Despite their highly dissimilar flavours, garlic, horseradish and cinnamon each can have a fiery taste. . This pungency has been attributed to chemicals that activate a specific ion-channel protein, known as transient receptor potential ankyrin 1 (TRPA1). Activated TRPA1 facilitates the flow of ions into the endings of specialized neurons in the mouth and skin. This excites the neurons, resulting in local inflammation and a sensation of burning pain. The activation of many ion channels by chemicals involves a readily reversible binding of the stimulating molecules. New evidence shows that the spicy compounds found in the foods listed above activate TRPA1 in a different, and often more sustained, manner, by covalently attaching themselves to cysteine amino-acid residues on the channel protein. . The TRPAl channel can be activated by an array of chemical and physical signals. There are a number of food constituents that act on TRPA1, as well as acrolein (an environmental irritant) and icilin (a substance that evokes a cooling sensation), each quite different structurally. TRPA1 can also be activated by several other cell-communication pathways, including G-protein-coupled signalling pathways and membrane depolarisation . All of this may be involved in cold-sensing and mechanosensation’.
The question is how can one channel be activated by so many and such varied stimuli? Macpherson and his colleagues in Nature 2007, vol 445, p 541 and discussed by Caterina Nature 2007 vol 445 p 491 note that many TRPAI stimuli are electrophiles capable of reacting with cysteine residues (albeit by different mechanisms). This property, rather than chemical geometry, night account for their shared activation of TPA1. They showed that the cysteine-reactive agonists must enter the cell before activating TRPAl. Covalent modification of these cysteines, individually or, together or in combination with others accounts for the activation of TRPAI by electrophiles.. Only-one cysteine residue was identified as crucial, possibly reflecting subtle differences between species in overall TRPAI structure. Aside from the newly identified sites of electrophile action, the TRPAI N-terminal domain contains 18 ankyrin repeat motifs and an EF-hand domain. These two features have been proposed to contribute to the regulation of the channel by mechanical force and calcium ions, respectively. The duration of covalent TRPA1 activation differs between species. The lifetimes of covalent cysteine modifications can vary, depending on the modifying agent and on environmental factors such as pH or redox state and lead to differences in the duration, magnitude and qualitative features of TRPA1-mediated food pungency, inflammation and pain. Electrophilic agents can be damaging to DNA and proteins, one possible step towards cancer.
Many of the cysteine-reactive compounds that activate TRPA1 induce the expression of the enzymes that detoxify them protecting against such injury.
- Martin Eastwood