Menthol metabolic pathways

According to McConkey et al. (2000), several enzymes of the menthol biosynthetic pathway appear to originate from widely divergent genetic resources in primarily metabolism, which would make their products less sign cant as chemotaxonomic characteristics. 

FIGURE 19.201 Metabolic pathways of menthol (137), menthone (149),p-cymene (178), thymol (179), car-vacrol methyl ether (201), and carvotanacetone (47) by microorganisms and rabbit. 

Metabolic pathways of menthol (137), menthone (149), thymol (179), and carvacrol methyl ether (202) are summarized in Figure 19.201. Menthol (137) is generally hydroxylated to give 1-hydroxy-(138), 2-hydroxy- (140), 4-hydroxy- (141), 6-hydroxy- (139), 7-hydroxy- (143), 8-hydroxy- (142), and 9-hydroxymenthol (144) and 1,8-dihydroxy- (146) and 7,8-dihydroxymenthol (148) (Asakawa et al., 1991 Takahashi et al., 1994 Van der Werf et al., 1997). Racemic menthyl acetate and menthyl chloroacetate are hydrolyzed asymmetrically by an esterase of microorganisms (Brit Patent, 1970 Moroe et al., 1971 Watanabe and Inagaki, 1977a,b). Menthone (149) is reductively metabolized to 137 and oxidatively metabolized to 3,7-dimethyl-6-hydroxyoctanoic acid (152), 3,7-dimethyl-... 

FIGURE 14.64 Metabolic pathways of (—)-menthol (137b) hy Aspergillus niger. (Modified from Asakawa, Y. et al., 1991. Phytochemistry, 30 3981-3987.)... 

In plant plastids, GGPP is formed from products of glycolysis and is eight enzymatic steps away from central glucose metabolism. The MEP pathway (reviewed in recent literature - ) operates in plastids in plants and is a preferred source (non-mevalonate) of phosphate-activated prenyl units (IPPs) for plastid iso-prenoid accumulation, such as the phytol tail of chlorophyll, the backbones of carotenoids, and the cores of monoterpenes such as menthol, hnalool, and iridoids, diterpenes such as taxadiene, and the side chains of bioactive prenylated terpenophe-nolics such as humulone, lupulone, and xanthohumol. The mevalonic pathway to IPP that operates in the cytoplasm is the source of the carbon chains in isoprenes such as the polyisoprene, rubber, and the sesquiterpenes such as caryophyllene. 

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