Les bases chimiques de l’allergie de contact

Antioxidant Activity and Skin Sensitization of Eugenol and Isoeugenol: Two Sides of the Same Coin?

Eugenol and isoeugenol are well acknowledged to possess antioxidant and thus cytoprotective activities. Yet both compounds are also important skin sensitizers, compelling the cosmetics and fragrance industries to notify their presence in manufactured products. While they are structurally very similar, they show significant differences in their sensitization properties. Consequently, eugenol and isoeugenol have been the subject of many mechanistic studies where the final oxidation forms, electrophilic ortho-quinone and quinone methide, are blamed as the reactive species forming an antigenic complex with nucleophilic residues of skin proteins, inducing skin sensitization. However, radical mechanisms could compete with such an electrophilic-nucleophilic pathway. The antioxidant activity results from neutralizing reactive oxygen radicals by the release of the phenolic hydrogen atom. The so-formed phenoxyl radicals can then fully delocalize upon the structure, becoming potentially reactive toward skin proteins at several positions. To obtain in-depth insights into such reactivity, we investigated in situ the formation of radicals from eugenol and isoeugenol using electron paramagnetic resonance combined with spin trapping in reconstructed human epidermis (RHE), mimicking human skin and closer to what may happen in vivo. Two modes of radical initiation were used, exposing RHE to (i) horseradish peroxidase (HRP), complementing RHE metabolic capacities, and mimicking peroxidases present in vivo or (ii) solar light using a AM 1.5 solar simulator. In both experimental approaches, where the antioxidant character of both compounds is revealed, oxygen- and carbon-centered radicals were formed in RHE. Our hypothesis is that such carbon radicals are relevant candidates to form antigenic entities prior to conversion into electrophilic quinones. On this basis, these studies suggest that pro- or prehapten fingerprints could be advanced depending on the radical initiation method. The introduction of HRP suggested that eugenol and isoeugenol behave as prohaptens, while when exposed to light, a prehapten nature could be highlighted.

Electrochemistry/mass spectrometry as a tool in the investigation of the potent skin sensitizer p-phenylenediamine and its reactivity toward nucleophiles

RATIONALE

Although para-phenylenediamine (PPD) is known to cause severe allergic contact dermatitis in consequence of autoxidation and/or skin metabolism pathways, it is commonly utilized as an ingredient in permanent hair dyes. The aim of this work was to simultaneously accelerate the autoxidation process and to simulate the metabolic activation of PPD using a purely instrumental system.

METHODS

Electrochemistry (EC) in combination with electrospray ionization mass spectrometry (ESI-MS) was used in this study to assess the skin-sensitizing potential of PPD. Online and offline coupled EC/ESI-MS experiments were carried out and the emerging oxidation products were investigated. In a second approach, these primary species were allowed to react with the nucleophiles glutathione (GSH), cysteine (Cys), potassium cyanide (KCN) and lysine (Lys) in order to evaluate their reactivity.

RESULTS

The reactive p-phenylene quinone diimine (PPQD), which can form upon autoxidation and/or skin metabolism of PPD, was effectively generated in a simple EC cell next to further oxidation products, including the trimeric product Bandrowski's Base (BB). Conjugation with GSH and Cys was successfully proven, but no adducts with KCN or Lys were observed. Furthermore, the application of different concentration ratios between PPD and nucleophile was shown to play a crucial role concerning the type of oxidation products and adducts being formed.

CONCLUSIONS

It was found that EC/MS is a well-suited approach for the targeted generation of reactive haptens such as PPQD while avoiding detection problems due to the complexity of matrices encountered when conducting conventional in vitro or in vivo experiments.

Electrochemistry/liquid chromatography/mass spectrometry to demonstrate irreversible binding of the skin allergen p-phenylenediamine to proteins

RATIONALE

para-Phenylenediamine (PPD) is a potent and well-known allergen, which is commonly used in hair or fur dyes and can cause severe allergic contact dermatitis. In this work, the skin-sensitizing potential of PPD with respect to the conjugation of proteins was evaluated using an approach without animal testing.

METHODS

Electrochemistry (EC) coupled offline to liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS) was employed to convert the pre-hapten PPD into its reactive hapten analogs. A previous study had already shown that this purely instrumental method is suitable for accelerating and simulating the various oxidation processes, which PPD may undergo, and that the emerging products are prone to react with soft thiol groups of small nucleophiles like glutathione and cysteine.

RESULTS

This investigation was extended by successfully demonstrating adduct formation between EC-generated PPD oxidation products and the three proteins β-lactoglobulin A (β-LGA), human serum albumin and human hemoglobin. A tryptic digest of modified β-LGA provided evidence for irreversible protein binding of monomeric PPD, a PPD dimer and the PPD trimer known as Bandrowski's Base. It was shown that the main oxidation product p-phenylene quinone diimine, and the reactive oligomerized species, primarily attack the free thiol function of proteins rather than other nucleophilic amino acid residues.

CONCLUSIONS

The pre-hapten PPD was efficiently activated upon EC oxidation and the resulting species were further reacted with different proteins leading to diverse hapten-protein complexes. Thereby, problems related to the complex matrix present in conventional in vitro or in vivo methods could effectively be avoided.