Cinnamic compound metabolism in human skin and the role metabolism may play in determining relative sensitisation potency

Background: trans-Cinnamaldehyde and trans-cinnamic alcohol cause allergic contact dermatitis (ACD) in humans; cinnamaldehyde is a more potent sensitiser than cinnamic alcohol. These two chemicals are principal constituents of the European Standard 'Fragrance Mix', as used in patch testing diagnostics of sensitisation to fragrances by clinical dermatologists. As contact sensitisers; are usually protein reactive compounds, it is hypothesised that cinnamic alcohol (not protein-reactive) is a 'prohapten' that requires metabolic activation, presumably by cutaneous oxidoreductases, to the protein-reactive cinnamaldehyde (a 'hapten'). It is postulated that cinnamaidehyde can be detoxified by aldehyde dehydrogenase (ALDH) to cinnamic acid and/or by alcohol dehydrogenase (ADH) to cinnamic alcohol. Hence, a variety of metabolic pathways may contribute to the relative exposures and hence sensitising potencies of cinnamic alcohol and cinnamaidehyde. Objective: To evaluate the extent of cinnamaidehyde and cinnamic alcohol metabolism in human skin and provide evidence for the role of cutaneous ADH and ALDH in such metabolism. Methods: The extent of cinnamic alcohol and aldehyde metabolism was investigated in human skin homogenates and sub-cellular fractions. A high performance liquid chromatography method was used for analysis of skin sample extracts. Studies were conducted in the presence and absence of the ADH/ cytochrome P450 inhibitor 4-methylpyrazole and the cytosolic ALDH inhibitor, disulfiram. Results: Differential metabolism of cinnamic alcohol and cinnamaldehyde was observed in various subcellular fractions: skin cytosol was seen to be the major site of cinnamic compound metabolism. Significant metabolic inhibition was observed using 4-methylpyrazole and disulfiram in whole skin homogenates and cytosolic fractions only. Conclusions: This study has demonstrated that cutaneous ADH and ALDH activities, located within defined subcellular compartments, play important roles in the activation and detoxification of CAlc and CAld in skin. Such findings are important to the development of computational hazard prediction toots for sensitisation (e.g. the DEREK program) and also to dermatologists in understanding observed interindividual differences, cross-reactivities or co-sensitisation to different cinnamic compounds in the clinic.