Metabolic Labeling for Protein Quantification

The first reports of metabolic labeling appeared in 1999. In this procedure, yeast cells were grown in two media, one of which used N-emiehed media. The two yeast cultures were combined and the proteins of interest were digested with trypsin before mass speetrometry analysis. The labeling by use of an N-ammonium salt results in the eomplete labeling of amino acid and could be quantified with mass spectrometry. The corresponding mass shift between the unlabeled and the labeled form of the peptide requires high resolution mass spectrometry for analysis. 

Ong et al. introduced a method termed SILAC (stable isotope labeling by amino acids in cell). In SILAC, two cell-culture populations are grown under identical conditions, except that one is supplied with the labeled amino acids (e.g. arginine with six atoms) and the other is with the non-labeled. After five or six doublings, two kinds of amino acids have fully incorporated into proteins. Every peptide pair is separated by the mass shift by the labeled amino acid. This approach cannot be applied to tissues or body fluids, and is 

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