O

10-Hydroxyfenchone

FIGURE 8.10 Proposed metabolism of fenchone in human liver microsomes. (Adapted from Miyazawa, M. and K. Gyoubu, 2006. Biol. Pharm. Bull, 29: 2354-2358; Jahrmann, R., 2007. Metabolismus von Monterpenen und Sesquiterpenen in Mensch und Säugetier: Bedeutung für die pharmazeutische Praxis. MPharm. diploma thesis, University of Vienna, Austria.)

in vitro experiments using human liver microsomes, identical biotransformation products should also be found in blood or urine samples of humans after dietary intake of fenchone-containing products.

8.2.10 Geraniol

Geraniol is a monoterpene alcohol with pronounced concentrations in the essential oil of Cymbopogon winteranus Jowitt (12-25%). It is also found in small quantities in rose, palmarosa, citronella, geranium, lemon, and many other essential oils. It has a rose-like odor and is commonly used in perfumes and in the flavor industry (Chadha and Madyastha, 1984). In a study of Chadha and Madyastha, several metabolites could be identified in rat urine after oral administration (Chadha and Madyastha, 1984). Geraniol can be either metabolized to 8-hydroxygeraniol and via 8-carboxygeraniol to 3,7-dimethyl-2,6-octenedioic acid (Hildebrandt's acid) or directly oxidized to geranic acid and 3-hydroxycitronelic acid (Figure 8.11). Formation of 8-hydroxygeraniol and 8-carboxygeraniol are due to selective oxidation of the C-8 in geraniol. The 8-hydroxylation of geraniol also occurs in higher plants where it is the first step in the biosynthesis of indole alkaloids.

8.2.11 Limonene

The monocyclic monoterpene (+)- and (-)-limonene enantiomers have been shown to be present in orange peel (Citrus aurantium L. sp. aurantium) and other plants and are extensively used as fragrances in household products and components of artificial essential oils. The (+)-limonene isomeric form is more abundantly present in plants than the racemic mixture and the (-)-limonene isomeric form (Wichtel, 2002). It has previously been shown that (+) limonene has chemopreventive activities in experimental animal models including rats and mice (Crowell et al., 1992). Because of the greater importance of (+)-limonene in the food and fragrance industry, only its metabolism and not that of (-)-limonene is described below. Several research groups have successfully described the biotransformation of (+)-limonene in vitro (rat and human liver microsomes) and in vivo (rat, mice, guinea

8-Hydroxygeraniol Geranic acid

Rat in vivo

Rat in vivo

8-Carboxygeraniol Hildebrandt's acid 3-Hydroxycitronelic acid

FIGURE 8.11 Proposed metabolism of geraniol in rats. (Adapted from Chadha, A. and M.K. Madyastha, 1984. Xenobiotica, 14: 365-374; Jahrmann, R., 2007. Metabolismus von Monterpenen und Sesquiterpenen in Mensch und Säugetier: Bedeutung für die pharmazeutische Praxis. MPharm. diploma thesis, University of Vienna, Austria.)

8-Carboxygeraniol Hildebrandt's acid 3-Hydroxycitronelic acid

FIGURE 8.11 Proposed metabolism of geraniol in rats. (Adapted from Chadha, A. and M.K. Madyastha, 1984. Xenobiotica, 14: 365-374; Jahrmann, R., 2007. Metabolismus von Monterpenen und Sesquiterpenen in Mensch und Säugetier: Bedeutung für die pharmazeutische Praxis. MPharm. diploma thesis, University of Vienna, Austria.)

pigs, dogs, rabbits, human volunteers, and patients). As shown in Figure 8.12, (+)-limonene is extensively biotransformed to several metabolites whereas in humans the main biotransformation products are perillyl alcohol; perillic acid; p-mentha-1,8-dien-carboxylic acid (an isomer of perillic acid); cis-dihydroperillic acid; trans-dihydroperillic acid; limonene; 1,2-diol; limonene-10-ol; limonene-8,9-diol; several glucuronides of perillic acid; dihydroperillic acid; and limonene-10-ol (Crowell et al., 1992; Miyazawa et al., 2002; Shimada et al., 2002).

8.2.12 Linalool

Linalool can be obtained naturally by fractional distillation and subsequent rectification from oils of the Cajenne rosewood, Brazil rosewood, Mexican linaloe, and coriander seed. The far highest concentration of linalool is found in the essential oil of Ocimum basilicum (up to 75%). Pure linalool possesses a fresh, clean, mild, light floral odor with a slight citrus impression and is used in large quantities in soap and detergent products (Wichtel, 2002). Although linalool is used in large quantities in the fragrance industry, there are no data available about its biotransformation in humans. In rat, however, linalool is metabolized by cytochrome P450 (CYP) isoenzymes to dihydrolinalool and tetrahydrolina-lool and to 8-hydroxylinalool, which is further oxidized to 8-carboxylinalool (Figure 8.13). CYP-derived metabolites are then converted to glucuronide conjugates (Chadha and Madyastha, 1984).

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