Carbon- and Oxygen-Centered Radicals Are Equally Important Haptens of Allylic Hydroperoxides in Allergic Contact Dermatitis

Abstract

Limonene is one of the most commonly used fragrance compounds in western countries today. When exposed to air, it autoxidises, forming hydroperoxides that are strong contact allergens. To cause allergic contact dermatitis (ACD), the hydroperoxides are considered to bind covalently to proteins in the skin via a radical pathway. Consequently, the nature and reactions of the radicals formed from the hydroperoxides are important. We have examined the radical formation from, and sensitizing potential of, three allylic hydroperoxides. Two of these are found in the oxidation mixture of limonene, while the third is a synthetic structural analogue. The identity of the radicals formed from these hydroperoxides has been studied in radical trapping experiments. Chemical trapping experiments were performed using 5,10,15,20-tetraphenyl-21 H,23 H-porphine iron(III) chloride [Fe(III)TPPCl 3] as an initiator and 1,1,3,3-tetramethylisoindolin-2-yloxyl as a radical trapper. Electron paramagnetic resonance experiments using photolysis for initiation were performed with and without 5-diethoxy-phosphoryl-5-methyl-1-pyrroline N-oxide. Our results demonstrate the ability of the studied hydroperoxides to form peroxyl, allyloxyl, and oxiranylcarbinyl radicals. These radicals can potentially react with proteins to form immunogenic hapten-protein complexes relevant for ACD. The sensitizing potency of the hydroperoxides was studied in the murine local lymph node assay. All three hydroperoxides were found to be potent sensitizers with some variations, which can be related to the identity and quantity of the radicals formed. The results indicate that both carbon- and oxygen-centered radicals are important intermediates in the formation of hapten-protein complexes and that the sensitizing potency of the hydroperoxides is related to their structures.

A sensitive method for determination of allergenic fragrance terpene hydroperoxides using liquid chromatography coupled with tandem mass spectrometry

“…1 H-and 13 C-NMR data of LinOOH 1, and LinOAcOOH 2 (both in a 5:3 ratio of tertiary hydroperoxide/secondary hydroperoxide) and LimOOH 3 agree with literature data [16,23,24].…”

“…The monoterpenes were purified by distillation (≥97% according to GC/MS) before use. The hydroperoxides were synthesized and purified according to earlier experience [16,23,24]. Identity and purity of the analytes were examined with NMR (>96% purity), and the data agreed with published reports.…”

Acute Limonene Toxicity in Escherichia coli Is Caused by Limonene Hydroperoxide and Alleviated by a Point Mutation in Alkyl Hydroperoxidase AhpC

“…A large fraction of this literature focuses on the allergenic properties of limonene, which were shown to be due to limonene hydroperoxide (25,(36)(37)(38). This direct parallel to bacterial toxicity suggests a general cellular damage mechanism for limonene hydroperoxide.…”

Sensitization potential and potency of terpene hydroperoxides in the cocultured activation test method

“…12,13 The sensitizing potency based on the murine local lymph node assay (LLNA) is categorized into extreme (EC3 value of ≤0.2%), strong (>0.2% to ≤2%), and moderate (>2%). 14 The allylic hydroperoxides derived from air-oxidized linalool, 15 limonene, 16 and citronellol, 17 as well as the other air-oxidized compounds, were evaluated in the LLNA, and the results showed that the hydroperoxides are the most potent sensitizers among all oxidation products derived from air oxidation of the parent terpenes. Consequently, limonene- 16 and the mixture of linalool hydroperoxides 15 are categorized as strong sensitizers, and the mixture of citronellol hydroperoxides is classified as a moderate sensitizer 17 (Table 2).…”