Metabolism of Monoterpenes

Abstract

ABSTRACr The 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 cyclopen-tane). Analysis of the disposition of (+G3Hjcamphor applied to mid-stem 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 offici-nalis), 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 gluco-sylation 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 phy-tosterols 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 acid, l-carboxymethyl-3-hydroxy-2,2,3-trimethyl cyclo-pentane (Fig. 1), which is presumed to be the transport derivative in this species (6). In this communication we describe experiments which confirm the transport of the bis-glucose derivative from leaves to roots, and which indicate that carbon from the terpenoid moiety is ultimately reutilized in the synthesis of acyl lipids and phytosterols of the root membranes. Thus, although the pathways for the catabolism of (-)-menthone and (+)-cam-phor in peppermint and sage, respectively, differ in detail, the fate of the terpenoid and the overall function of the catabolic process appear to be the same in both species. MATERIALS AND METHODS Plant Material, Substrates, and Reagents. Common sage (Sal-via officinalis L.) plants were grown from seed under controlled conditions described previously (6). Unless otherwise specified, tissues of flowering plants were used for all experiments.