Metabolism of Monoterpenes

Abstract

Previous studies have shown that the monoterpene ketone I1G-'3H1-menthone is reduced to the epimeric alcohols i-menthol and d-neomenthol in leaf discs of flowering peppermint (Mentha piperita L.), and that a portion of the menthol is converted to menthyl acetate while the bulk of the neomenthol is transformed to neomenthyl-,B--glucoside (Croteau, Martinkus 1979 Plant Physiol 64: 169-175 2Although the systematic name for l-menthone is (5R,2S)-trans-5-methyl-2-(1-methylethyl)cyclohexanone, we have utilized here the more common nomenclature based on numbering of thep-menthane system (i.e. menthone = p-menthan-3-one) in which the methyl-substituted carbon is IR and the isopropyl-substituted carbon is 4S.'Author to whom inquiries should be made.variety of sources, including tracer studies and analyses of both short term and long term variation in monoterpene content (23, 24). Rapid and permanent turnover of monoterpenes has been shown to occur in flowering peppermint plants (Mentha piperita L.), and during this period of apparent catabolism, at least some of the otherwise undamaged leaf oil glands are emptied of their contents (3, 9). Coincident with the decrease in monoterpene content of peppermint leaves is the conversion of the major monoterpene constituent, l-menthone, to i-menthol and to lesser quantities of 1-menthyl acetate and d-neomenthol (4). Similar metabolic processes occur in other Mentha species (14). Detailed studies of the metabolism of l-[G-3H]menthone in peppermint leaf discs confirmed earlier observations that menthone was converted to menthol and menthyl acetate, and furthermore revealed that a significant proportion of the 1-menthone was transformed to dneomenthyl-,8-D-glucoside (7,9) (Fig. 1). Little neomenthyl acetate or menthyl-,f-D-glucoside was formed from [G-3HJmenthone, indicating a high degree of specificity in the metabolic disposition of the epimeric reduction products of the ketone (i.e. menthol and menthyl acetate accumulate in the volatile oil, whereas neomenthol is specifically converted to the water-soluble glucoside). Further analytical studies and in vivo tracer studies indicated that roughly half of the 1-menthone metabolized was converted to 1-menthol (of which approximately 20%o was acetylated), while the remaining half was transformed to d-neomenthol (of which nearly all was glucosylated) (9). When l-[G-3H]menthone was applied to leaves ofintact mint plants, the resulting [3Hlneomenthyl glucoside could be detected in the roots, and was shown to undergo subsequent conversion to unidentified polar products at this location (9,18). While a number of non-iridoid monoterpenyl glycosides have been reported in plants (2,5,12,19, 26,28,30,32,33), including a recent report on the occurrence of l-menthyl-fB-Dglucoside in the rhizomes of Japanese peppermint (Mentha arvensis Mal. x M. piperita L.) (27), and the suggestion made that such glycosides are transport derivatives (1 1, 15), studies with peppermint were the first to implicate glycosylation of monoterpenols directly a...

Metabolism of Monoterpenes

“…Lyophilization of the methanol-soluble products revealed that this material contained little 3H20 (<2%), while acid hydrolysis followed by recovery of (3H]neomenthol indicated the presence of relatively little neomenthyl glucoside (<1%). The latter observation is in sharp contrast to comparable experiments with peppermint leaves in which the glucoside is the major product of neomenthol metabolism (27). Preliminary examination by TLC of the methanol-soluble fraction (silica gel G with ethermethanol [1:1, v/vJ) and of the nonvolatile, ether-solubles (silica gel G with hexane:ethyl acetate [4:1, v/v]) indicated the presence in both cases of several products of diverse polarity.…”

“…1). The high degree of selectivity in the metabolic disposition of the epimeric reduction products of the ketone was subsequently shown to result from compartmentation (27). Examination of the specificity and location of each enzyme of the sequence revealed the NADPH-dependent, menthol-specific dehydrogenase and the relatively nonspecific acetyl CoA-dependent acetyl transferase to reside primarily in the epidermis (presumably the epidermal oil glands), whereas the NADPH-dependent, neomenthol-specific dehydrogenase and associated UDP-glucose:monoterpenol glucosyl transferase were located almost exclusively in the mesophyll (14,18,23,27).…”

Metabolism of Monoterpenes: Conversion of l-Menthone to l-Menthol and d-Neomenthol by Stereospecific Dehydrogenases from Peppermint (Mentha piperita) Leaves

“…In peppermint, the metabolic disposition of the diastereomeric reduction products of 1-menthone is highly specific in that only 1-menthyl acetate (with little d-neomenthyl acetate) and d-neomenthyl-,f-D-glucoside (with little l-menthyl-fi-D-glucoside) are formed (6,12). In vivo experiments, using labeled CO2, menthone, menthol and neomenthol as precursors, provided compelling evidence that compartmentation effects, and not the substrate specificity of the transferases, were responsible for the selectivity of pathways observed; this conclusion was confirmed by isolation and characterization of the acetyl CoA:monoterpenol acetyltransferase and the UDP-glucose:monoterpenol glucosyltransferase involved (5,18). The nature of the terpenyl moiety of the acetate and ,B-D-glucoside is, thus, determined at the menthone reduction step by compartmentation of these stereospecific reactions with the appropriate, relatively nonselective, transferase.…”

“…1) (6,12,18). Based on sequential analysis of the terpene composition of several other species (7, 8, 10, 1 1, 20, 21) 2Although the systematic name for 1-menthone is (5R,2S)-trans-5-methyl-2-(1-methylethyl)cyclohexanone, we have utilized here the more common nomenclature based on numbering of thep-menthane system (i.e.…”

“…1) (6,12,18 Finely powdered (NH4)2SO4 was added to the 105,000g supernatant (at 0-4°C) over a period of 30 min until 30%o saturation (0.176 g/ml) was obtained. Precipitated protein was removed by centrifugation at 27,000g (10 min), and the supernatant was brought to 80%1o saturation with (NH4)2SO4 (0.561 g/ml), as before.…”