Bioassay-directed fractionation of clove terpenes from the plant Eugenia caryophyllata has led to the isolation of the following five active known compounds: beta-caryophyllene [1], beta-caryophyllene oxide [2], alpha-humulene [3], alpha-humulene epoxide I [4], and eugenol [5]. Their structures were determined on the basis of spectral analysis (hreims, 1H and 13C nmr). These compounds showed significant activity as inducers of the detoxifying enzyme glutathione S-transferase in the mouse liver and small intestine. The ability of natural anticarcinogens to induce detoxifying enzymes has been found to correlate with their activity in the inhibition of chemical carcinogenesis. Thus, these sesquiterpenes show promise as potential anticarcinogenic agents.

Citrus limonoids were studied as inducers of the detoxifying enzyme system glutathione S-transferase in mice. Among eight limonoids studied, nomilin was shown to be the most potent inducer in the liver and in the small intestinal mucosa. In the forestomach, the inducing activity was low but the enzyme activity was significantly elevated when compared with that of the control. In the lung and colon no appreciable enzyme activity was induced by nomilin. Three other limonoids, obacunone, isoobacunoic acid, and ichangin, were shown to be enzyme inducers in the liver and some other tissues studied. Limonin, on the other hand, showed only marginal activity as an enzyme inducer in the small intestinal mucosa and was inactive in all other tissues studied. Limonol showed small inducing activity in the forestomach while deoxylimonin was not active in any of the tissues examined. Since anticarcinogens such as nomilin and kahweol are potent inducers of detoxifying enzymes, citrus limonoids such as obacunone, isoobacunoic acid, and others may be effective inhibitors of chemically induced carcinogenesis.Limonoids are a group of chemically related triterpene derivatives found in the Rutaceae and Meliaceae families.

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Bioassay-directed fractionation of dill weed oil and caraway oil, respectively, from the plants Anethum graveolens L. and Carum carvi L. (Umbelliferae) has led to the isolation of three monoterpenes, anethofuran (1), carvone (2), and limonene (3). Their structures were determined on the basis of spectral analysis. These compounds induced the detoxifying enzyme glutathione S-transferase in several mouse target tissues. The alpha,beta-unsaturated ketone system in carvone appeared to be critical for the high enzyme-inducing activity.

Two citrus limonoids, limonin and nomilin, were tested for their effects on the development of 7,12-dimethylbenz[a]anthracene (DMBA)-induced buccal pouch epidermoid carcinomas. Forty-five female Syrian hamsters were divided into three equal groups. The left buccal pouches of the animals in each group were pretreated topically with two applications of dimethylsulfoxide (DMSO) (group I), a 2.5% solution of limonin dissolved in DMSO (group II) or a 2.5% solution of nomilin dissolved in DMSO (group III). After this initial treatment, 11 hamsters from each group were selected. The left buccal pouches of these animals were painted 2 or 3 times weekly with a 0.5% solution of DMBA in mineral oil. On alternate days the pouches were painted with DMSO (I), the 2.5% solution of limonin (II) or the 2.5% solution of nomilin (III). The 12 remaining hamsters were used as controls and were painted with mineral oil and DMSO (I), mineral oil and the 2.5% solution of limonin (II), or mineral oil and the 2.5% solution of nomilin (III). After 15 weeks the hamsters were killed, the pouches were excised and the tumors were counted and measured. Tumors of variable size were common in the animals treated with DMBA. However, the animals receiving topical applications of limonin exhibited a 60% reduction in tumor burden. Further comparisons between groups I and II showed that this reduction in tumor burden was due to a 20% decrease in tumor number and a 50% decrease in tumor mass. The results for group III showed that nomilin was considerably less effective as an inhibitor of DMBA-induced neoplasia.