Relative Quantification: N15 & O18

N15 and O18 isotope labeling is very similar to SILAC with the exception that for cellular protein labeling oxygen or nitrogen isotopes are feed into the system. After a number of cell divisions, each instance of this particular element will be replaced by its isotope version. At this point proteins are extracted and digested with a protease. The 2 samples are then mixed and are subjected to LC-MSMS analysis to identify the peptide sequences. Since the peptides originating from 2 different samples are identical in all regards with the exception of their masses, their masses are used to quantify these peptides in the sample.