovine and porcine submaxillary mucins (BSM and PSM), and NH2-terminal glyco-octapeptides obtained by cyanogen bromide cleavage of human erythrocyte glycophorin A. In addition, desialylated PSM-glycopeptides were more potent inhibitors than untreated PSM-glycopeptides. Among monosaccharides and their glycosides, only methyl N-acetyl-alpha-galactosaminide inhibited lectin binding at a high concentration, but a synthetic oligosaccharide, O-beta-galactopyranosyl-(1----3)-O-(2-acetamido-2-deoxy-alpha-D- galactopyranosyl)-N-tosyl-L-serine, was a strong inhibitor.

Identification of the major human sialoglycoprotein from red cells, glycophorin AM, as the receptor for Escherichia coli IH 11165 and characterization of the receptor site

The pyelonephritogenic Escherichia coli strain 1 H 11165 specifically agglutinates human erythrocytes carrying the M blood group antigen. The polymorphic forms of this antigen, M and N, are located in the NH2-terminal region of the major human red-cell sialoglycoprotein, glycophorin A. Radioactively labeled glycophorin A from M cells specifically bound to the bacteria. Purified glycophorin AM, but not glycophorin AN, efficiently inhibited for binding. Mild periodate treatment oxidized the NH2-terminal serine in glycophorin AM and this resulted in loss of binding to the bacteria. High concentrations of serine and alkali-labile oligosaccharides derived from glycophorin AM inhibited the binding, whereas the synthetic M-specific NH2-terminal pentapeptide Ser-Ser-Thr-Thr-Gly did not. Neuraminidase treatment of glycophorin AM did not destroy the binding. The most efficient inhibition of binding was observed with the N-terminal glyco-octapeptide obtained from glycophorin AM by CNBr cleavage. This peptide contains both the essential serine residue and the alkali-labile oligosaccharides, which both are recognized by the bacterium.

Monoclonal antibodies specific for the M- and N-forms of human glycophorin A

Four mouse monoclonal antibodies directed against the red cell membrane protein glycophorin A have been isolated and characterized. They are produced by hybridomas derived from SP2/0 myeloma cells and spleen cells from Biozzi mice immunized with a mixture of human erythrocytes from homozygous blood group M and N individuals. These antibodies recognize and bind to purified glycophorin A and to glycophorin on the red cell surface. All are of the IgGl, kappa light chain subclass and bind to determinants presented on the 39 amino acid, trypsin-sensitive, N-terminal peptide of glycophorin A. Three display differential specificities for the two allelic forms of glycophorin A; two are exquisitely specific for the M-form and one preferentially binds the N-form. Treatment of red cells with neuraminidase, which removes N-acetylneuraminic acid from glycophorin A, abolishes the binding of these three antibodies. The binding of the N-specific antibody is also sensitive to modification of the amino-terminal residue of the antigen. The fourth antibody binds equally well to both the M- and N-forms as well as to neuraminidase-treated red cells; thus it recognizes a public, N-acetylneuraminic acid independent glycophorin A determinant.

13C-N.m.r.-spectral study of the mode of binding of Gd3+ to various glycopeptides

Natural-abundance, 13C-n.m.r. spectroscopy was used to study the mode of binding of Gd3+ to mono-O-glycosylated L-serine and tripeptides variously composed of Gly and L-Thr. When the amino and carboxyl groups of the amino acid are not blocked, strong interaction of Gd3+ with them is observed; this is also readily apparent with some related, nonglycosylated peptides. When the amino and carboxyl groups of the amino acid are blocked, noticeable interaction of Gd3+ with the glycosidic oxygen atom (O-3) and O-2' for the glycopeptide containing alpha-D-Galp, and with O-3 and N-2' for the glycopeptide containing alpha-D-GalpNAc, is observed. Weak interactions are also possible with O-4' and O-6' of the glycosyl groups. Although the amino acids were protected, these metal ion-carbohydrate interactions may still be mediated, to some extent, by the acetyl protecting the amino group and by the ester group on the amino acid.

Blood group antigens: synthesis of TN glycopeptide related to human glycophorin AM

Three 2-acetamido-2-deoxy-alpha-D-galactopyranose residues attached to Ser2, Thr3 and Thr4 of the amino-terminal portion of glycophorin AM are responsible for the so-called TN blood group specificity. In a continuation of earlier work, this report describes the first chemical synthesis of the triglycosylated pentapeptide H2N-Ser1-Ser2*-Thr3*-Thr4*-Gly5-OH, in which (*) represents the 2-acetamido-2-deoxy-alpha-D-galactopyranosyl residue. This compound constitutes the G1-G5 sequence of the amino-terminal portion of human glycophorin AM, the main erythrocyte membrane glycoprotein. The above compound was obtained by a stepwise peptide coupling strategy alternatively using aminoacids and adequately protected and/or activated O-glycosyl-aminoacids. Since the desired sequence possesses both unglycosylated and glycosylated serine this route was preferred to that in which the glycosylation is carried out on the preformed pentapeptide H2N-Ser-Ser-Thr-Thr-Gly-OH. Carbohydrate residues were introduced into the sequence as 2-azido-2-deoxy-alpha-D-galactopyranosyl-L-serine and L-threonine derivatives. The azido functions were further converted into the corresponding acetamido groups by treatment of the final triglyco-pentapeptide with sodium borohydride in the presence of nickel chloride followed by acetylation.

Specific 13C reductive methylation of glycophorin A. Possible relation of the N-terminal amino acid and the lysine residues to MN blood group specificities

Heterozygous and homozygous glycophorin A were partially and fully reductively methylated with 13C-enriched formaldehyde in the presence of sodium cyanoborohydride. Total reductive methylation modified the five lysine residues (to produce N epsilon,N-[13C]dimethyl lysine) and the N-terminal amino acid residues (N alpha,N-[13C]dimethyl serine and leucine) of glycophorins AM and AN, respectively. 13C-NMR spectra of these species indicated that the 13C-enriched methyl carbons of the five lysyl derivatives all occur at 44.1 ppm downfield from Me4Si. Titration results indicate that the pK alpha of these methylated lysines is greater than 10. The chemical shift equivalent methyl resonances of the 13C-enriched methylated N-terminal Leu derivative were found to occur at 42.8 ppm downfield from Me4Si and exhibited a normal pH titration behavior (pK alpha approximately 7.4). The methyl resonances of the N alpha,N-[13C]dimethyl Ser derivative, on the other hand, were found to exhibit chemical shift nonequivalence, indicating rotational constraints about the C alpha-N bond. The linewidths of the two methyl resonances were also found to be considerably different; this phenomenon could be eliminated by running spectra of the sample (pH approximately 5.0) at elevated temperatures (75 degrees C). This result suggested that for the N alpha,N-[13C]dimethyl Ser derivative of glycophorin AM, hindered rotation must occur about one of the N alpha-13CH3 bonds. This structural difference at the N-terminal residue of glycophorins AM and AN may be related to the MN blood group determinants displayed by these related glycoproteins.

The chimpanzee M blood-group antigen is a variant of the human M-N glycoproteins

Chimpanzee erythrocytes express strong M but weak, occasional N blood-group activity, as detected by anti-M and anti-N reagents. We have found that the M activity is carried by a major membrane glycoprotein that is similar but not identical to the human MM glycoprotein (glycophorin A). We have isolated and characterized this glycoprotein from erythrocyte membranes of four individual chimpanzees. The purified glycoproteins strongly inhibited agglutination of M cells by rabbit anti-human M sera and only weakly inhibited the agglutination of N cells by rabbit anti-human N sera. They also displayed medium-to-strong inhibitory activity against chimpanzee iso- and crossimmune antisera tested with chimpanzee erythrocytes of various V-A-B-D and Wc specificities, which are known as chimpanzee extensions of the human type M-N system and the Miltenberger counterpart, respectively. Each glycoprotein was cleaved with CNBr into three fragments, whose size, solubility, and composition were analogous to those obtained by similar treatment of the human M-N antigens. The amino-terminal fragment was found to be a glycooctapeptide whose amino acid composition and partial sequence indicated that it is an intermediate form of the human M and N glycooctapeptides. Its carbohydrate content comprised two threonine-linked saccharide units that, although similar in composition to the human threonine-linked units, were fewer in number than the three units found in the corresponding human glycooctapeptides. Structural similarities to the human antigens strongly suggest that the amino terminus bears the major antigenic determinants of the molecule, and the occurrence in this region of numerous, albeit rare, variants among humans and in chimpanzees indicates that the corresponding coding sequence of the structural gene is particularly susceptible to mutational events. We conclude that the chimpanzee M gene product is a variant of the human type and that the chimpanzee gene is an allele of the human polymorphic M-N locus.

13C-N.M.R.-spectral study of the binding of Gd3+ to glycophorin

Natural-abundance, 13C-n.m.r. spectroscopy was used to study the binding of Gd3+ to glycophorin, and also to the tetrasaccharides isolated from glycophorin after treatment of the glycoprotein with NaOH-NaBH4. Gd3+ binds to the tetrasaccharide (both in the isolated, reduced form and when still attached to the native glycoprotein), and, especially, to the alpha-NeuAc residues. In order to cause severe line-broadening of the 13C resonances of alpha-NeuAc, the ratios of the alpha-NeuAc residues of glycophorin, and of the isolated, reduced tetrasaccharide, to Gd3+ were much higher than that needed for causing similar broadening for 2-O-methyl-alpha-NeuAc-Gd3+ solutions. These results indicate that the other carbohydrate residues of the tetrasaccharide may be involved in the binding of Gd3+, producing a stronger metal-ion-binding effect.