Metabolism of Monoterpenes

Abstract

ABSTRACT(-)-Menthone, the major monoterpene component of the essential oil of maturing peppermint (Meathapiperita L.) laves (6 micromoles per leaf) is rapl metabolzed at the onset of flowering with a concomitant rise in the level of (-)-menthol (to about 2 micromoles per leaf). Exogenous (-)-HG3Hlmenthone is converted into (-)-I3HImenthol as the major steam-volatile product in kaf discs in flowering peppermint (10% of incorporated tracer); however, the major portion of the incorporated tracer (86%) resided in the novolatile metabolites of (-)-IG-'Hlmenthone. Acid hydrolysis of the nonvolatile material released over half of the radioactivity to the steamvolatile fraction, and the major component of this fraction was identified as (+)-neomenthol by radiochromatographic analysis and by synthesis of crystalline derivatives, thus suggesting the presence of a neomenthyl glycoside. Thin layer chromatography, ion exchange chromatography, and gel permeation chromatography on Bio-Gel P-2 allowed the purification of the putative neomenthyl glycoside, and these results suggested that the glycoside contained a single, neutral sugar residue. Hydrolysis of the purified glycoside, followed by reduction of the resulting sugar moiety with NaB%H4, generated a single labeled product that was subsequentiy identified as glucitol by radio gas-liquid chromatography of both the hexatrimethylsilyl ether and hexaacetate derivative, and by crystallization to constant specific radioactivity of both the alditol and the corresponding hexabenzoate. These results, along with studies on the hydrolysis of the glycoside by specific glycosidases, strongly suggest that (+)-neomenthyl-/8-D-glucoside is a major metabolite of (-)-menthone in flowering peppermint. This is the first report on the occurrence of a neomenthyl glycoside, and the first evidence implicating glycosylation as an early step in monoterpene catabolismL While monoterpenes were once regarded as metabolically inert "secondary products" (17), considerable evidence, based on both short term tracer studies and long term periodic analyses, now indicates that monoterpenes are, in fact, metabolically active and subject to rapid turnover in plants (15 Evidence for monoterpene turnover in peppermint (Mentha piperita L.) has been obtained by periodic analysis of plants grown under controlled conditions (7). In these studies, midstem leaves accumulated high levels of (-)-menthone until the time of floral initiation, and then lost much of the menthone. Previous evidence has indicated that (-)-menthone can be converted to (-)-menthol (4,5,18) and (-)-menthyl acetate (9) in mint and, during the period of menthone catabolism, a portion of the menthone appeared to be converted to these metabolites. However, the quantity of menthol and menthyl acetate formed was not sufficient to account for the amount ofmenthone lost, and no other metabolites of menthone have been identified. Here we describe, for the first time, the identification of (+)-neomenthyl-,6-D-glucoside as a major metabolite of (-)-mentho...

Metabolism of Monoterpenes

“…Coincident with the overall decrease in monoterpene content of peppermint leaves is the conversion of the major monoterpene constituent, I-menthone, to i-menthol and to lesser quantities of l-menthyl acetate and d-neomenthol3 (5). Detailed examination of the metabolism of l-[G-3H]menthone in peppermint leafdiscs confirmed earlier observations that menthone was converted to menthol and menthyl acetate, and additionally revealed that up to half of the l-menthone metabolized was transformed via neomenthol to d-neomenthol-fl-D-glucoside (14,17) (Fig. 1).…”

“…Rapid turnover of monoterpenes has been shown to occur in mature leaves of flowering peppermint plants (Mentha piperita L.) (4,17 (14,17) (Fig. 1).…”

“…Rapid turnover of monoterpenes has been shown to occur in mature leaves of flowering peppermint plants (Mentha piperita L.) (4,17). During this period ofactive metabolism, at least some of the leaf oil glands are emptied of their terpenoid contents.…”

“…Each step in the main reaction sequence, from hydrolysis of neomenthyl glucoside to lactonization of menthone, was demonstrated in cell-free extracts from the rhizomes of flowering mint plants. The lactonization step is of particular significance in providing a means of cleaving the p-menthane ring to afford an acyclic carbon skeleton that can be further degraded by modifications of the well-known #-oxidation sequence.Rapid turnover of monoterpenes has been shown to occur in mature leaves of flowering peppermint plants (Mentha piperita L.) (4,17 (14,17) (Fig. 1).…”

Temperature and Photoperiod Influence Trichome Density and Sesquiterpene Content of Lycopersicon hirsutum f. hirsutum

“…Higher light intensity and duration usually promote terpene accumulation (8,16), and cool nights have been found to increase monoterpene concentration in peppermint (Mentha piperita L.) before flowering (12). Monoterpene concentrations are generally higher in young leaves and peak at or before flowering (3,12), with increased rates of catabolism at the time of flowering (4,5).…”