ABSTRACT: Chemical skin and respiratory allergies affect many people around the world and are becoming a major public health problem. To date our knowledge on the process of protein haptenation is still limited and mainly derived from studies performed in solution using either amino acids, model peptides, isolated proteins or cell lysates. In order to better understand chemical interactions occurring between respiratory allergens and the skin, we have investigated the reactivity of phthalic anhydride 1 (PA), a chemical respiratory allergen, towards a reconstructed human epidermis model. This study was performed using a new approach combining reconstructed human epidermis (RHE) as a skin model, HRMAS NMR technique to investigate the in situ chemical reactivity and LC-MS/MS to identify modified epidermal proteins. The reaction of PA appeared to be quite fast in RHE compared to model experiments because after 30 min of incubation, no residual signal corresponding to (13C)-1 could be detected. The major product formed could be assigned to phthalic acid, the hydrolysis product of PA. Two amide type adducts on lysine residues were observed and after 8h of incubation, we also observed the formation of an imide type cyclized adducts with lysine. In parallel, RHE samples topically exposed to phthalic anhydride (13C)-1 were analyzed using the shotgun proteomics method. Thus, 948 different proteins were extracted and identified, 135 of which being modified by PA i.e. 14.2% of the extracted proteome. A total of 886 amino acids were modified by PA, of which 211 modifications were validated by fragmentation spectra. We thus identified 475 modified lysine residues (154 validated modifications associated with 122 proteins i.e. 12.9% of the proteome), 183 modified His residues (22 validated modifications associated with 42 proteins i.e. 4.4% of the proteome), 119 modified Tyr residues (30 validated modifications associated with 58 proteins i.e. 6.1% of the proteome) and 109 modified Arg residues (5 validated modifications associated with 25 proteins i.e. 2.6% of the proteome). The rate of modified residues, as a proportion of the total number of modifiable nucleophilic residues in RHE, was rather low (1%) and PA modifications were mainly observed on lysine residues with the formation of amide type adducts with a mass increase of +149 Da and a small number of imide type adducts with an increase mass of +131 Da. At the protein level, modified proteins were mainly type I and Type II keratins and other proteins which are abundant in the epidermis such as protein S100A, Caspase 14, annexin A2, serpin B3, Fatty-acid binding protein 5, histone H2, H3, H4 etc. However, the most modified protein, mainly on histidine residues, was filaggrin, a protein that is sparse (0.0266 mol%) and rich in histidine.
Lauriane Kuhn 
