Mass spectrometric approaches to the confirmation of maduramicin alpha in chicken fat

Detection of arginine dimethylated peptides by parallel precursor ion scanning mass spectrometry in positive ion mode

Dimethylation at arginine residues has been shown to be central in cellular processes such as signal transduction, transcription activation, and protein sorting. The two methyl groups are either placed symmetric or asymmetric on the zeta standing nitrogen atoms of the arginine side chain. Here, we introduce a novel method that enables the localization of dimethylarginine (DMA) residues in gel-separated proteins at a level of sensitivity of better than 1 pmol and that allows one to distinguish between the isomeric symmetric and asymmetric position of the methyl groups. The method utilizes two side-chain fragments of DMA, the dimethylammonium ion (m/z 46.06) and the dimethylcarbodiimidium ion (m/z 71.06), for positive ion mode precursor ion scanning. Dimethylcarbodiimidium ions (m/z 71.06) are produced by symmetric as well as asymmetric dimethylarginine but are observed more strongly for symmetric DMA. It is utilized here in the precursor of m/z 71 scan to indicate the presence of DMA in a peptide. The dimethylammonium ion (m/z 46.06) is specific for asymmetric DMA and is utilized here in the precursor of m/z 46 scan. The positive ion mode allows for the identification of the protein by peptide sequencing and simultaneous detection and localization of the modified residues. The analysis can be conducted on any mass spectrometer capable of precursor ion scanning. However, the high resolution of a quadrupole TOF instrument is beneficial to assign the accurate charge state of the often highly charged precursors. Using the precursor of m/z 71 scan, we found FUS/TLS and Sam68 to be DMA-containing proteins. We discovered at least 20 DMA sites in FUS/TLS. In MS/MS, we observed neutral loss of dimethylamine (m/z 45.05) from which it follows that the dimethylation in FUS/TLS is asymmetric. Monitoring in parallel the fragments m/z 46.06 and 71.06 in precursor ion scans and peptide sequencing, we identified at least nine asymmetric DMA modifications in Sam68. The parallel monitoring of fragments in precursor ion scans is a versatile tool to specify the nature of protein modifications in cases where a single fragment is not conclusive.

Quantitation and facilitated de novo sequencing of proteins by isotopic N-terminal labeling of peptides with a fragmentation-directing moiety

We describe a method for comparative quantitation and de novo peptide sequencing of proteins separated either by standard chromatographic methods or by one- and two-dimensional polyacrylamide gel electrophoresis. The approach is based on the use of an isotopically labeled reagent to quantitate (by mass spectrometry) the ratio of peptides from digests of a protein being expressed under different conditions. The method allows quantitation of the changes occurring in spots or bands that contain more than one protein and has a greater dynamic range than most staining methods. Since the reagent carries a fixed positive charge under acidic conditions and labels only the N-terminal of peptides, the interpretation of tandem mass spectra to obtain sequence information is greatly simplified. The sequences can easily be extracted for homology searches instead of using indirect mass spectral-based searches and are independent of posttranslational modifications.

Analysis of antibiotics by liquid chromatography-mass spectrometry

The current status of the application of LC-MS in the analysis of antibiotic and antibacterial compounds is reviewed. The main application area of LC-MS in this field is the confirmation of identity in animal food products for human consumption at maximum residue levels, set by the regulatory authorities. LC-MS is found to play an important role in the determination and confirmation of sulphonamides, beta-lactam antibiotics, (fluoro) quinolone antibiotics, as well as various other groups including aminoglycosides, chloramphenicol, ionophore antibiotics, 5-nitrofuran derived compounds and macrolide antibiotics. Currently available data on these compound classes are reviewed, with special emphasis on important aspects especially relevant to LC-MS and on the mass spectral information obtained.

Development of an ELISA for maduramicin and determination of the depletion kinetics of maduramicin residues in poultry

Polyclonal antibodies against the coccidiostat maduramicin were produced by the immunization of rabbits with a conjugate between maduramicin and human serum albumin. The antibodies exhibited minimal cross-reactivity with structurally similar ionophoric coccidiostats (monensin, salinomycin, narasin and laslocid). An enzyme-linked immunosorbent assay (ELISA) was developed for maduramicin in poultry tissue using these antibodies. The ELISA had a limit of detection of approximately 0.02 ng/g in tissue (ng/ml in serum) and a limit of determination of 1.0 ng/g in tissue (ng/ml in serum). The half-life of maduramicin was measured in broiler chickens that had been treated with a therapeutic dose of maduramicin for 14 days. Following withdrawal of the medicated feed, the mean half-lives of maduramicin were 20, 39 and 13 h in liver, muscle and serum, respectively. At the recommended withdrawal period (5 days) the mean residual maduramicin concentration in liver was 1.6 ng/g, in muscle was 1.5 ng/g and in serum was 0.05 ng/ml.

Applications of liquid chromatography-mass spectrometry interfacing systems in food analysis: pesticide, drug and toxic substance residues

This paper reviews applications of different LC-MS techniques for the determination of xenobiotic substances in foods. Specific examples of contaminants discussed are pesticides, herbicides, insecticides and drugs; concerning toxic substances, mycotoxins, phycotoxins, cyanobacterial toxins, mutagenic and heterocyclic amines and beta-carbolines, arsenic, tin and inorganic halogen compounds, packaging materials and various epoxy resins are considered. Advantages and limitations are outlined for the different LC-MS interfacing systems (particle beam, thermospray, atmospheric pressure ionization with electrospray, ionspray and heated pneumatic nebulizer). The impact of developments in instrumental analysis on methodology and the limitations of the various LC-MS methods are discussed. Further, the coupling of LC with element-selective detection systems such as inductively coupled plasma mass spectrometry is discussed, with emphasis on speciation of trace toxic elements in foods.