Stable isotope standards and capture

Anderson NL, Anderson NG, Haines LR, et al. Mass spectrometric quantitation of peptides and proteins using stable isotope standards and capture by anti-peptide antibodies (SISCAPA)./. Proteome Res. (2004) 3 235-244. 

Stable Isotope Standards and Capture by Anti-Peptide Antibodies... 

Immunoenrichment can be applied at the peptide level, particularly in the stable isotope standards and capture by anti-peptide antibodies (SISCAPA) method (140). Target peptides together with their stable isotope-labeled counterparts (used as quantitation references) are enriched by antibodies that are developed against antigens with almost the same sequences. The antibodies are often covalently attached to magnetic beads for sample cleanup. This technique boasts an improved limit of quantitation and increased inter-laboratory reproducibility (141). 

The most used current method for measuring the concentration of low-abundance proteins in plasma is termed SISCAPA, which stands for stable-isotope standards and capture by antipeptide antibodies [37]. In this method, which is a hybrid of mass spectrometry and immimoassay, proteins are first digested with a protease into peptides. After digestion, isotope labeled internal standards are spiked into the sample to allow ratiometric quantitation. The mixture is then exposed to antibodies that have been raised to bind to the desired peptides more than one antipeptide antibody can be used in this step if a multiplex assay is desired. Washing steps are used to remove the excess nontarget peptides, and the peptides are then eluted from the antibodies and analyzed by LC-MS/MS. 

Symbol Ca atomic number 20 atomic weight 40.078 a Group IIA (Group 2) alkaline-earth metaUic element ionic radius 1.06 A (Ca2+) electron configuration [Ar]4s2 valence state +2 standard electrode potential, E° = -2.87V stable isotopes and their abundance Ca-40 (97.00%), Ca-44 (2.06%) Ca-42 (0.64%), Ca-48 (0.18%), Ca-43 (0.145%), and Ca-46 (0.003%) also the element has six unstable isotopes of which Ca-41 has the longest half-life, l.lxlO yr (decay mode electron capture), and Ca-38 has shortest half life 0.66 sec (P-decay). 

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