Identification of intermediates and enzymes involved in the early steps of artemisinin biosynthesis in Artemisia annua.

An important group of antimalarial drugs consists of the endoperoxide sesquiterpene lactone artemisinin and its derivatives. Only little is known about the biosynthesis of artemisinin in Artemisia annua L., particularly about the early enzymatic steps between amorpha-4,11-diene and dihydroartemisinic acid. Analyses of the terpenoids from A. annua leaves and gland secretory cells revealed the presence of the oxygenated amorpha-4,11-diene derivatives artemisinic alcohol, dihydroartemisinic alcohol, artemisinic aldehyde, dihydroartemisinic aldehyde and dihydroartemisinic acid. We also demonstrated the presence of a number of biosynthetic enzymes such as the amorpha-4,11-diene synthase and the--so far unknown--amorpha-4,11-diene hydroxylase as well as artemisinic alcohol and dihydroartemisinic aldehyde dehydrogenase activities in both leaves and glandular trichomes. From these results, we hypothesise that the early steps in artemisinin biosynthesis involve amorpha-4,11-diene hydroxylation to artemisinic alcohol, followed by oxidation to artemisinic aldehyde, reduction of the C11-C13 double bond to dihydroartemisinic aldehyde and oxidation to dihydroartemisinic acid.

Germacrenes from fresh costus roots.

Four germacrenes, previously shown to be intermediates in sesquiterpene lactone biosynthesis, were isolated from fresh costus roots (Saussurea lappa). The structures of (+)-germacrene A, germacra-1(10),4,11(13)-trien-12-ol, germacra-1(10),4,11(13)-trien-12-al, and germacra-1(10),4,11(13)-trien-12-oic acid were deduced by a combination of spectral data and chemical transformations. Heating of these compounds yields (-)-beta-elemene, (-)-elema-1,3,11(13)-trien-12-ol, (-)-elema-1,3,11(13)-trien-12-al, and elema-1,3,11(13)-trien-12-oic acid respectively, in addition to small amounts of their diastereomers. Acid induced cyclisation of the germacrenes yields selinene, costol, costal, and costic acid respectively. It is highly probable that the elemenes reported in literature for costus root oil are artefacts.

Biosynthesis of germacrene A carboxylic acid in chicory roots. Demonstration of a cytochrome P450 (+)-germacrene a hydroxylase and NADP+-dependent sesquiterpenoid dehydrogenase(s) involved in sesquiterpene lactone biosynthesis.

Sprouts of chicory (Cichorium intybus), a vegetable grown in the dark, have a slightly bitter taste associated with the presence of guaianolides, eudesmanolides, and germacranolides. The committed step in the biosynthesis of these compounds is catalyzed by a (+)-germacrene A synthase. Formation of the lactone ring is the postulated next step in biosynthesis of the germacrene-derived sesquiterpene lactones. The present study confirms this hypothesis by isolation of enzyme activities from chicory roots that introduce a carboxylic acid function in the germacrene A isopropenyl side chain, which is necessary for lactone ring formation. (+)-germacrene A is hydroxylated to germacra-1(10),4,11(13)-trien-12-ol by a cytochrome P450 enzyme, and is subsequently oxidized to germacra-1(10),4,11(13)-trien-12-oic acid by NADP+-dependent dehydrogenase(s). Both oxidized germacrenes were detected as their Cope-rearrangement products elema-1,3,11(13)-trien-12-ol and elema-1,3,11(13)-trien-12-oic acid, respectively. The cyclization products of germacra-1(10),4,11(13)-trien-12-ol, i.e. costol, were also observed. The (+)-germacrene A hydroxylase is inhibited by carbon monoxide (blue-light reversible), has an optimum pH at 8.0, and hydroxylates beta-elemene with a modest degree of enantioselectivity.

Amorpha-4,11-diene synthase catalyses the first probable step in artemisinin biosynthesis.

The endoperoxide sesquiterpene lactone artemisinin and its derivatives are a promising new group of drugs against malaria. Artemisinin is a constituent of the annual herb Artemisia annua L. So far only the later steps in artemisinin biosynthesis--from artemisinic acid--have been elucidated and the expected olefinic sesquiterpene intermediate has never been demonstrated. In pentane extracts of A. annua leaves we detected a sesquiterpene with the mass spectrum of amorpha-4,11-diene. Synthesis of amorpha-4,11-diene from artemisinic acid confirmed the identity. In addition we identified several sesquiterpene synthases of which one of the major activities catalysed the formation of amorpha-4,11-diene from farnesyl diphosphate. This enzyme was partially purified and shows the typical characteristics of sesquiterpene synthases, such as a broad pH optimum around 6.5-7.0, a molecular mass of 56 kDa, and a K(m) of 0.6 microM. The structure and configuration of amorpha-4,11-diene, its low content in A. annua and the high activity of amorpha-4,11-diene synthase all support that amorpha-4,11-diene is the likely olefinic sesquiterpene intermediate in the biosynthesis of artemisinin.

Metabolism of the insecticide teflubenzuron in rats.

1. The metabolic fate of the insecticide teflubenzuron, orally dosed to the male Wistar rat, was investigated. Particular attention was paid to the metabolic fate of the benzoyl and aniline moiety after hydrolysis of the urea bridge. 2. The 0-48-h urinary and faecal metabolic patterns and recoveries showed that for a dose range of 4-53 mumol (1.5-20 mg) teflubenzuron, 90% of the dose was excreted in the faeces mainly in unmodified form, approximately 4.6% was absorbed from the lumen and excreted in the urine, and 5.4% was retained in the body. Metabolites excreted in the urine could be identified as benzoate and aniline derivatives originating from the two aromatic rings of teflubenzuron liberated from the parent molecule by hydrolysis of the urea bridge. 3. The amount of urinary benzoate-type metabolites was about eight times the amount of aniline-type metabolites, indicating significant differences in efficiency of urinary excretion of the benzoate moiety as compared with the aniline ring. 4. To investigate further the possible reason underlying this difference in urinary excretion efficiency between the two aromatic derivatives formed from teflubenzuron, dose-recovery studies of these aniline- and benzoate-type metabolites were performed. These studies confirmed the discrepancy observed between the urinary recovery of the benzoyl and the aniline moiety of teflubenzuron. 5. Additional results of the present study indicate that the above discrepancy can be explained by the fact that the benzoate derivative is excreted mainly in its unmetabolized form, whereas the aniline derivative needs additional phase I and II modifications before it can be excreted from the body, the former being a relatively slow reaction. Furthermore, conversion of the halogenated aniline derivative in phase I metabolism might result in a reactive benzoquinone-type or N-oxidized primary metabolite, which can be retained in the body due to reaction with cellular macromolecules.