Application of MALDI-TOF-mass spectrometry to proteome analysis using stain-free gel electrophoresis.

The combination of MALDI-TOF-mass spectrometry with gel electrophoretic separation using protein visualization by staining procedures involving such as Coomassie Brilliant Blue has been established as a widely used approach in proteomics. Although this approach has been shown to present high detection sensitivity, drawbacks and limitations frequently arise from the significant background in the mass spectrometric analysis. In this chapter we describe an approach for the application of MALDI-MS to the mass spectrometric identification of proteins from one-dimensional (1D) and two-dimensional (2D) gel electrophoretic separation, using stain-free detection and visualization based on native protein fluorescence. Using the native fluorescence of aromatic protein amino acids with UV transmission at 343 nm as a fast gel imaging system, unstained protein spots are localized and, upon excision from gels, can be proteolytically digested and analyzed by MALDI-MS. Following the initial development and testing with standard proteins, applications of the stain-free gel electrophoretic detection approach to mass spectrometric identification of biological proteins from 2D-gel separations clearly show the feasibility and efficiency of this combination, as illustrated by a proteomics study of porcine skeleton muscle proteins. Major advantages of the stain-free gel detection approach with MALDI-MS analysis are (1) rapid analysis of proteins from 1D- and 2D-gel separation without destaining required prior to proteolytic digestion, (2) the low detection limits of proteins attained, and (3) low background in the MALDI-MS analysis.

Online immunocapture ICP-MS for the determination of the metalloprotein ceruloplasmin in human serum.

OBJECTIVE: The human copper-protein ceruloplasmin (Cp) is the major copper-containing protein in the human body. The accurate determination of Cp is mandatory for the reliable diagnosis of several diseases. However, the analysis of Cp has proven to be difficult. The aim of our work was a proof of concept for the determination of a metalloprotein-based on online immunocapture ICP-MS. The immuno-affinity step is responsible for the enrichment and isolation of the analyte from serum, whereas the compound-independent quantitation with ICP-MS delivers the sensitivity, precision, and large dynamic range. Off-line ELISA (enzyme-linked immunosorbent assay) was used in parallel to confirm the elution profile of the analyte with a structure-selective method. The total protein elution was observed with the 32S mass trace. The ICP-MS signals were normalized on a 59Co signal. RESULTS: The human copper-protein Cp could be selectively determined. This was shown with pure Cp and with a sample of human serum. The good correlation with off-line ELISA shows that Cp could be captured and eluted selectively from the anti-Cp affinity column and subsequently determined by the copper signal of ICP-MS.

When is mass spectrometry combined with affinity approaches essential? A case study of tyrosine nitration in proteins.

Tyrosine nitration in proteins occurs under physiologic conditions and is increased at disease conditions associated with oxidative stress, such as inflammation and Alzheimer's disease. Identification and quantification of tyrosine-nitrations are crucial for understanding nitration mechanism(s) and their functional consequences. Mass spectrometry (MS) is best suited to identify nitration sites, but is hampered by low stabilities and modification levels and possible structural changes induced by nitration. In this insight, we discuss methods for identifying and quantifying nitration sites by proteolytic affinity extraction using nitrotyrosine (NT)-specific antibodies, in combination with electrospray-MS. The efficiency of this approach is illustrated by identification of specific nitration sites in two proteins in eosinophil granules from several biological samples, eosinophil-cationic protein (ECP) and eosinophil-derived neurotoxin (EDN). Affinity extraction combined with Edman sequencing enabled the quantification of nitration levels, which were found to be 8 % and 15 % for ECP and EDN, respectively. Structure modeling utilizing available crystal structures and affinity studies using synthetic NT-peptides suggest a tyrosine nitration sequence motif comprising positively charged residues in the vicinity of the NT- residue, located at specific surface- accessible sites of the protein structure. Affinities of Tyr-nitrated peptides from ECP and EDN to NT-antibodies, determined by online bioaffinity- MS, provided nanomolar K(D) values. In contrast, false-positive identifications of nitrations were obtained in proteins from cystic fibrosis patients upon using NT-specific antibodies, and were shown to be hydroxy-tyrosine modifications. These results demonstrate affinity- mass spectrometry approaches to be essential for unequivocal identification of biological tyrosine nitrations.

Identification, affinity characterisation and biological interactions of lectin-like peptide-carbohydrate complexes derived from human TNF-alpha using high-resolution mass spectrometry.

A cyclic disulfide heptadecapeptide (TIP17ox; 2) derived from the lectin-like 17-amino acid domain of human tumor necrosis factor-alpha [TNF-alpha (100-116)] was synthesised and demonstrated to bind specifically to N,N-diacetylchitobiose, a disaccharide present in many glycan structures of glycoproteins. Although the TIP domain forms a loop structure in the native TNF-alpha protein, we show in this study by high-resolution ESI-FTICR mass spectrometry that a homologous linear heptadecapeptide (TIP17rd; 1) binds with comparable affinity to chitobiose, suggesting that cyclisation is not essential for carbohydrate binding. ESI-FTICR-MS was used as an efficient tool for the direct molecular characterisation of TIP peptide-carbohydrate complexes. The specific binding of the TNF-TIP domain to chitobiose and other carbohydrate motifs in glycoproteins may explain the high proteolytic stability of these peptides in biological fluids. A considerably higher proteolytic stability in human plasma was found by mass spectrometric analysis for the cyclic TIP peptide 2, compared to the linear peptide 1. Furthermore, affinity-proteomics studies using immobilised cyclic TIP peptide 2 provided the identification of specific interacting glycoproteins in plasma.