Biosynthesis ajmalicine

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)... 

Although ajmalicine (39) is not on the pathway to the bisindole alkaloids, it is a compound of substantial commercial interest, and several of the intermediates in its formation are probable intermediates in the extended biosynthetic pathway. This work is therefore reviewed for the purpose of completeness of studies on C. roseus. Considerable progress has been made on the biosynthesis of ajmalicine (39), and the studies on the formation of strictosidine (33) and cathenamine (76) have already been described. One of the preparations described by Scott and Lee was a supernatant from a suspension of young seedlings of C. roseus which af-... 

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). 

Cathenamine (100) has been identified as an early intermediate in terpenoid indole alkaloid biosynthesis (cf. Vol. 8, p. 27). It has also been isolated from Guettarda eximia. Another alkaloid, 4,21-dehydrogeissoschizine (99), has now been isolated from this plant it is readily converted into (100) in alkaline solution.29 On incubation with an enzyme preparation from Catharanthus roseus cell cultures in the presence of NADPH at pH 7, (99) was converted into ajmalicine (102), 19-ep/-ajmalicine (103), and tetrahydroalstonine (104), which are the normal products with this enzyme preparation. In the absence of NADPH, cathenamine (100) accumulated.30 The reaction to give (100) proceeded linearly with time, and was dependent on the concentration of protein and substrate. No conversion occurred in the absence of enzyme. 

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... 

Polz, L., Schiibel, H. and Stockigt, J. (1986) Characterisation of 2p(R)-17-0-acetylajmalan acetylesterase a specific enzyme involved in the biosynthesis of the Rauwolfa alkaloid ajmalicine. Z. Naturforsch., 42c, 333 2. 

Schmidt, D. and Stockigt, J. (1995) Enzymic formation of the sarpagan-bridge a key step in the biosynthesis of sarpagine-ajmalicine-type alkaloids. Planta Med., 61, 254-8. 

Fig. 38 The biosynthesis of ajmaline, ajmalicine, a-yohimbine, reserpine, and rescinnamine.

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. ... 

Biosynthesis of oxindole alkaloids in Mitragyna species was studied by Shellard and Houghton, who fed ajmalicine (80) and 3-isoajmalicine to young plants of Sri Lankan M. parvifolia. Both precursors were incorporated into mitraphylline (149) and isomitraphylline (148) produced by M. parvifolia (97). This and a similar series of biosynthetic experiments led to the modification of a previously postulated hypothesis for the biosynthesis of oxindole alkaloids. This modified biosynthethic pathway to oxindole alkaloids in Mitragyna species is presented in Scheme 11. 

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. 

Judging from the literature it seems unlikely that the biosynthesis of tryptophan or tryptamine are the rate-limiting steps in the formation of serpentine and ajmalicine in cell suspension cultures of C. roseus. The fact that addition of secologanin in particular has a positive effect on alkaloid production indicates that the terpenoid pathway might be a better candidate for the rate-limiting step. 

Majerus and Pareilleux (775) observed that a sudden drop in pH from 6.0 to 5.0 resulted in enhanced excretion of tryptamine and ajmalicine in a continuous culture of immobilized C. roseus cells. From the literature it becomes clear that the pH of the culture influences alkaloid transport between cells and medium. Whether the external pH also affects alkaloid biosynthesis is still a matter of investigation. 

Terpenoid Indole Alkaloids.—It has been confirmed recently for ajmalicine (84), vindoline (89), and catharanthine (90), in Catharanthus roseus Vinca rosea) that strictosidine (79), and not vincoside (86), is the key intermediate in terpenoid alkaloid biosynthesis cf. ref. 9). 

C21H22N2O3, Mr 350.42, amorphous, [a]D -52° (CHCI3). 0 unstable, reactive alkaloid, existing as /d -immonium salts, isolated from Guettarda exi-mia (Rubiaceae) and synthesized enzymatically with enzymes from Catharanthus roseus cell suspension cultures C. plays a key role in the biosynthesis of monoterpenoid indole alkaloids, e.g., the hetero-yohimbine alkaloids such as ajmalicine, 19-epi-aj-malicine, and tetrahydroalstonine (see Alstonia alkaloids). 

Most of the recent progress in alkaloid enzymology has been made possible by purified enzymes that catalyze the respective steps of alkaloid biosynthesis. This is illustrated by the application of these techniques to alkaloid biosynthesis. For example, formation of ajmalicine (1) from its precursors, tryptamine (2) and secologanin (3), involves four key intermediates (Fig. 1.1). 

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