Allyl sulphur compounds
Onions, leeks, garlic and other Allium species synthesize cysteine sulphoxide derivatives
(allyl sulphur compounds) as a mechanism of protection against attack by pests. When the plant cells are damaged, the enzyme alliinase is released. It catalyses a series of reactions to form thiosulphinates, and a variety of organic mono-, di- and trisulphates and thiols. In onions the major product is thiopropanal-S-oxide, a potent lachrymator.
These allyl sulphur compounds lower the activity of microsomal cytochrome P450 by three different mechanisms:
- They act as substrates, and inhibit the enzyme by mechanism-dependent (suicide) inhibition (section 22.214.171.124).
- They antagonize the induction of cytochrome P450 by ethanol.
- They cause down-regulation of translation of mRNA, with no effect on transcription.
They also cause increased clearance of potential carcinogens and metabolites, by induction of glutathione S-transferases.
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