Catharanthine biosynthesis

The effect of light on alkaloid production in these cultures was also evaluated (7/9). More catharanthine was produced in the light than in the dark, and the same observation was made for ajmalicine (39). Knobloch et al. 120) examined the production of anthocyanins, ajmalicine (39), and serpentine (40) in cell suspension cultures and found that although serpentine levels increased 18-fold in the light, ajmalicine levels decreased 50%. In the work of Goodbody et al. 119) the biosynthesis of vindoline (3)... 

The extremely low yield of vincristine (2) from intact plants has made pursuit of its biosynthesis a very challenging problem, which at this point in time remains unsolved. Kutney et al. have used both anhydrovinblastine (8) (227) and catharanthine N-oxide (107) (233) as precursors to vincristine (2) in a cell-free preparation, but incorporation levels were extremely low. Therefore, the question of whether vinblastine (1) is an in vivo, as well as an in vitro, precursor remains to be answered. Several possibilities exist for the overall oxidation of vinblastine (1) to vincristine (2), including a direct oxidation of the A-methyl group or oxidative loss of the N-methyl group followed by N-formylation. 

Terpenoid Indole Alkaloids.—Important recent work has defined strictosidine (97) as a key intermediate in the biosynthesis of terpenoid indole alkaloids with both 3a- and 3/3-configurations. Some of this work, published earlier in preliminary form (cf. Vol. 9, p. 18), is now available in a full paper.26 In addition to those alkaloids examined earlier, strychnine, gelsemine, vincadifformine, isoreserpiline, aricine, isoreserpinine, and ajmaline have been shown to derive from strictosidine (data are also included for ajmalicine, for catharanthine, and for vindoline which had been reported earlier). 

Over seven-hundred references are concerned with the isolation and chemistry of alkaloids, and approximately one-third of these are devoted to synthesis and biosynthesis. It is perhaps in these areas that the most notable research is to be found. Although it is probably invidious to attempt the exercise, a personal selection of highlights would include new results on the biosynthesis of quinoliz-idine alkaloids (p. 4), the first synthesis of an eleven-membered macrocyclic pyrrolizidine diester (p. 49), the synthesis of Poranthera alkaloids (p. 68), and, in the indole field, the synthesis of tryptoquivalines G and L (p. 152), of a chiral intermediate in the construction of heteroyohimbine alkaloids (p. 167), and of a catharanthine intermediate, using palladium catalysts (p. 186). 

Extensive studies to quantitate the production of indole alkaloids in Catharanthus roseus hairy root cultures have revealed that they accumulate several compounds including ajmalicine, serpentine, catharanthine, tabersonine, horhammericine, and lochnericine.27, 28 The presence of tabersonine in hairy roots has raised speculations that this intermediate in vindoline biosynthesis, together with catharanthine, is transported from this potential site of biosynthesis through the vasculature to the stem and to the leaves where tabersonine is further elaborated into vindoline within laticifers and/or idioblasts.26 However, oxidized derivatives of tabersonine, such as horhammericine and lochnericine, are present at 5 to 15 times the levels of tabersonine in hairy roots,27 and presumably this prevents their transport and/or use for vindoline biosynthesis. In this context, it would be interesting to... 

Another all-carbon Diels-Alder reaction is proposed for the biosynthesis of the indole alkaloids tabersonine 1-6 and catharanthine 1-7 of the Aspidosperma and Iboga family [28-31]. The compounds are formed via strictosidine 1-3, the first nitrogen-containing precursor of the monoterpenoid indole alkaloids, and stemmadenine 1-4, which is cleaved to give the proposed intermediate dehy-drosecodine 1-5 with an acrylate and a 1,3-butadiene moiety (Scheme 1-1). 

Figure 2.11 Biosynthesis of vindoline, catharanthine and the dimeric alkaloids vinblastine and vincristine. T16H, tabersonine-16-hydroxylase OMT, 5-adenosylmethionine 16-hydroxy-tabersonine-O-methyltransferase NMT, 5-adenosylmethionine 16-methoxy-2,3-dihydro-3-hydroxymethyltabersonine-/ /-methyltransferase D4H, desacetoxy-vindoline-4-dioxygenase DAT, acetylcoenzyme A 4-0-deacetylvindoline-4-0-acetyltransferase.

This reaction has been extensively studied in the case of the chlopromazine radical (R -mediated aminopyrine (S) oxidation [41], a typical reaction for xenobiotics, as well as in the case of the vindoline radical (R -mediated catharanthine (S) oxidation [42], a key reaction in the biosynthesis of the anticancer drugs, vinblastine and vincristine, which are obtained from Catharanthus roseus. 

Terpenoid Indole Alkaloids.—Crude preparations from Catharanthus roseus seedlings and from tissue cultures have been shown to be capable of synthesizing in good yield from tryptamine (110) and secologanin (111) with added co-factors, geisso-schizine (112) and ajmalicine (113), representatives of the first major class of terpenoid indole alkaloids (Corynanthe). Geissoschizine was metabolized to ajmalicine and several other unidentified alkaloids (for reviews of biosynthesis in intact plants see ref. 106). The catabolic turnover of vindoline and catharanthine in C. roseus has been studied. ... 

The biosynthesis of the isoprenoid moiety of terpenoid alkaloids has been reviewed. It has been found ° that catharanthine (which accumulates in Vinca rosea) inhibited a membrane-bound monoxygenase that oxidized geraniol at C-10. The inhibition was reversible and non-competitive in solubilized preparations of the enzyme and hence was probably not due to disruption of membranes. Other alkaloids that were produced as end-products were less inhibitory and catharanthine may mediate feed-back control of alkaloid biosynthesis in vivo. 

Although it can be presumed that dimeric alkaloids like vincaleukoblastine (149) are formed in vivo by joining bases like catharanthine (150) and vindoline (151) together, biosynthetic evidence for such a view has been lacking. This contrasts with the biosynthesis of (150) and (151), which is fairly well understood. 

Fig. (2). Biosynthesis of vinblastine from the monomeric precursors catharanthine and vindoline. Anhydrovinblastine is the direct product of the dimerization reaction and the precursor of the anticancer drugs. Shaded areas indicate the structural differences between the precursor catharanthine and the deavamine part of...

Fig. (5). Biosynthesis of catharanthine and tabersonine from strictosidine, the central precursor of all terpenoid indole alkaloids. SGD strictosidine p-D-glucosidade.

The search of the enzyme responsible for the dimerization reaction, i.e. for the biosynthesis of anhydrovinblastine, resulted in the finding that peroxidase-like activities extracted from cell suspension cultures were capable of performing the coupling of catharanthine and vindoline into anhydrovinblastine [125-127]. Horseradish peroxidase, a commercial plant peroxidase, was also capable of performing the coupling reaction [128]. 

Terpenoid indole alkaloid biosynthesis actually starts with the coupling of tryptamine and secologanin (Fig. 12). In the next step, a glucosidase splits off the sugar moiety and the reactive dialdehyde formed is further converted through different pathways to a cascade of products, including ajmalicine, catharanthine, tabersonine, and vindoline. 

The knowledge of the biosynthesis of catharanthine is very limited only some feeding experiments with labeled precursors have been described, quite some years ago. Qureshi and Scott (227-229) reported that catharanthine is formed from tabersonine fed to the plant. However, other groups have not been able to confirm these results (230-232). Corynantheine aldehyde (229) and geissoschizine fed to C. roseus plants were reported to be incorporated into catharanthine (233). From these experiments it is believed that the pathway goes from strictosidine via 4,21-dehydrogeissoschizine, stemmadenine, and dehydrosecodine (Fig. 16). Based on the structures, the involvement of tabersonine in the catharanthine pathway is not likely, despite the reports of its incorporation. So far, nothing is known about the enzymes involved in this pathway. 

The biosynthesis of the terpenoid indole alkaloids in C. roseus has been studied extensively, but still the pathway has not yet been completely elucidated on the level of the intermediates. Particularly, the secoiridoid pathway, and the different pathways after strictosidine leading to, for example, tabersonine and catharanthine are not yet completely known. On the level of the enzymes, certain steps have now been quite well characterized, but others remain unknown. The conversion of loganin into secologanin is one of the intriguing unresolved problems, although it is not a rate-limiting step. Even possible intermediates and the chemical mechanism behind this conversion are not clear, despite quite extensive studies. 

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