Cause Clarification of Cysteine Oxidation by Active Species Generated during the Oxidation Process of Cinnamaldehyde and Impact on an In Chemico Alternative Method for Skin Sensitization Using a Nucleophilic Reagent Containing Cysteine

Abstract

Aldehydes comprise a major portion of skin sensitizers because they can react with both cysteine and lysine. Moreover, cinnamaldehyde (CA) is a typical moderate sensitizer and is often used in an alternative test method for skin sensitization. The amino acid derivative reactivity assay (ADRA) is an in chemico test method that evaluates the reactivity of cysteine derivatives (N-(2-(1naphthyl)acetyl)-L-cysteine, NAC) and lysine derivatives with the test chemicals and uses CA as a proficiency substance. We found that NAC depletion for CA was only 10−20% when CA was used directly from the reagent bottle, although it increased to almost 100% when stored after being aliquoted from the reagent bottle. It was also found that this was due to the air oxidation of NAC itself rather than the reaction of NAC with CA, indicating that this result simply shows an increase in apparent reactivity. Aldehydes are known to produce active species, such as radicals, during air oxidation. Therefore, we investigated whether radicals were generated under storage conditions using the radical scavenger OH-TEMPO. LC/MS/MS analysis revealed that CA and OH-TEMPO complexes were produced during the air oxidation of CA. In the results of five aldehydes, similar to CA, active species were not generated as significantly as CA. Collectively, during the evaluation of the aldehydes, it can be seen that careful measures need to be taken to prevent the aldehydes from oxidizing during storage, indicating that assessment without preventing air oxidation carries an increased risk of overestimation compared with the intrinsic skin sensitization potency.

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“…2 Importantly, cysteine possesses strong reducing capacity due to its active amino and carboxyl groups, making it susceptible to oxidation by oxygen gas. 3 Additionally, cysteine serves various physiological functions, for example, the prevention and treatment of radioactive hazard, 4 maintenance of sulfhydrase activity in the skin, treatment of skin diseases, [5][6][7] detoxication, 8 and so forth. These properties make cysteine applicable in diverse fields, such as: food, 9 cosmetics, 10 biochemical studies, 11 and so on.…”