Metabolism of Monoterpenes

Croteau, Rodney; Sood, V.K.; Renstrøm, Britta; Bhushan, Ravi

doi:10.1104/pp.76.3.647

Cited by 35 publications

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References 19 publications

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“…Previous investigations have indicated that peppermint monoterpenes can be metabolically degraded at later stages of leaf development (Croteau, 1988). A catabolic pathway was described involving the sequential reduction and glucosylation of menthone to neomenthol glucoside (Croteau and Martinkus, 1979), which is transported to the rhizome and degraded (Croteau et al, 1984;Croteau and Sood, 1985). Although the present study provided no evidence for monoterpene catabolism, the large variances in monoterpene incorporation after pulse labeling (Fig.…”

Section: Rate Of Monoterpene Loss Is Negligible Throughout Leaf Develcontrasting

confidence: 47%

Regulation of Monoterpene Accumulation in Leaves of Peppermint

2000

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Plants synthesize numerous classes of natural products that accumulate during development and are thought to function as constitutive defenses against herbivores and pathogens. However, little information is available about how the levels of such defenses are regulated. We measured the accumulation of monoterpenes, a model group of constitutive defenses, in peppermint (Mentha ؋ piperita L.) leaves and investigated several physiological processes that could regulate their accumulation: the rate of biosynthesis, the rate of metabolic loss, and the rate of volatilization. Monoterpene accumulation was found to be restricted to leaves of 12 to 20 d of age, the period of maximal leaf expansion. The rate of monoterpene biosynthesis determined by 14 CO 2 incorporation was closely correlated with monoterpene accumulation, as determined by gas chromatographic analysis, and appeared to be the principal factor controlling the monoterpene level of peppermint leaves. No significant catabolic losses of monoterpenes were detected throughout leaf development, and monoterpene volatilization was found to occur at a very low rate, which, on a monthly basis, represented less than 1% of the total pool of stored monoterpenes. The composition of volatilized monoterpenes differed significantly from that of the total plant monoterpene pool, suggesting that these volatilized products may arise from a separate secretory system. With the demonstration that the rate of biosynthesis is the chief process that determines monoterpene accumulation in peppermint, efforts to improve production in this species can now focus on the genes, enzymes, and cell differentiation processes that regulate monoterpene biosynthesis.

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Section: Rate Of Monoterpene Loss Is Negligible Throughout Leaf Develcontrasting

confidence: 47%

Regulation of Monoterpene Accumulation in Leaves of Peppermint

2000

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“…1) (16). The lactone, in turn, is metabolized to several unidentified nonvolatile polar and nonpolar products.…”

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Metabolism of Monoterpenes

Croteau

Sood²

1985

Plant Physiol.

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l-Menthone of peppermint leaves is reduced to d-neomenthol which is glucosylated and transported to the rhizome, whereupon the jf-D-glucoside is hydrolyzed, the aglycone oxidized back to I-menthone, and this ketone converted to l-3,4-menthone lactone. I-IG-3H1-3,4-Menthone lactone and its labeled progenitors, when incubated with excised mint rhizomes, gave rise to nonvolatile lipids as well as polar metabolites. The lipids thus generated consisted of labeled squalene and phytosterols in the nonsaponifiable fraction and C14-C26 fatty acids in the saponifiable fraction. These results imply degradation of the terpenoid to acetylcoenzyme A and reduced pyridine nucleotide, and reincorporation of label via these products. Starch and soluble carbohydrates were also found to be labeled; however, chemical degradation of the [Hjglucose obtained on hydrolysis of starch indicated the presence of tritium only on interior carbons, suggesting that labeling had occurred yia reduced pyridine nucleotides. Analysis of the labeled organic acids revealed the presence of several hydroxy methylacyl intermediates suggesting the operation of a modified j-oxidation pathway in the degradation of the acyclic terpenoid skeleton. The results indicate that monoterpenes transported to the rhizome are oxidized to yield acetyl-coenzyme A and reduced pyridine nucleotides, and suggest that metabolic turnover of monoterpenes in mint represents a mechanism for recycling carbon and energy from foliar terpenes into other metabolites of the rhizome.Monoterpenes accumulated in mature leaves of flowering peppermint plants (Mentha piperita L.) undergo metabolic turnover by a mechanism involving, as an early step, the reduction of the major monoterpene component, /-menthone2, to the epimeric alcohols /-menthol and d-neomenthol (7, 23). The latter alcohol is preferentially converted to the /3-D-glucoside and transported to the rhizome ( Fig. 1) (15). The kinetics of d-neomenthol-/3-Dglucoside synthesis and transport have been determined (16), and the location and properties of each enzyme of the metabolic sequence in leaves have been examined (13,17,21,23 2 Although the systematic name for /-menthone is (5R,2S)-trans-5-methyl-2-( 1-methylethyl)cyclohexanone, we have utilized here the more common nomenclature based on numbering of the p-menthane system (i.e. menthone = p-menthan-3-one) in which the methyl-substituted carbon is 1 R and the isopropyl-substituted carbon is 4S.On reaching the rhizome, the glucoside is hydrolyzed and the aglycone oxidized back to menthone, which undergoes oxygenation to yield 1-3,4-menthone lactone (i.e., the biological equivalent of the Baeyer-Villiger reaction) ( Fig. 1) (16). The lactone, in turn, is metabolized to several unidentified nonvolatile polar and nonpolar products. Each enzyme of the metabolic sequence from neomenthyl glucoside to menthone lactone has been demonstrated in cell-free preparations from mint rhizomes, and tentative evidence has also been obtained for the enzymic activation of the lactone to the...

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“…In peppermint root, a sufficient number of metabolites, formed subsequent to the lactonization of the monoterpene ketone, could be identified to permit elucidation of a degradative pathway (12,13) (24,25). The microbial pathway also involves lactonization as a means of carbon ringopening (24,25), but further details ofthis pathway are presently unclear (20).…”

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“…The decline in content of the monterpene ketone (-)-menthone in the leaf oil of peppermint (Mentha piperita) has been shown to result from the enzymic reduction of the ketone to (+)-neomenthol, followed by glucosylation of this alcohol and subsequent transport ofthe resulting fl-D-glucoside from leaves to roots (11). On reaching the rhizome and roots, the glucoside is hydrolyzed, the aglycone oxidized ' back to (-)-menthone, and this ketone converted to (-)3,4-menthone lactone (13). The lactone undergoes a modified #-oxidation sequence at this site, analogous to that employed by microorganisms in the degradation of acyclic terpenoids (2), to yield acetyl CoA and reduced pyridine nucleotides which are subsequently utilized in the biosynthesis of acyl lipids and phytosterols of the root membranes (12).…”

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“…The preparations of (+)-[G-3H]camphor (20 Ci/mol) and (+)-[G-3H]neomenthyl-j%D-glucoside (10 Ci/mol) have been described (10,13 (8), and the product was isolated by combination of solvent extraction and microscale steam distillation (9). Following an assessment of specific activity by a combination of aliquot counting and capillary GLC analysis (about 3 gCi of product at specific activity about 0.15 Ci/mol were obtained from 5 mCi '4CO2), the (+)-[U-'4C]camphor was converted to the corresponding 1,2-campholide by peracid oxidation (6,12), and the lactone was isolated by TLC.…”

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Metabolism of Monoterpenes

Croteau¹,

El-Bialy²,

Dehal³

1987

Plant Physiol.

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ABSTRACrThe bicyclic monoterpene ketone (+)-camphor undergoes lactonization to 1,2-campholide in mature sage (Sah'ia officinalis L.) leaves followed by conversion to the jO-Dfglucoside-6-0-glucose ester of the corresponding hydroxy acid (l-carboxymethyl-3-hydroxy-2,2,3-trimethyl cyclopentane). Analysis of the disposition of (+G3Hjcamphor applied to midstem leaves of intact flowering plants allowed the kinetics of synthesis of the bis-glucose derivative and its transport from leaf to root to be determined, and pve strong indication that the transport derivative was subsequently metabolized in the root. Root extracts were shown to possess j@ glucosidase and acyl glucose esterase activities, and studies with (+)-1,2[U-4"Ccampholide as substrate, using excised root segments, revealed that the terpenoid was converted to lipid materials. Localization studies confirmed the radiolabeled lipids to reside in the membranous fractions of root extracts, and analysis of this material indicated the presence of labeled phytosterols and labeled fatty acids (C,4 to C20) of acyl lipids.Although it was not possible to detail the metabolic steps between 1,2-campholide and the acyl lipids and phytosterols derived therefrom because of the lack of readily detectable intermediates, it seemed likely that the monoterpene lactone was degraded to acetyl CoA which was reincorporated into root membrane components via standard acyl lipid and isoprenoid biosynthetic pathways. Monoterpene catabolism thus appears to represent a salvage mechanism for recycling mobile carbon from senescing oil glands on the leaves to the roots.The monoterpene content of the leaf oil of sage (Salvia officinalis), and of several other plants of the Lamiaceae (Labiatae), has been shown to decrease on maturity (3,14,15,21) an observation considered to represent monoterpene turnover rather than simple leaching or evaporative losses of these relatively volatile materials (16,18). The decline in content of the monterpene ketone (-)-menthone in the leaf oil of peppermint (Mentha piperita) has been shown to result from the enzymic reduction of the ketone to (+)-neomenthol, followed by glucosylation of this alcohol and subsequent transport ofthe resulting fl-D-glucoside from leaves to roots (11 back to (-)-menthone, and this ketone converted to (-)3,4-menthone lactone (13). The lactone undergoes a modified #-oxidation sequence at this site, analogous to that employed by microorganisms in the degradation of acyclic terpenoids (2), to yield acetyl CoA and reduced pyridine nucleotides which are subsequently utilized in the biosynthesis of acyl lipids and phytosterols of the root membranes (12). These results suggest that metabolic turnover ofmonoterpenes in mint represents a salvage mechanism for recycling mobile carbon and energy from foliar terpenes into other metabolites of the rhizome.In mature sage leaves, the monoterpene ketone (+)-camphor undergoes lactonization to 1,2-campholide followed by conversion to the fl-D-glucoside-6-O-glucose ester of the corresponding hydroxy ...

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Metabolism of Monoterpenes

Cited by 35 publications

References 19 publications

Regulation of Monoterpene Accumulation in Leaves of Peppermint

Regulation of Monoterpene Accumulation in Leaves of Peppermint

Metabolism of Monoterpenes

Metabolism of Monoterpenes

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