A Mass Spectrometry Strategy for Protein Quantification Based on the Differential Alkylation of Cysteines Using Iodoacetamide and Acrylamide

Mass spectrometry has become the most prominent yet evolving technology in quantitative proteomics. Today, a number of label-free and label-based approaches are available for the relative and absolute quantification of proteins and peptides. However, the label-based methods rely solely on the employment of stable isotopes, which are expensive and often limited in availability. Here we propose a label-based quantification strategy, where the mass difference is identified by the differential alkylation of cysteines using iodoacetamide and acrylamide. The alkylation reactions were performed under identical experimental conditions; therefore, the method can be easily integrated into standard proteomic workflows. Using high-resolution mass spectrometry, the feasibility of this approach was assessed with a set of tryptic peptides of human serum albumin. Several critical questions, such as the efficiency of labeling and the effect of the differential alkylation on the peptide retention and fragmentation, were addressed. The concentration of the quality control samples calculated against the calibration curves were within the ±20% acceptance range. It was also demonstrated that heavy labeled peptides exhibit a similar extraction recovery and matrix effect to light ones. Consequently, the approach presented here may be a viable and cost-effective alternative of stable isotope labeling strategies for the quantification of cysteine-containing proteins.

Metabolite analysis, isolation and purity assessment using various liquid chromatographic techniques combined with radioactivity detection

During the discovery of metabolic routes of a drug candidate, radioactively labeled substances are administered. This study reports the multidimensional application of overpressured layer chromatography (OPLC) and high-performance liquid chromatography (HPLC) coupled with online or off-line nondestructive radioactivity detection methods in metabolism studies. Among these methods, digital autoradiography and flow-cell radioactivity detectors (RD) using solid scintillators are used. In this study, the hyphenation of OPLC with RD is reported. The application of the OPLC-RD technique is demonstrated on a metabolism study as well as the multidimensional chromatographic selectivity using normal-phase OPLC for the separation in the first dimension, followed by reversed-phase HPLC-RD, which provides additional selectivity to the separation. Information regarding the identity of radiolabeled metabolites and data obtained from spectroscopic methods could be advantageously used during structure elucidation.