Ute Bahr

Matrix-assisted ultraviolet laser desorption of non-volatile compounds

Abstract The use of a matrix for pulsed ultraviolet laser desorption mass spectrometry is shown to extend its applicability into the range of larger, thermally labile biomolecules. The matrix compounds tested show strong resonance absorption at the laser wavelength of 266 nm and can be either solid or liquid. Characteristic features of the matrix-assisted LD mass spectra are high quasimolecular ion yield with little or no fragmentation and only a few signals in the low mass range. The matrix effect is discussed in the light of three possible mechanisms: an effective and controllable energy transfer of the laser energy to the condensed phase by predominantly one-photon absorption, the creation of a uniform and soft disintegration of the condensed phase at moderate irradiances independent of the analytes individual properties, and an enhancement of ionization yield by protonation via excited state molecules.

Identification of (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate as a major activator for human γδ T cells in Escherichia coli

The gcpE and lytB gene products control the terminal steps of isoprenoid biosynthesis via the 2‐C‐methyl‐D‐erythritol 4‐phosphate pathway in Escherichia coli. In lytB‐deficient mutants, a highly immunogenic compound accumulates significantly, compared to wild‐type E. coli, but is apparently absent in gcpE‐deficient mutants. Here, this compound was purified from E. coli ΔlytB mutants by preparative anion exchange chromatography, and identified by mass spectrometry, 1H, 13C and 31P NMR spectroscopy, and NOESY analysis as (E)‐4‐hydroxy‐3‐methyl‐but‐2‐enyl pyrophosphate (HMB‐PP). HMB‐PP is 104 times more potent in activating human Vγ9/Vδ2 T cells than isopentenyl pyrophosphate.

UV laser matrix desorption/ionization mass spectrometry of proteins in the 100 000 dalton range

Abstract Ultraviolet laser desorption/ionization out of an absorbing matrix has been successfully used to generate molecular ions of proteins in the mass range up to 120 000 dalton. The actual upper mass limit of generated ions is most probably set by ineffective ion detection rather than the ion formation process. Molecular weight determination with a time-of-flight mass spectrometer is facilitated by intense signals of multiply charged and cluster molecular ions. No fragment ions were observed in the mass range above 1000 dalton. Cluster ions were observed up to a mass of 200 000 dalton. The accuracy of mass determination so far is better than 0.5%; 20–100 ng of protein were used for preparation, and less than 1 pg was consumed for a complete mass spectrum. Because of the ease of preparation and the measuring time of just a few minutes this technique should become a valuable tool for molecular weight determination of biopolymers.

Principles and applications of matrix-assisted UV-laser desorption/ionization mass spectrometry

Abstract Matrix-assisted laser desorption/ionization mass spectrometry (LDI MS) has shown its potential for desorbing ions of biomolecules with relative molecular masses ( M r ) up to about 300 000 daltons. Determinations of M r with an accuracy of 0.01–0.2% from about 1 pmol of sample are possible. Examples are shown for proteins, glycoproteins, oligonucleotides and carbohydrates. Examples of the combination of LDI MS with biochemical methods are shown, including the determination of the carbohydrate content of glycoproteins, the identification of disulphide-bonded subunits by cleavage of the light and heavy chains of a monoclonal antibody, the detection of the time course of enzymatic reactions and the analysis of proteins bound to a PVDF membrane.

Gas-Phase cationization and protonation of neutrals generated by matrix-assisted laser desorption.

The ionization mechanisms involved in matrix-assisted ultraviolet laser desorption/ionization (MALDI) were studied with a time-of-flight mass spectrometer. When protonated or cationized quasimolecular ions generated by MALDI are not extracted promptly, their abundance is a function of the delay time between laser irradiation and ion extraction, maximizing at an optimum delay time (DTM) of a few hundred nanoseconds. The ion abundance at DTM exceeds that of prompt extraction by a factor of 2 or more. Increasing the cation density near the sample surface reduces the DTM, whereas increasing the desorption laser irradiance has the opposite effect. The enhancement suggests extensive gas-phase ion-molecule reactions after irradiation by the desorption laser has ceased.

Laser desorption ionization mass spectrometry of large biomolecules

The development of new ionization techniques has revolutionized the field of mass spectrometry of large biomolecules within the last two years. Matrix-assisted UV-laser desorption ionization is a new technique which today enables molecular ion generation and molecular weight determination of proteins with high sensitivity up to several hundred thousand daltons

Analysis of biopolymers by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry is an analytical technique enabling the mass analysis of biopolymers with masses up to at least 300,000 Da. Incorporation of analyte in a matrix consisting of small highly absorbing organic molecules and excitation with short pulses of intense laser light enables the production of intact molecule ions to be analyzed in a time-of-flight mass spectrometer. Mass accuracies of up to 0.01% can be achieved from sample amounts of 1 pmol or less. Proteins, glycoproteins, oligonucleotides and oligosaccharides have been analyzed. The short analysis time of several minutes makes the method well suited for combination with other biochemical methods.

3-Aminoquinoline Acting as Matrix and Derivatizing Agent for MALDI MS Analysis of Oligosaccharides

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a widely used method in oligosaccharide analysis. Underivatized oligosaccharides are not well-suited for that purpose due to their low ionization efficiency; however, derivatization requires tedious sample purification steps which may lead to sample losses, thereby decreasing its benefit. On-target derivatization performed by the matrix 3-aminoquinoline does not require such purification and yields Schiff bases which can be measured in positive and negative ion mode from one single spot. In negative ion mode, spectra from anionic adducts of the derivatives can be acquired from 1 fmol of oligosaccharide. Furthermore, postsource decay (PSD) fragmentation in positive and negative ion mode is enhanced, providing information on oligosaccharide sequence, linkage, and branching. Optimization of reaction conditions and matrix solution led to a complete and reproducible derivatization for all tested standard oligosaccharides. Finally, the method was applied to trifucosyllacto-N-hexaose and trifucosyl-para-lacto-N-hexaose, two isomers occurring in human breast milk samples, which were easily identified and distinguished.

Metabolism of boswellic acids in vitro and in vivo

Boswellia serrata resin dry extract is among the few herbal remedies designated with an orphan drug status for the treatment of peritumoral brain edema. In addition, boswellic acids (BAs), the main active ingredients of B. serrata extracts, have potent anti-inflammatory properties, and may represent promising agents for the treatment of inflammatory diseases. Pharmacokinetic studies, however, revealed poor bioavailability, especially of 11-keto-β-boswellic acid (KBA) and 3-acetyl-11-keto-β-boswellic acid (AKBA), the most potent BAs. To address the question of whether BAs are extensively metabolized, we determined the metabolic stability of KBA and AKBA in vitro, investigated the in vitro metabolism of BAs, and compared the metabolic profiles of KBA and AKBA with those obtained in rats in vivo. In rat liver microsomes and hepatocytes as well as in human liver microsomes, we found that KBA but not AKBA undergoes extensive phase I metabolism. Oxidation to hydroxylated metabolites is the principal metabolic route. In vitro, KBA yielded metabolic profiles similar to those obtained in vivo in rat plasma and liver, whereas no metabolites of AKBA could be identified in vivo. Furthermore, AKBA is not deacetylated to KBA. This study indicates that the efficacy of B. serrata extract may be enhanced by increasing the bioavailability of AKBA.

The Bowen–Conradi syndrome protein Nep1 (Emg1) has a dual role in eukaryotic ribosome biogenesis, as an essential assembly factor and in the methylation of Ψ1191 in yeast 18S rRNA

The Nep1 (Emg1) SPOUT-class methyltransferase is an essential ribosome assembly factor and the human Bowen–Conradi syndrome (BCS) is caused by a specific Nep1D86G mutation. We recently showed in vitro that Methanocaldococcus jannaschii Nep1 is a sequence-specific pseudouridine-N1-methyltransferase. Here, we show that in yeast the in vivo target site for Nep1-catalyzed methylation is located within loop 35 of the 18S rRNA that contains the unique hypermodification of U1191 to 1-methyl-3-(3-amino-3-carboxypropyl)-pseudouri-dine (m1acp3Ψ). Specific 14C-methionine labelling of 18S rRNA in yeast mutants showed that Nep1 is not required for acp-modification but suggested a function in Ψ1191 methylation. ESI MS analysis of acp-modified Ψ-nucleosides in a Δnep1-mutant showed that Nep1 catalyzes the Ψ1191 methylation in vivo. Remarkably, the restored growth of a nep1-1ts mutant upon addition of S-adenosylmethionine was even observed after preventing U1191 methylation in a Δsnr35 mutant. This strongly suggests a dual Nep1 function, as Ψ1191-methyltransferase and ribosome assembly factor. Interestingly, the Nep1 methyltransferase activity is not affected upon introduction of the BCS mutation. Instead, the mutated protein shows enhanced dimerization propensity and increased affinity for its RNA-target in vitro. Furthermore, the BCS mutation prevents nucleolar accumulation of Nep1, which could be the reason for reduced growth in yeast and the Bowen-Conradi syndrome.