Surface enhanced laser desorption ionization SELDI

Wright, G. L., Cazares, L. H., Leung, S.-M., Nasim, S., Adam, B.-L., Yip, T.-T., Schellhammer, P. F., Gong, L., and Vlahou, A. (2000). ProteinChip surface enhanced laser desorption/ionization (SELDI) mass spectrometry a novel protein biochip technology for detection of prostate cancer biomarkers in complex protein mixtures. Prostate Cancer and Prostatic Diseases 2, 264-276. 

Also for MALDI, there is a special case worth mentioning. Surface-enhanced laser desorption/ionization (SELDI) is a technique that utilizes special sample plates [196, 197]. These have different modified surfaces, for example, hydrophobic, anionic, or antibody treated. Which type of surface to select depends on the application. After application of analyte the surface is washed according to a protocol leaving only the desired components on the target. Finally, a MALDI matrix is applied before analysis in the spectrometer. See Chapter 12 for an application example of SELDI. 

Surface enhanced laser desorption/ionization (SELDI) is a distinctive form of laser desorption ionization where the target plays an active role in the sample preparation procedure and ionization process [49]. Depending on the chemical or biochemical treatment, the SELDI surface acts as solid phase extraction or an affinity probe. Chromatographic surface is used for sample fractionation and purification of biological samples prior to direct analysis by laser desorption/ ionization. SELDI is mainly applied for protein profiling and in biomarker discovery by comparing protein profiles from control and patient groups. 

Matrix-assisted laser desorption ionization (MALDI) and surface-enhanced laser desorption ionization (SELDI) have been used online with TOF-MS for protein differential profiles of intact or hydrolyzed biological matrices in proteomics. The potential use of affinity chips, grafted with specific Ab towards the drug compound for MALDI or SELDI, will bring sensitive and selective tools for macromolecules. Specific Ab towards either the intact protein or several signature peptides... 

FIGURE 7 Group B streptococcus infection-induced differential protein expression in nonhuman primate (A) and human (B) amniotic fluid samples by surface-enhanced laser desorption/ionization (SELDI-TOF MS) using normal-phase protein chip arrays. Spectrum from 2.5 to 15 kDa collected at 235 nm laser intensity. Detailed spectra show increased expression of the 3.5 and 10.8 kDa peaks between control and infected. Arrows indicate the unique peaks represented by polypeptides overexpressed in infection. 

The Bik glycoprotein has an isoelectric point (p/) of 2.1 and is composed of a peptide and two glycoconjugate portions [30]. The predicted peptide sequence of Bik is helpful to understanding protein function. The peptide has Kunitz-binding domains I and II attached to the N-terminal peptide tail [31-33] (Fig. 2). Protein mass spectrometry by surface-enhanced laser desorption ionization (SELDI) in combination with detection by Bik antibodies has demonstrated that substantial variation exists in the Bik molecule [13,14]. 

The wide application of newly developed advanced Surface MALDI MS methods, such as laser desorption ionization (LDI), surface-enhanced laser desorption ionization (SELDI), direct ionization on silicon surface (DIOS), etc. have two shortcomings. First, problems with MALDI-TOF MS sample preparation for nano-sized systems have not been completely worked out. Second, the reliability and reproducibility of experimental data concerning similar bio-active systems is lacking. Sometimes molecular, associative and fragment ions cannot, be observed in the MALDI mass spectra. We are currently working on overcoming these difficulties with some encouraging results.13,14... 

On-probe purification using derivatized MALDI probe surfaces has been described to simplify the sample preparation process. Various developments in this field have allowed the introduction of new techniques such as the surface-enhanced laser desorption ionization (SELDI) [42], The surface of the probe plays an active role in binding the analyte by hydrophobic or electrostatic interactions, while contaminants are rinsed away. In the same way, this technique uses targets with covalently coupled antibodies directed against a protein, allowing its purification from biological samples as urine or plasma. Subsequent addition of a droplet of matrix solution allows MALDI analysis. 

A potential change in the area of MALDI involves the development of Surface-Enhanced Laser Desorption Ionization (SELDI). " This technique, developed by Ciphergen Biosystems (http //www.ciphergen.com/), combines affinity purification and MALDI on the target. The most common... 

At present, a variety of label-free detection strategies are available for biomedical research, such as surface plasmon resonance (SPR), CNTs, cantilevers, surface-enhanced laser desorption ionization (SELDI)-time of flight (TOF)-mass spectrometry (MS), microfluidic purification chips, immunosensors... 

Wright, G.L. Cazares, L.H. Leung, S.M. et al. ProteinChip Surface Enhanced Laser Desorption/ionization (SELDI) Mass Spectrometry A Novel Protein Biochip Technology for Detection of Prostate Cancer Biomarkers in Complex Protein Mixtures, Prostate Cancer and Prostatic Dis., 2, 264-276 (2000). 

A recent advancement is the coupling of MS with LCM for identihcation of tumor markers specihc for cancer [89]. Specific cells of pathological interest are selected with LCM under direct microscopic visualization, laser captured, and removed from the tissues. The extracted lysate from the cells is applied directly to spots on the substrate of a surface-enhanced laser desorption/ionization (SELDI) MS, ionized, and desorbed from the surface, and a time-of-flight (ToF) proteomic profile of the entire cellular system is observed. This method has been recently applied to toxicology [90]. 

Several modifications of MALDI have been developed to couple additional sampling and reaction capabilities to this technique. Surface-enhanced laser desorption ionization (SELDI) is one type of modified MALDI and describes an ionization process that involves reacting a sample with an enhanced surface. With SELDI, the sample interacts with a surface modified with some chemical functionality prior to laser desorption ionization and mass analysis. For example, an analyte could bind with receptors or affinity media on the surface, and be selectively captured and sampled by laser desorption. A SELDI surface can be modified for chemical (hydrophobic, ionic, immunoaffinity) or biochemical (antibody, DNA, enzyme, receptor) interactions with the sample. This technique can act as another dimension of separation or sample cleanup for analytes in complex matrices. As discussed before, one disadvantage of MALDI is that the matrix (usually a substituted cinnamic acid) that is mixed with the sample can directly interfere with the analysis of small molecules. There have been several areas of research to overcome this issue.Direct ionization on silicon (DIOS) is an example of a modification of MADLI that eliminates the matrix. In this case, analytes are captured on a silicon surface prior to laser desorption and ionization. Other examples of matrix-free laser desorption techniques include the use of siloxane or carbon-based polymers. 

The almost endless possibilities of mass spectrometry have been increased by yet another hopeful variant SELDI (surface-enhanced laser desorption ionization). SELDI combines protein chips with a UV-MALDl-TOF. The chips are solid aluminum strips coated with cationic or anionic ion exchangers, with hydrophile or hydrophobic molecules. Chips with activated surfaces are also available. These bind proteins covalently via their amino groups and enable you to coat chips with antibodies or receptors as you need them. Every chip has eight coated holes with a diameter of 1 mm. You apply the sample into the holes. Part of the pro-teins/peptides is adsorbed. The remainder is washed off. The adsorbed proteins/peptides are transferred into matrix and can then be analyzed in the mass spectrometer (Figure 7.8). What can SELDI do better than MALDI ... 

Surface-Enhanced Laser Desorption Ionization (SELDI)... 

Surface enhanced laser desorption/ionization (SELDI) SELDI is a variant of MALDI in which the MALDI probe is derivatized with various substances that have affinity for the analyte. The probes are then used to extract the analyte directly from mixtures thus avoiding sample loss through more complicated cleanup procedures. Contaminants can... 

Surface-enhanced laser desorption ionization (SELDI) uses so-called protein chips for the detection of peptides and proteins from complex biological fluids such as blood or urine, often for the identification of diagnostic biomarkers for specific carcinomas [156, 157]. These protein chips can contain various media for positive or negative ion exchange or reverse-phase chromatography, as well as specific antibodies or DNA. The functionaUzed surface is immobihzed on a MALDl sample plate for the selective enrichment of constituents of the complex mixture applied, whereas the not bound supernatant is removed by washing. Unfortunately, a large number of unsubstantiated claims for the detection of disease-related biomarkers has discredited this approach, mostly as a result of poor mass spectrometric performance. 

In 1993, Hutchens and co-workers described surface-enhanced laser desorption/ionization (SELDI) technique, an affinity technology, which has progressed over the last decade to become a powerful analytical, an on-plate approach (Hutchens and Yip 1993). SELDI is a distinctive form of laser desorption/ionization (LDI) mass spectrometry in which the EDI probe plays an active role in the homogenization, preconcentration, amplification, purification, extraction, enrichment digestion, derivatization, synthesis, separation, and detection with complementary techniques, prior to the desorption and ionization of the analytes by MALDI (Merchant and Weinberger 2000). The principle of this approach is very simple. Biomolecules are captured by adsorption, partition, electrostatic interaction, or affinity chromatography on a solid-phase protein chip surface. Although SELDI provides a unique sample preparation platform, it is similar to MALDI-MS in that a laser... 

Silica gel can be derivatized in multiple ways, e.g., by covalently binding ligands via Si-O bonds to its surface. Both surface-enhanced laser desorption/ionization (SELDI) and material-enhanced laser desorption/ionization (MELDI) make use of the presence of metal complexes on a silica gel surface to selectively adsorb target compounds via conplex formation ftom solution to a target surface [197]. Besides silica gel, also cellulose or glyddyl methacrylate particles, and even diamond powder have been enployed as carriers for the metal conplex-fiinctionalized groups [198,199]. 

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