ITRAQ-labeled peptide

There are several examples of peptidomic studies using iTRAQ labeling. One study identified substrates of prolyl peptidases in mouse models, and another explored the effect of diurnal variation on the endogenous peptide composition of human parotid saliva (64,65). 

Reacting the peptides in each sample with an individual, mixed-label reagent adds 145 Da to each peptide. Addition of the same mass allows selection of differentially labeled peptides using a single miz in the first analyzer of a QqQ, QTOF, or TOF/TOF MS/MS instrument. Ion traps cannot be used in iTRAQ analyses because of the low masses of the reporter ions... 

Isobaric Tags for Relative and Absolute Quantitation represents recent technology advancement, where four specific reagents label peptides in up to four samples. While the labeled peptides have the same mass, each label fragments into a different sized, characteristic tag that can be identified, and quantified in the tandem mass spectra In Arabidopsis iTRAQ has been used to map the organellarproteomes [123]. Another study applied iTRAQ to describe early changes to the phosphoproteome oiArabidopsis during defense response [124]. 

It has been demonstrated also that the iTRAQ tandem mass spectrometric quantitative analysis strategy can be used in conjunction with the quadrupole ion trap by performing multiple stages of mass analysis (that is, MS ) [125], For example, chemical derivatization with the iTRAQ reagent not only labels the N-terminus of a peptide, but the lysine side chain also. Thus, tryptic peptides with a modified lysine residue present at the C-terminus will produce a yj product ion at m/z 291 following ClD-tandem mass spectrometry. To generate the low m/z iTRAQ reporter ions required for quantitation, the yj product ion is isolated and subjected to data-dependent CID-MS. Using this approach, peptide identification is achieved in the MS/MS scan, while quantitation is achieved via MS. ... 

The technique of isobaric tag for relative and absolute quantification (iTRAQ) is based on covalently labeling the amines at N-termini and the side chains of peptides with isotope-coded tags after the proteins have been isolated form biological systems, denatured, and enzymatically digested. Here the isotope labeling is carried out at a much later stage than in both SILAC and ICAT. iTRAQ is an MS/MS technique that uses CID-derived ions for both the identification and quantification of proteins. 

---