At the start of June, OriGene released its library of 5,000 heavy isotope–labeled full-length human proteins for use as quantitative internal standards in single reaction monitoring and multiple reaction monitoring (SRM/MRM) MS. SRM/MRM MS provides absolute quantitative protein measurements, which are necessary for proteomics research applications such as the identification of protein isoforms, discovery of lower abundance proteins and monitoring of protein post-translational modifications. In SRM/MRM MS, proteins of interest are paired with standardized heavy isotope–labeled proteins, and the light-heavy pairs undergo numerous enrichment and purification steps, followed by tandem MS analysis. However, there has been a lack of standardized heavy-labeled proteins due to the cost and time needed to develop full-length cDNA clones, which are used to generate the heavy-isotope labeled proteins. According to Walter Tian, vice president of Marketing at OriGene, the company was able to introduce these proteins because of its history: “Over the last 14 years, OriGene has focused its resources on building the world largest collection of full-length human cDNA clones. From this 25,000 clone collection, OriGene has expressed 12,000 human proteins in the human HEK293 cell line. Out of the 12,000, we have purified 5,000 proteins.” OriGene’s labeled protein standards cost $980 for 10 µg.

Currently, absolute quantitative protein measurements are made using labeled peptides as standards, which are less accurate than labeled protein standards, according to Mr. Tian. “Protein quantification using labeled protein standards allows for measurement of all variations for the whole [SRM/MRM] process, thus producing more accurate results. In contrast, labeled peptides are usually added only at the step of trypsin digestion.” Trypsin digestion of proteins takes place after sample enrichment and purification, which is done through gel separation or LC. By using labeled proteins, protein changes during the LC step can now be accounted for with SRM/MRM MS analysis.