PSAQ™: the ultimate in quantification.

In the field of small compounds analysis, stable isotope dilution assays are extensively used in combination with liquid chromatography-mass spectrometry (LC-MS) to provide confident quantification results. Over the last decade, the principle of isotope dilution has been adopted by the proteomics community in order to accurately quantify proteins in biological samples. In these experiments, a protein's concentration is deduced from the ratio between the MS signal of a tryptic peptide and that of a stable-isotope-labeled analog, which serves as an internal standard.
The first isotope dilution standards introduced in proteomics were chemically synthesized peptides incorporating a stable-isotope-tagged amino acid. These isotopically labeled peptide standards, which are currently widely used, are generally added to samples after protein isolation and digestion.
Thus, if protein enrichment is necessary, they do not allow correction for protein losses that may occur during sample prefractionation; nor do they allow the tryptic digestion yield to be taken into account. To reduce these limitations we have developed the PSAQ™ (Protein Standard Absolute Quantification) strategy using full-length stable-isotope-labeled proteins as quantification standards. These standards and the target proteins share identical biochemical properties. This allows standards to be spiked into samples at an early stage of the analytical process. As a result, the PSAQ method provides highly accurate quantification results, including for samples requiring extensive biochemical prefractionation (such as serum/plasma samples).