Allyl Sulfur Compounds

Members of the allium family (onions, garlic, and leeks) contain cysteine sulfoxide derivatives (allyl sulfur compounds), such as allicin and alliin (see Figure 14.9). When the plant cells are damaged, the enzyme alliinase is released from vacuoles and catalyzes the formation of thiosulfinates and thiols, including the lachrymator thiopropanal S-oxide. Their function in the plant is presumably to provide protection against attack by pests.

allicin o nh,+

Figure 14.9. Allyl sulfur compounds allicin and alliin. Relative molecular masses (Mr): allicin, 162.3; and alliin, 177.2.

The allyl sulfur compounds have two actions that may protect against the development of cancer:

1. They reduce the activation of many procarcinogens to the active carcinogen by lowering the activity of microsomal cytochrome P450 enzymes. They achieve this by acting as partial substrates of the enzyme, leading to mechanism-dependent inhibition; antagonizing the induction of cytochrome P450 by ethanol and various other compounds; and by decreasing the translation of mRNA, with no effect on transcription.

2. They increase the metabolic clearance of potential carcinogens and their metabolites, by induction of glutathione S-transferases.

The allyl sulfur compounds of garlic also have an anticoagulant action (by inhibiting platelet coagulability) and inhibit cholesterol synthesis by inactivating HMG-CoA reductase.

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