Separation of gamma-globulins from porcine plasma by reversed micellar extraction

Factors affecting the separation of gamma-globulins from porcine plasma using reversed micelles were screened based on a fractional factorial design. The optimal processing conditions for obtaining the maximum yield of gamma-globulins under various constraints of product purity were determined using response surface methodology (RSM) and nonlinear programming. Results showed that the pH and sodium chloride concentration of the aqueous phase, and the concentration of surfactant (bis-(2-ethylhexyl) sulfosuccinate sodium salt, AOT) of the organic phase were the most important factors affecting the extraction performance. An eighty-five percent product purity and ninety-seven percent yield were obtained under the extraction conditions of 400 mM NaCl, 350 mM AOT, and pH 7.0. The extract exhibited immunological reactivity against anti-pig IgG.

The extracytoplasmic domain of the erythropoietin receptor forms a monomeric complex with erythropoietin

We have generated a truncated form of the erythropoietin receptor (EPO-R), the extracytoplasmic ligand-binding domain, that is secreted from a transfected Chinese hamster ovary (CHO) cell line. The truncated receptor is readily purified from CHO conditioned media as a 33-Kd glycosylated protein, which is converted to a 25-Kd species upon treatment with protein N-glycan glycosidase F. Cross-linking of radioiodinated EPO to the secreted receptor yielded a complex of 72 Kd. Also, the growth of the EPO-dependent cell line, FDCPE, was inhibited in a dose-responsive manner by the truncated receptor. The complex of the secreted receptor and EPO was isolated by gel filtration and shown to be a one-to-one complex of the receptor and growth factor by quantitative amino terminal sequencing. Finally, analysis of the interaction of the receptor and growth factor by gel filtration indicated an apparent dissociation constant of 1.1 nmol/L for the truncated receptor.

The distribution of glycan structures in individual N-glycosylation sites in animal and plant glycoproteins

Glycopeptides representing each individual N-glycosylation site in six animal and plant glycoproteins (ovoinhibitor and ovotransferrin, orosomucoid, antitrypsin, phaseolin, and phytohemagglutinin) have been isolated and compared by mass spectrometric analysis. Since the isolation step separates each individual peptide regardless of the nature of the glycan attached to it, it is possible to observe the entire spectrum of glycans associated with each site from the mass spectrum of the corresponding glycopeptide. The three glycosylation sites in ovoinhibitor have very similar but not identical glycans; they are significantly different from those observed in the single site of ovotransferrin. The three sites in serum antitrypsin also have quite similar glycans, whereas the five sites in orosomucoid show considerable variation in both the nature and the relative amount of glycans. The two plant glycoproteins each have two sites with very different glycan structures. Except for the first and third glycosylation sites of antitrypsin which were found to have remarkably homogeneous glycans (97 and 90% of a biantennary complex structure), all the individual glycosylation sites contained heterogeneous mixtures of glycan structures. The results support the proposition that each N-linked glycan in a glycoprotein is affected by its unique protein environment to such an extent that each one may be displayed to the processing enzymes as a unique structural entity. On the basis of a limited number of observations of the glycan interfering with chymotryptic but not tryptic cleavage in the proximity of the glycan attachment site, it is proposed that hydrophobic interactions between the protein and the glycan may be involved in the conformational modulation of the glycans.

New methods for rapid separation and detection of oligosaccharides from glycoproteins

Ion exchange chromatography at high pH with pulsed amperometric detection of the eluted glycans permitted resolution of the eight major components in the mixture of asparagine-linked glycans derived from the single glycosylation site of ovalbumin. The individual glycans were first partially separated according to size, and were characterized by fast atom bombardment-mass spectrometry and specific enzymatic degradation with beta-galactosidase and endoglycosidase H; subnanomolar quantities of all eight components could subsequently be unequivocally identified in the elution diagram. To ascertain that the chromatographic separation of the ovalbumin glycan mixture was not restricted to the asparagine-linked glycans, it was established that the corresponding mixture of reducing oligosaccharides (asparagine removed) or Asn-oligosaccharides blocked at the alpha-amino group with biotin gave very similar resolution of the eight glycans. In the absence of pure reference compounds, the quantification of the different glycans by the amperometric detection system was evaluated by comparing the electrochemical signal to the molecular ion peak intensity in the mass spectrometer. With one exception, the two methods were in good agreement, which suggests that the amperometric detection system yields a valid quantitative estimate for most of these chemically related compounds.

The covalent structure of individual N-linked glycopeptides from ovomucoid and asialofetuin

In order to explore whether individual N-linked glycans in a given glycoprotein may be processed to different end products and at the same time prepare a number of well characterized glycopeptides as substrates for glycopeptide hydrolases, we have prepared the individual glycopeptides representing the four major glycosylation sites in ovomucoid and the three sites in asialofetuin. The individual glycopeptides were characterized by amino acid sequence determination before and after removal of the glycan by peptide:N-glycanase (amidase), and the liberated glycans were subjected to mass spectrometric analysis. As expected from available sugar analyses of the individual glycans in ovomucoid, no major differences were detected between the four glycosylation sites in this glycoprotein, but a definite trend toward less processed (less extensively branched) species was observed in going from site 1 to 4. In fetuin, for which the glycan pool is known to be made up of about two-thirds triantennary and one-third biantennary structures, the analysis of the three glycopeptides gave triantennary to biantennary ratios of 75/25, 67/33, and 70/30, respectively, demonstrating that the three sites are processed to a very similar, albeit perhaps not identical, extent. All the glycopeptides obtained in these studies, including the CNBr-produced glycopeptide from ovalbumin, were purified by a set series of steps, gel filtration on Sephadex G-50 followed by ion-exchange chromatography on DE52 and/or reverse phase high performance liquid chromatography. Based on the results, these procedures appear to have general application for the preparation of glycopeptides.

Purification and characterization of two glycopeptide hydrolases from jack beans

Two glycopeptide hydrolases, an endo-beta-N-acetylglucosaminidase and peptide:N-glycanase (amidase), have been isolated from defatted jack bean meal by standard procedures involving differential solubility and column chromatography. The purified products appear to be free of contaminating proteases and exoglycosidases, and their substrate specificity has been explored with regard to both glycan and peptide structure of the substrates. The endoglycosidase appears to be specific for high mannose glycans; no hydrolysis of either hybrid or complex glycans has been observed. It shows limited activity with two intact glycoproteins, ribonuclease B and yeast invertase, and gives optimal rate with glycopeptides. Free glycan-Asn derivatives are poor substrates in comparison with glycopeptides or glycan-Asn derivatives where the alpha-amino group has been dansylated. The amidase will liberate both high mannose, hybrid, and asialo-complex glycans from both proteins and peptides, but many glycans in intact proteins or in long peptides are resistant to the amidase and become active as substrates only after further proteolytic cleavage. The best substrates appear to be those with the glycosylated asparagine no more than 4-5 residues in from either the NH2- or COOH-terminal end of the peptide. Sialylated glycans do not appear to be released by the amidase.

Effects of the protein matrix on glycan processing in glycoproteins

In the biosynthesis of glycoproteins containing asparagine-linked glycans, a number of regulatory factors must be involved in converting the single glycan precursor into the variety of different final structures observed in different eukaryotic species. Among these factors are the kind of glycan-processing enzymes available in the Golgi apparatus of different cells, the specificity and regulatory properties of these enzymes, and the unique properties of the protein matrix in which a given glycan resides during the biosynthetic processing. In examining the role of this latter regulatory factor, we have considered a simplified model in which a few key steps are common to all cells, regardless of the nature of the processing enzymes available. The protein-bound oligomannose precursor Man8GlcNAc2-, arriving in the Golgi after the initial trimming in the endoplasmic reticulum (ER), first undergoes a series of preprocessing steps to yield Man5GlcNAc2- in animals and plants or Man13-15GlcNAc2- in yeast. At this stage the key commitment step--to process or not to process--determines whether the above intermediates will remain as unprocessed oligomannose structures or be initiated into a new series of reactions to yield processed structures characteristic of the organisms involved (complex or hybrid for vertebrates, polymannose for yeast, xylosylated glycans for plants and some invertebrates, or Man3GlcNAc2- structures for other invertebrates). It is proposed that this commitment step, along with the obligatory preprocessing steps, is regulated primarily by each glycan's unique exposure on its protein matrix. Subsequent processing steps leading to complex or hybrid structures, fucosylation, extent of branching, and specific structures at the nonreducing terminals are most likely determined primarily by the enzyme makeup of the individual processing machineries, but with the protein matrix still playing a significant role.