Liquid chromatography "on-flow" 1H nuclear magnetic resonance on native glycosphingolipid mixtures together with gas chromatography/mass spectrometry on the released oligosaccharides for screening and characterisation of carbohydrate-based antigens from pig lungs

NMR spectroscopy as a tool to close the gap on metabolite characterization under MIST

Withdrawals from the market due to unforeseen adverse events have triggered changes in the way therapeutics are discovered and developed. This has resulted in an emphasis on truly understanding the efficacy and toxicity profile of new chemical entities (NCE) and the contributions of their metabolites to on-target pharmacology and off-target receptor-mediated toxicology. Members of the pharmaceutical industry, scientific community and regulatory agencies have held dialogues with respect to metabolites in safety testing (MIST); and both the US FDA and International Conference on Harmonisation have issued guidances with respect to when and how to characterize metabolites for human safety testing. This review provides a brief overview of NMR spectroscopy as applied to the structure elucidation and quantification of drug metabolites within the drug discovery and development process. It covers advances in this technique, including cryogenic cooling of detection circuitry for enhanced sensitivity, hyphenated LC-NMR techniques, improved dynamic range through new solvent-suppression pulse sequences and quantitation. These applications add to the already diverse NMR toolkit and further anchor NMR as a technique that is directly applicable to meeting the requirements of MIST guidelines.

Comparison of LC-NMR and conventional NMR for structure elucidation in drug metabolism studies

Liquid chromatography-nuclear magnetic resonance (LC-NMR) has proven to be a useful technique for the structure elucidation of novel metabolites from pharmaceutical compounds. Proponents of LC-NMR tout the advantage of eliminating the step of a separate chromatographic isolation. However, the advantages of directly coupling NMR and HPLC instrumentation must be weighed against compromises in performance made to each technique to achieve a hyphenated system. While significant advances have been made in LC-NMR technology, a strong case can be made that HPLC purification of metabolites followed by conventional tube NMR is equally useful. It is relatively rare that one approach will be successful and the other not. The fundamental consideration is whether there is sufficient chromatographic expertise in the NMR laboratory to adequately design and execute appropriate experiments such that a pure chromatographic peak will be produced in the hyphenated system. Due to speed and sensitivity differences between NMR spectroscopy and mass spectrometry, liquid chromatography/mass spectrometry (LC/MS) continues to be the front-line approach for the structure elucidation of metabolites.

Phytochemical analysis of the Southern Australian marine alga, Plocamium mertensii using HPLC-NMR

INTRODUCTION

Over the last decade HPLC-NMR has become a robust analytical technique that has been applied to a wide range of studies, particularly plant extracts. There have been only a few applications of the use of HPLC-NMR to profile marine natural product extracts and no studies involving marine algae. The marine alga selected for this study belongs to the genus Plocamium, which is a well known source of polyhalogenated monoterpenes.

OBJECTIVE

To chemically profile the marine alga P. mertensii, using a combination of on-line (HPLC-NMR) and off-line approaches.

METHODOLOGY

P. mertensii was extracted with 3:1 methanol-dichloromethane and subsequently partitioned into dichloromethane and methanol-soluble fractions. The methanol partition was exclusively profiled by HPLC-NMR (on-flow, stop-flow and time-slice experiments) whilst the dichloromethane partition was investigated by conventional isolation and purification procedures.

RESULTS

HPLC-NMR analysis of the methanol partition partially identified the presence of the major compounds 7, 13, 27 and 37, the structures of which were unequivocally elucidated by off-line characterisation of the dichloromethane partition. Two additional minor metabolites (3 and 8) present in the dichloromethane partition were only tentatively identified as these co-occurred in a mixture with compounds 7 and 13. As a result of this study a number of chemical shift reassignments were made for compound 37.

CONCLUSION

This is one of the few reports describing the application of HPLC-NMR to rapidly profile or dereplicate a marine organism and the first application of HPLC-NMR to successfully profile the chemistry of a marine alga.

Glycolipid binding epitopes involved in adherence of the periodontitis-associated bacterium Porphyromonas gingivalis

The ability of the periodontal pathogen Porphyromonas gingivalis to use different glycolipid structures as receptors has previously been demonstrated. The bacterium adhered to acid and nonacid glycolipids originating from human organs and to nonacid glycolipids of porcine origin. The aim of the present study was to analyze these binding epitopes by structural characterization. Glycolipid fractions with positive bacterial binding from e.g. human and porcine origin, were purified by the high performance liquid chromatography technique and thereafter used in bacterial overlay assays with (35)S-labeled P. gingivalis. Purified fractions with positive binding were structurally characterized by proton nuclear magnetic resonance spectroscopy. Complementing thin-layer chromatograms and bacterial overlay assays with pure reference glycolipid fractions and competition experiments with lactose were performed to define potential receptors. The P. gingivalis binding epitopes, including cerebrosides with nonhydroxy fatty acids, lactosylceramide with hydroxy fatty acids, sulfatides, lacto-, neolacto- and gangliotetraosylceramides, are in several instances similar to those found for other bacteria, e.g. H. pylori, H. influenzae and N. meningitidis. In addition P. gingivalis also bound to the Galalpha4Gal epitope of the globo series of glycolipids. In the future these results may be valuable for development of new treatment strategies, such as anti-adhesion therapies and vaccines specifically directed against P. gingivalis infection.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

Analysis of N-glycans of pathological tau: possible occurrence of aberrant processing of tau in Alzheimer's disease

In a previous study [Wang et al. (1996) Nat. Med. 2, 871-875], Wang et al. found (i) that abnormally hyperphosphorylated tau (AD P-tau) isolated from Alzheimer's disease (AD) brain as paired helical filaments (PHF)-tau and as cytosolic AD P-tau but not tau from normal brain were stained by lectins, and (ii) that on in vitro deglycosylation the PHF untwisted into sheets of thin straight filaments, suggesting that tau only in AD brains is glycosylated. To elucidate the primary structure of N-glycans, we comparatively analyzed the N-glycan structures obtained from PHF-tau and AD P-tau. More than half of N-glycans found in PHF-tau and AD P-tau were different. High mannose-type sugar chains and truncated N-glycans were found in both taus in addition to a small amount of sialylated bi- and triantennary sugar chains. More truncated glycans were richer in PHF-tau than AD P-tau. This enrichment of more truncated glycans in PHF might be involved in promoting the assembly and or stabilizing the pathological fibrils in AD.

Combination of matrix solid-phase dispersion extraction and direct on-line liquid chromatography-nuclear magnetic resonance spectroscopy-tandem mass spectrometry as a new efficient approach for the rapid screening of natural products: application to the total asterosaponin fraction of the starfish Asterias rubens

A combination of matrix solid-phase dispersion extraction (MSPD) and LC-NMR-MS hyphenation is proposed as a rapid screening method of natural products for unknown compounds. In this report, this new analytical approach is applied for the first time. MSPD represents a significant simplification compared to classical extraction procedures and is thus an excellent complement to the fast and powerful LC-NMR-MS: MSPD yields extracts suitable for LC-NMR-MS in one simple preparation step, while LC-NMR-MS yields a wealth of information in one single chromatographic run. The suitability of this technique to characterise glycosidic compounds in the molecular mass range of 1200 to 1400 a.m.u. is demonstrated. The information on the number of exchangeable protons provided by an additional back-exchange experiment proved to be particularly valuable for structural elucidation. The possibility of semi-quantitative LC-NMR measurements through methyl signals H(3)-18 and 19 of the steroidal skeleton is demonstrated and is ensuingly used to provide relative quantitative data of the steroid oligosaccharide fraction.

Total synthesis of the natural antigen involved in the hyperacute rejection response to xenotransplants

The major glycosphingolipid in pig vascular endothelium is the ceramide pentasaccharide Gal alpha(1 --> 3)Gal beta(1 --> 4)GlcNAc beta(1 --> 3)Gal beta(1 --> 4)Glc beta(1 --> 0)Cer (1), which binds specifically to human anti-Gal antibody and is involved in the hyperacute rejection response in xenotransplantation from pig to man. The synthesis of 1 and its methyl glycoside 2 is described.