iTRAQ (isobaric tag for relative and absolute quantitation)

iTRAQ is a multiplex protein labelling for mass spectrometry-based protein quantification. It is available as 4-plex and 8-plex labels (we only use 4plex labelling at present). iTRAQ reagents label primary amines (N-terminal amino group and epsilon amino group of lysines) either of intact proteins or proteolytically digested proteins. After labelling samples are pooled and further processed for mass spectrometric analysis.

The iTRAQ labels are isobaric and can not be distinguished by MS only. All differentially labelled peptides of the same species are merged in one peak. Upon collision induced dissociation the tags release low mass reporter fragment ions with a tag-specific mass of either m/z 114, 115, 116 and 117 for the 4-plex label. The relative peak intensities of the different reporter ions in the CID (collision induced dissociation) MS/MS spectrum are used for relative protein quantification.