Epidemiological evidence indicates that a high dietary intake of plants of the Allium family, such as garlic and onions, decreases the risk of
cancer in humans. It has been suggested that this effect is due to the ability of the aliphatic mono-, di-, tri-, and tetrasulfides derived from these vegetables to increase tissue activities of Phase 2 detoxification
enzymes. In contrast, toxic effects have been recorded in domestic and farm animals after the consumption of garlic or onions, involving oxidative damage to erythrocytes and consequent
hemolytic anemia. This effect again has been attributed to the aliphatic
sulfides. In the present study, the ability of
sulfides derived from garlic and onions to generate "
active oxygen" species and cause oxidative damage to erythrocytes in vitro has been compared, together with their ability to cause
hemolytic anemia and increase the activity of the Phase 2
enzymes quinone reductase (QR) and
glutathione S-transferase (GST) in rats. Monosulfides were without significant effect on any parameter. Di-, tri-, and tetrasulfides generated
hydrogen peroxide in the presence of GSH and
hemoglobin and caused oxidative damage to erythrocytes in vitro. The activity decreased in the order of tetra- > tri- >
disulfide, with the
allyl compounds being more potent than the propyl. In vivo, both allyl and propyl tri- and tetrasulfides were powerful
hemolytic agents. In contrast, only the allyl
sulfides increased the activities of QR and GST; the propyl derivatives were completely without effect. Allyl and propyl tri- and tetrasulfides, thus, may contribute to the toxic effects of Allium vegetables, while only the allyl derivatives are effective in increasing tissue activities of
cancer-protective
enzymes.