**3.3.3 Analysis using isotope labeled intact proteins**

Very recently, isotope-labeled intact proteins have been successfully used for MS based protein quantification (Brun et al., 2007; Janecki et al., 2007; Peng et al., 2004). This approach is based on the use of isotope labeled full-length proteins produced by recombinant DNA techniques as internal standards for quantification. The advantage of this approach is that it is unnecessary to take into account the efficiency of hydrolysis/proteolysis that is the main limitation of amino acid or peptide analysis approaches. As the target analyte and the internal standard are chemically identical, they act exactly the same way during not only pre-analytical treatments such as proteolysis, but also LC/MS measurement. For this reason, this analysis can provide more accurate quantitative analysis than the other MS-based approaches described above, minimizing the influences of experimental variations. However, the difficulty of preparation of isotopically labeled protein has been an obstacle. The following is one representative example of how to quantify a certain protein in a complex biological sample using an isotope-labeled intact protein. Janecki et al accurately measured the level of the human alcohol dehydrogenase (ADH1C1) in liver extracts using in-vitro expressed labeled ADH1C1 (Janecki et al., 2007).

