*2.1.3. Stable isotope labeling by amino acids in cell culture (SILAC)*

SILAC, an acronym to stable isotope labeling by amino acids in cell culture, is a procedure where an essential a.a. has been replaced by its stable isotope counterpart in the cells' growth medium, making this "heavy" amino acid incorporated into all expressed proteins [45]. This causes the growth of two populations of cells: the ones growing in "light" medium containing the natural isotope in the amino acids and the ones growing in "heavy" medium containing stable isotope-labeled amino acids [33, 46]. After complete labeling, equal amounts of labeled and unlabeled cells or protein extracts are mixed in the cell population. The samples are then digested into peptides and then analyzed with mass spectrometry. The quantification of SILAC is thus based on the ratio of introduced isotope-labeled peptides to unlabeled peptides [46]. The many advantages of SILAC are its easy implementation, reasonable quantitative accuracy, and high reproducibility [46].
