Isolation and characterization of a presumed biosynthetic precursor of camptothecin from extracts of Camptotheca acuminata

Divergent camptothecin biosynthetic pathway in Ophiorrhiza pumila

BACKGROUND

The anticancer drug camptothecin (CPT), first isolated from Camptotheca acuminata, was subsequently discovered in unrelated plants, including Ophiorrhiza pumila. Unlike known monoterpene indole alkaloids, CPT in C. acuminata is biosynthesized via the key intermediate strictosidinic acid, but how O. pumila synthesizes CPT has not been determined.

RESULTS

In this study, we used nontargeted metabolite profiling to show that 3α-(S)-strictosidine and 3-(S), 21-(S)-strictosidinic acid coexist in O. pumila. After identifying the enzymes OpLAMT, OpSLS, and OpSTR as participants in CPT biosynthesis, we compared these enzymes to their homologues from two other representative CPT-producing plants, C. acuminata and Nothapodytes nimmoniana, to elucidate their phylogenetic relationship. Finally, using labelled intermediates to resolve the CPT biosynthesis pathway in O. pumila, we showed that 3α-(S)-strictosidine, not 3-(S), 21-(S)-strictosidinic acid, is the exclusive intermediate in CPT biosynthesis.

CONCLUSIONS

In our study, we found that O. pumila, another representative CPT-producing plant, exhibits metabolite diversity in its central intermediates consisting of both 3-(S), 21-(S)-strictosidinic acid and 3α-(S)-strictosidine and utilizes 3α-(S)-strictosidine as the exclusive intermediate in the CPT biosynthetic pathway, which differs from C. acuminata. Our results show that enzymes likely to be involved in CPT biosynthesis in O. pumila, C. acuminata, and N. nimmoniana have evolved divergently. Overall, our new data regarding CPT biosynthesis in O. pumila suggest evolutionary divergence in CPT-producing plants. These results shed new light on CPT biosynthesis and pave the way towards its industrial production through enzymatic or metabolic engineering approaches.

Minor alkaloids from an aqueous extract of the hook-bearing stem of Uncaria rhynchophylla

Seven new monoterpene alkaloids (1-7), along with 18 known analogues, were isolated from an aqueous decoction of the hook-bearing stems of Uncaria rhynchophylla (Gou-teng). Their structures were determined by spectroscopic data analysis and electronic circular dichroism (ECD) calculations. Compound 1 is the first monoterpene 22-norindoloquinolizidine alkaloid with a ketene unit, while 2 and 3 are unusual indoloquinolizidine alkaloids having an oxazinane ring.[Formula: see text].

Distribution of camptothecin biosynthetic intermediates and identification the rate-limiting step of camptothecin biosynthesis

Two key biosynthetic intermediates (pumiloside and strictosamide) of camptothecin were isolated. A high performance liquid chromatography-ultraviolet (HPLC-UV) method was developed to determine four main alkaloid compounds (pumiloside, strictosamide, camptothecin and 10-hydroxycamptothecin) and estimate two minor compounds (deoxypumiloside, 9-methoxycamptothecin) simultaneously in different parts of Camptotheca acuminata, with a good linearity and R² > 0.999 for all curves. The results indicated that there was a positive correlation between the two key intermediates (strictosamide and pumiloside) and camptothecin in vivo. The speculation that the root was the synthetic position of camptothecin in vivo was confirmed. The rate-limiting step of camptothecin biosynthesis was estimated the step from pumiloside to deoxypumiloside based on its concentration fall sharply.

Bifunctional Cytochrome P450 Enzymes Involved in Camptothecin Biosynthesis

Camptothecin (CAM) is a well-known, complex, plant-derived antitumor monoterpenoid indole alkaloid (MIA). Featuring a unique pentacyclic pyrroloquinoline scaffold, CAM is biosynthetically distinct from the other known MIAs, such as antitumor vincristine and vinblastine. Herein, CaCYP72A565 and CaCYP72A610 enzymes involved in the biosynthesis of the monoterpenoid moiety of CAM were cloned from CAM-producing  Camptotheca acuminata. Heterologous overexpression and functional characterization assays showed that CaCYP72As catalyzes two consecutive reactions, the stereoselective hydroxylation at C-7 of 7-deoxyloganic acid and the subsequent carbon-carbon (C-C) bond cleavage between C-7 and C-8 of iridoid glucoside, to generate the intramolecular cyclopentane ring-opening secoiridoid glucoside. Comparative metabolite profiling analyses suggested that C. acuminata synthesizes loganic acid, secologanic acid, and strictosidinic acid as its MIA carboxylic acid intermediates. CaCYP72As are novel bifunctional enzymes that catalyze stereoselective hydroxylation and subsequent C-C bond cleavage reactions to give a ring-opening product with two functional groups, an aldehyde and a double bond.

Suppression of camptothecin biosynthetic genes results in metabolic modification of secondary products in hairy roots of Ophiorrhiza pumila

Camptothecin is a monoterpenoid indole alkaloid that exhibits anti-tumor activity. In Ophiorrhiza pumila, production of camptothecin and its related alkaloids was high in the hairy roots, but not in the cell suspension culture derived from hairy roots. To identify the intermediates in camptothecin biosynthesis, expression of genes encoding tryptophan decarboxylase (TDC) and secologanin synthase (SLS), the two enzymes catalyzing the early steps in camptothecin biosynthesis, were suppressed in the hairy roots of O. pumila by RNA interference (RNAi), and metabolite changes were investigated. In most TDC- and SLS-suppressed lines, accumulation of camptothecin and related alkaloids, strictosidine, strictosamide, pumiloside, and deoxypumiloside was reduced. The accumulation levels of secologanin exhibited a strong negative correlation with the expression level of TDC, and that of loganin exhibited a negative correlation with the expression level of SLS. Some hairy root-specific chromatographic peaks detected by liquid chromatography Fourier transform ion cyclotron resonance mass spectrometry (LC-FTICR-MS) exhibited positive or negative correlation with TDC expression, suggesting their possible involvement in camptothecin biosynthesis.

1-(1-Hy-droxy-eth-yl)-7,8-dihydro-indolo[2,3-a]pyridine-[3,4-g]quinolizin-5(13H)-one (angustoline) monohydrate from Nauclea subdita (Rubiaceae)

THE TITLE COMPOUND (TRIVIAL NAME: angustoline monohydrate), C(20)H(17)N(3)O(2)·H(2)O, features a fused-ring system formed by one five- and four six-membered rings. The nearly planar benzimidazole portion (r.m.s. deviation = 0.008 Å) and the nearly planar 2,7-naphthyridin-1-one portion (r.m.s. deviation = 0.022 Å) of the fused-ring system are slightly twisted, with a dihedral angle of 9.47 (8)°, owing to the tetra-hedral nature of the two methyl-ene linkages in the central six-membered ring. The secondary N atom acts as a hydrogen-bond donor to the water molecule of crystallization. In the crystal, the amino and hy-droxy groups, and the water mol-ecule are engaged in hydrogen bonding, generating a three-dimensional network.

Paratunamides A-D, oxindole alkaloids from Cinnamodendron axillare

Four new oxindole alkaloids, paratunamides A-D (1-4), containing a secologanin unit, were isolated from the bark of Cinnamodendron axillare, and their structures and relative configurations were elucidated by spectroscopic data. The absolute configuration at C-7 in 1-4 was assigned as S, S, R, and S, respectively, on the basis of the CD spectra.

Camptothecin: current perspectives

This review provides a detailed discussion of recent advances in the medicinal chemistry of camptothecin, a potent antitumor antibiotic. Two camptothecin analogues are presently approved for use in the clinic as antitumor agents and several others are in clinical trials. Camptothecin possesses a novel mechanism of action involving the inhibition of DNA relaxation by DNA topoisomerase I, and more specifically the stabilization of a covalent binary complex formed between topoisomerase I and DNA. This review summarizes the current status of studies of the mechanism of action of camptothecin, including topoisomerase I inhibition and additional cellular responses. Modern synthetic approaches to camptothecin and several of the semi-synthetic methods are also discussed. Finally, a systematic evaluation of novel and important analogues of camptothecin and their contribution to the current structure-activity profile are considered.

Camptothecin, over four decades of surprising findings

Camptothecin (CPT) is a modified monoterpene indole alkaloid produced by Camptotheca acuminata (Nyssaceae), Nothapodytes foetida, Pyrenacantha klaineana, Merrilliodendron megacarpum (Icacinaceae), Ophiorrhiza pumila (Rubiaceae), Ervatamia heyneana (Apocynaceae) and Mostuea brunonis (Gelsemiaceae), species belonging to unrelated orders of angiosperms. From the distribution of CPT and other secondary metabolites, it has been postulated that the genes encoding enzymes involved in their biosynthesis evolved early during evolution. These genes were presumably not lost during evolution but might have been "switched off" during a certain period of time and "switched on" again at some later point. The CPT derivatives, irinotecan and topotecan, are used throughout the world for the treatment of various cancers, and over a dozen more CPT analogues are currently at various stages of clinical development. The worldwide market size of irinotecan/topotecan in 2002 was estimated at about $750 million and at $1 billion by 2003. In spite of the rapid growth of the market, CPT is still harvested by extraction from bark and seeds of C. acuminata and N. foetida. All parts of C. acuminata contain some CPT, although the highest level is found in young leaves (approximately 4-5 mg g(-1) dry weight), approximately 50% higher than in seeds and 250% higher than in bark. The development of hairy root cultures of O. pumila and C. acuminata, and the cloning and characterization of genes encoding key enzymes of the pathway leading to CPT formation in plants has opened new possibilities to propose alternative and more sustainable production systems for this important alkaloid.