Limonene metabolism

Limonene was extensively metabolized by a variety of mammalian species [279, 290, 292, 301]. Its principal circulating metabolites identified in the rat were perillic acid and dihydroperillic acid. These components were effective inhibitors of isoprenylation and cellular proliferation in vitro [271]. 

As mentioned before, a Pseudomonas incognita was isolated by enrichment technique on the monoterpene alcohol linalool that was also able to grow on geraniol, nerol and limonene [36]. The metabolism of limonene by this bacterium was also investigated [37]. After fermentation the medium yielded as main product a crystallic acid, perillic acid, together with unmetabolised limonene, and some oxygenated compounds dihydrocarvone, carvone, carveol, p-menth-8-en-1 -ol-2-one, p-menth-8-ene-1,2-diol or p-menth-1 -ene-6,9-diol (structure not fully elucidated) and finally / -isopropenyl pimelic acid. 

To date over 30 plant terpenoid synthases have been cloned as cDNAs, and many of these were found to encode enzymes of secondary metabolism (43). Isolation and analysis of six genomic clones encoding monoterpene ((—)-pinene and (—)-limonene), sesquiterpene ((E)-a-bisabolene and S-selinene) and diterpene (abietadiene) synthases from Abies grandis, and a diterpene (taxadiene) synthase from Taxus brevifolia have been reported (44). Overexpression of a cotton farnesyl diphosphate synthase (EPPS) in transgenic Artemesia annua has resulted in 3- to 4-fold increase in the yield of the sesquiterpenoid anti-malarial drug, artemisinin, in hairy roots (45). 

D-Limonene was not mutagenic in four strains of Salmonella, did not significantly increase the number of trifluorothymidine-resistant cells in the mouse L5178Y/TK" assay, and did not induce chromosomal aberrations or sister chromatid exchanges in cultured CHO cells. All assays were conducted in the presence and absence of exogenous metabolic activation. 

When inhaled, limonene is readily metabolized, and a long half-life in blood was observed in a slow elimination phase, which indicates accumulation in adipose tissues. About 1% of the total uptake was removed unaltered in the air exhaled after the exposure, while approximately 0.003% was eliminated in the urine (Falk-Filipsson, 1993). 

Numerous in vivo and in vitro studies have been reported on the physiological effects of /(-limoneiie (Crowell and Gould, 1994). Most of the in vivo studies have been carried out with rats, because both humans and rats metabolize limonene in a similar manner (Crowell et al., 1994). The high therapeutic ratio of limonene in the chemotherapy of rodent cancers suggests that limonene may be an efficacious chemotherapeutic agent for human malignancies (Crowell et al., 1994 Elson and Yu, 1994). Table 5.12 summarizes some of the reported physiological actions deduced from in vivo and in vitro studies. 

Crowell, P.L., Elson, C.E., Bailey, H.H., Elegbede, A., Haag, D.J. and Gould, M.N. 1994. Human metabolism of experimental cancer therapeutic agent 4-limonene. Cancer Chemother. Pharmacol. 35 31-37. 

Comparative studies with acetaminophen indicate that metabolism in the hamster resembles more closely that in man than in mouse. d-Limonene (32) is metabolized in rat, rabbit and hamster preferentially to C-l carboxylic acid derivatives, whereas dog and man largely form the 8,9-diol, and the guinea pig metabolizes 32 by these two pathways to the same extent . Compound 41 is substantially oxidized by rat and guinea pig, but not man, to biphenylacetic acid this possibly explains the ineffectiveness of 41 in rheumatoid arthritis . Sedative-hypnotic 42 is N-hydroxvIated in the cat, a more... 

Limonene is metabolized predominantly to (+)-perillic acid (103 R = CO2H) by Pseudomonas incognita, presumably via C-7 hydroxylation, which has also been reported in the conversion of a-terpineol into 7-hydroxy-a-terpineol in further work (see Vol. 8, p. 26) on callus cultures of Nicotiana tabacum and is also presumed to be involved to account for the isolation of oleuropeic acid (140)... 

Alonso-Gutierrez, J. et al (2013) Metabolic engineering of Escherichia coli for limonene and perillyl alcohol production. Metab. Eng., 19, 33-41. 

Limonene as a by-product of orange juice production can be used for polymer production together with CO2 to form a polystyrol-like foam. Menthon can be used as a precursor for branched dicarbonic acids and alpha-pinen (a by-product of the forest industry) as precursor for cymol production or for enzymatic verbenone production. Other important terpenoids are phytosterols. They are used in the production of steroid hormones such as cortison or progesteron by Bayer Sobering Pharma. Probably the most important example where plant secondary metabolism and microbial fermentation are synergistically combined, steroid hormones, are produced from phytosterols. A fermentation process by Bayer Schering Pharma uses a plant metaboUte and converts it by a single-step microbial fermentation into the desired end product. 

Miyazawa, M., Shindo, M., and Shimada, T. (2002) Sex differences in the metabolism of (+) -and (-)-limonene enantiomers to carveol and perillyl alcohol derivatives by cytochrome P450 enzymes in rat liver micro-somes. 15(1) 15-20. 

FIGURE 9.15 Proposed metabolism of (+)-limonene in rats, rabbits, guinea pigs, dogs, and humans. (Adapted from Crowell, P.L. et al., Cancer Chemother. Pharmacol., 31, 205, 1992 Miyazawa, M. et al.. Drug Metab. Dispos., 30, 602, 2002 Shimada, T. et al.. Drug Metab. Pharmacokin., 17, 507, 2002.)... 

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