direct peptides are the precursors of proteins and have been studied in a variety of applications including proteomics, biomarker identification, protein-protein interactions and cell signaling. Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry is a robust and convenient analytical tool for measuring peptides and proteins, especially when they are complex or are present in small quantities in samples such as cells and tissues. This article reviews recent advances in MALDI-MS approaches to peptide profiling in cellular and tissue samples. This includes novel strategies for peptide sequencing in MALDI-TOF mass spectrometry and the use of postsource decay MALDI-MS to improve peptide sequencing in cold matrices.
Known skin and respiratory sensitizing chemicals are usually reacted with synthetic peptides containing the nucleophilic amino acid residues lysine and cysteine, in the so-called direct peptide reactivity assay (DPRA) framed by OECD Guideline 442. The test identifies dermal sensitizers based on their reactivity with these peptides, which are designed to mimic protein binding of chemical haptens with epidermal proteins. If a compound reacts with the peptides and thereby reduces their content, it is classified as a sensitizer. DPRA data are used to discriminate between sensitizers and non-sensitizers, allowing the prediction of skin and respiratory toxicity of substances.
A modified version of DPRA, referred to as a kinetic direct peptide reactivity assay or kDPRA, is also available. In this method, the reaction kinetics of a test substance with the peptide is evaluated by incubating the test substance and a model peptide for several different incubation times. The reactions are stopped by the addition of monobromobimane and the remaining unreacted peptide content is quantified. The kDPRA can be used to identify the time course of a chemical’s reactivity with a model peptide, and it is an excellent complement to DPRA for predicting the time to the first skin and/or respiratory sensitization.
The authors have recently performed a comparative in chemico study of the DPRA and kDPRA results of nine essential oils of Lamiaceae species (Lavandula angustifolia, Lavandula spiciata, Melissa officinalis, Mentha longifolia, Rosmarinus officinalis, Salvia officinalis, Thymus vulgaris, Verbena officinalis). In this case, it was found that the kDPRA and DPRA were able to predict the sensitization potential of most of the tested chemicals, irrespective of their reactivity with the model peptides. This is a significant advance in comparison to other peptide-based methods, and may be of value when assessing the allergenicity of plant-derived cosmetics.