Metal labeling for accurate multiplexed peptide quantification via matrix-assisted laser desorption/ionization mass spectrometry.

Two peptide quantification strategies, the isobaric tags for relative or absolute quantitation (iTRAQ) labeling methodology and a metal-chelate labeling approach, were compared using matrix-assisted laser desorption/ionization-TOF/TOF MS and MS/MS analysis. Amino- and cysteine-directed labeling using the rare earth metal chelator 1,4,7,10-tetraazacyclododecane-N,N',N″,N″'-tetraacetic acid (DOTA) were applied for relative quantification of single peptides and a six-protein mixture. For analyte ratios close to one, iTRAQ and amino-directed DOTA labeling delivered overall comparable results regarding accuracy and reproducibility. In contrast, the MS-based quantification via amino-directed lanthanide-DOTA tags was more accurate for analyte ratios ≥5 and offered an extended dynamic range of three orders of magnitude. Our results show that the amino-directed DOTA labeling is an alternative relative quantification tool offering advantages like flexible multiplexing possibilities and, in particular, large dynamic ranges, which should be useful in sophisticated, targeted issues, where the accurate determination of extremely different protein or peptide concentration becomes relevant.