isotope-coded affinity tag (ICAT)

In order to establish the relevance of a protein to an organism’s phenotype, it is important to be able to determine quantitative changes in its expression. This can be achieved by labelling with a compound (tag) which contains stable isotopes  such as ²H. One batch of the organism is tagged  with the heavy form of the label, containing the  stable isotopes (²H), and another batch is tagged  with the light form, containing ¹H. The reactions  are specific for cysteine residues within the  proteins. The labelled samples are mixed and  digested with trypsin and the isolated peptides are  analysed by LC/MS/MS, enabling the relative  abundances of each peptide pair, containing the  light and heavy label, to be determined. ICAT is a commercial term.