Ajmalicine from Catharanthus roseus

The rauwolfia alkaloids are now hardly ever prescribed in the UK, either as antihypertensives or as tranquillizers. Over a period of a few years, they have been rapidly superseded by synthetic alternatives. Reserpine has also been suggested to play a role in the promotion of breast cancers. Both ajmalicine (= raubasine) (Figure 6.76) and ajmaline (Figure 6.82) are used clinically in Europe, though not in the UK. Ajmalicine is employed as an antihypertensive, whilst ajmaline is of value in the treatment of cardiac arrhythmias. Ajmalicine is also extracted commercially from Catharanthus roseus (see page 357). 

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. 

Within the natural products field, the investigation of complete biosynthetic pathways at the enzyme level has been especially successful for plant alkaloids of the monoterpenoid indole alkaloid family generated from the monoterpene gluco-side secologanin and decarboxylation product of tryptophan, tryptamine [3-5]. The most comprehensive enzymatic data are now available for the alkaloids ajmalicine (raubasine) from Catharanthus roseus, and for ajmaline from Indian Rauvolfia serpentina [6] the latter alkaloid with a six-membered ring system bearing nine chiral carbon atoms. Entymatic data exsist also for vindoline, the vincaleucoblastin (VLB) precursor for which some studies on enzymatic coupling of vindoline and catharanthine exist in order to get the so-called dimeric Catharanthus indole-alkaloids VLB or vincristine [7-9] with pronounced anti-cancer activity [10, 11]. 

Fig. 1 First example of a complete biosynthetic pathway to an alkaloid (ajmalicine = raubasine) based on enzymatic reactions elucidated by isolation of the single enzymes from Catharanthus roseus cell suspension cultures...

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. ... 

Several important alkaloids have been isolated from Catharanthus roseus plants ajmalicine (34) and serpentine (35) from the root, and the dimeric alkaloids vinblastine (36) and vincristine (37) from the leaves. Therefore, much research has been done on production of these alkaloids by means of plant cell culture. Catharanthus roseus is, in fact, one of the most widely studied plants for the production of secondary metabolites in cell culture systems. We here discuss the two types of alkaloids separately. Table XXIX summarizes patents concerning the production of alkaloids by means of cell cultures of C. roseus. 

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). 

It is a common knowledge that plants are useful sources of many valuable medicines. Among these medicines several indole alkaloids can be found, e.g. reserpine and deserpidine from Rauwolfia serpentina ajmalicine and yohimbine from Corynanthe yohimhe vinblastine and vincristine from Catharanthus roseus just to mention a few of them. 

Asada M, Shuler ML (1989) Stimulation of ajmalicine production and exaetirai from Catharanthus roseus effects of adsorption in situ, elicitors, and alginate immohilization. Appl Microbiol Biotechnol 30 475-481. doi 10.1007/BF00263851... 

In the transcript of a lecture, Zenk has reviewed58 the enzymic synthesis of ajmalicine and its biogenetic intermediates from secologanin and tryptamine in cell clones of Catharanthus roseus. 

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... 

Isolation of the stereospecific strictosidine s)mthase (STS) and formation of strictosidine with the 3a-(S) configuration proved conclusively that this was the natural precursor of the terpenoid indole alkaloids. Strictosidine occurs naturally in Rhazya stricta and the synthase has been isolated from a number of other species Amsonia salicifolia, A. tabemaemontam, Catharanthus pusillus, C. roseus, Rauvolfia verticillata, R. vomitoria, R. serpentina, Rhazya orientalis and Voacanga africana. The enzyme has been purified to homogeneity from R. serpentina (Hampp and Zenk, 1988). A comparison of the activity of STS from C. roseus roots, the only portion of the plant to contain ajmalicine, with that present in plant cell cultures producing the same alkaloid demonstrated that the plant cell cultures are far more metabolically active (Ziegler and Facchini, 2008). 

Other cell suspension cultures of Catharanthus roseus synthesize and accumulate substantial amounts of both serpentine (8) and ajmalicine (7). Serpentine (8), the principal alkaloid of some cell cultures of Catharanthus roseus, is derived from ajmalicine (7) in contrast to some other indole alkaloids, serpentine is located inside the vacuole (Deus-Neumann and Zenk, 1984 Zenk et al 1985). Serpentine... 

Ajmalicine (7) was first isolated from yohimbe bark and, later, from Rauvolfia species. This compound is one of the principal alkaloids of Catharanthus roseus. Ajmalicine is found in members of both the Apocynaceae and Rubiaceae. Approximately 3600 kg are produced each year at a market price of about 2000/kg (Verpoorte et al., 1991). 

Lee CWT, Shuler ML (2000) The effect of inoculum density and conditioned medium on the production of ajmalicine and catharanthine from immobilized Catharanthus roseus cells. Biotechnol Bioeng 67 61-71. doi 10.1002/(SICI)1097-0290(20000105) 67 1 <61 AID-BIT7 >3.0.CO 2-J... 

Lee-Parson CWT, Shuler ML (2005) Sparge gas composition affects biomass and ajmalicine production from immobilized cell cultures of Catharanthus roseus. Enzyme MicrobTechnol 37 424-434. doi 10.1016/J.enzmictec.2005.02.016... 

Zhao J, Zhu WH, Hu Q (2000) Enhanced ajmalicine production in Catharanthus roseus cell cultures by combined elicitor treatment from shake-flask to 20-1 airlift biOTeactor. Biotechnol Lett 22 509-514. doi 10.1023/A 1005616920165. doi 10.1023/A 1005616920165 Morgan JA, Shanks JV (2000) Determination of metabolic rate-limitations by precursOT feeding in Catharanthus roseus hairy root cultures. J Biotechnol 79 137-145. doi 10.1016/ S0168-1656(00)00221-2... 

Ajtnalicine is found in plants belonging to Apocynaceae, namely, Catharanthus roseus, Rauwolfia serpentina, and Rauwolfia vomitoria. Ajmalicine was first isolated from yohimbe bark and is one of the principal alkaloids of Catharanthus roseus and widely used to treat circulatory disorders [6] (Fig. 20.3). Ajmalicine is... 

Yoshikawa T, Furuya T (1983) Regeneration and in vitro flowering of plants derived from callus cultures of opium poppy Papaver somniferum). Experientia 39 1031-1033 Zenk MH, El-Shagi H, Arens H, Stockigt J, Weiler EW, Deus B (1977) Formation of the indole alkaloids serpentine and ajmalicine in cell suspension cultures of Catharanthus roseus. In Barz W, Reinhard E, Zenk MH (eds) Plant tissue culture and its bio-technological application. Springer, Berlin Heidelberg New York, pp 27-43... 

---