Quantification of proteins using lanthanide labeling and HPLC/ICP-MS detection

Quantitative proteomics poses a great challenge and also offers considerable opportunities for analytical chemistry. Currently, the available methods for quantitative proteomics are mainly based on labeling by isotopes and quantification by biological mass spectrometers with soft-ionization sources. Recently, inductively coupled plasma-mass spectrometry (ICP-MS) has been introduced as an attractive complement to biological mass spectrometry for protein quantification. Here we developed a new method based on lanthanide labeling and ICP-MS detection for relative quantification of protein mixtures. The bifunctional reagent DTPAA was chosen as the element-labeling reagent for proteins. Two samples containing RNase A, cytochrome c, and lysozyme in different mixture ratios were labeled with two lanthanides, Ce and Sm, respectively. After separation with cation exchange chromatography, the proteins could be relatively quantified by comparison between signal intensities of Ce and Sm in ICP-MS. The intact proteins can be quantitatively analyzed without enzyme digestion. Because there are 17 lanthanides available for protein labeling, the developed method provides a possibility for high-throughput and top-down proteomics quantification by ICP-MS.