Some plant families are especially rich in alkaloids. The Dogbane botanical family (Apocynaceae Lindl., Juss.) is a good example (Table 2). This family is distributed worldwide, especially in tropical and sub-tropical areas. The Dogbane family is a large botanical taxa containing at least 150 genera and 1700 species. Alkaloids are especially abundant in the following
Table 2 General botanical characteristics of the Dogbane family
Botanical Forms and Parts
Some typical genera
Special characteristics Leaves
Sometimes succulents or cactus-like
Milky juice or latex, hairs
Opposite or verticillate with reduced stipules Pinnateveined
Regular, radial Calyx with 5 sepals Tubular corolla
Pollen grains usually tricolporate (dicolporate rarely)
Sometimes berry-like or drupe-like
Compressed with tufts of long hairs Albumen
genera: devil's-pepper (Rauvolfia L.), periwinkle (Catharanthus G. Don), milkwood (Tabernaemontana L.), strophanthus (Strophanthus DC.), voacanga (Voacanga U.) and alstonia (Alstonia R. Br.). The species belonging to these genera contain L-tryptophan-derived alkaloids (Figure 8). Indian snake-root (Rauvolfia serpentina) (Figure 9) contains reserpine and rescinnamine, the quinine tree (Rauwolfia capra) yields quinine, and iboga milkwood (Tabernaemontana iboga) produces iboganine. Deserpine has been isolated from the roots of Rauwolfia canescens60. This alkaloid differs from reserpine only by absence of a metoxy group but shows an interesting profile of biological activity. It has been employed in clinical practice for the treatment of hypertension and
as a tranquilizer and also as a controller of other cardiac disorders. Deserpine is a compound with limited availability from natural sources.
According to Varchi et al.60 reserpine usually occurs at about 0.10-0.16% of natural extracts and deserpine only at 0.04%. Furthermore, five new indole alkaloids (Nb-methylajmaline, Nb-methylisoajmaline, 3-hydroxysarpagine, yohimbic acid and isorauhimbic acid) were isolated from the dried roots of Rau-wolfia serpentina61. Srivastava et al.62 reported on alkaloids isolated from heynana milkwood (Tabernaemontana heyneana Wall.). They discovered ervatine, tabersonine, coronaridine, heyneanine, voacristine, voacristine hydrox-yindolenine, hydroxyibogamine and coronaridine hydroxyindolenine. These alkaloids show both bioimpact and uterotrophic activity. Moreover, Heij-den et al.63 have described the isolation of indole alkaloids from Tabernaemontana elegans, a species which occurs in southern part of Africa and is used in traditional medicine in Zimbabwe, Mozambique and Southern Africa. These alkaloids are apparicine, 16-S-hydroxy-16, 22-dihydro-apparicine, tubotaiwine, vobasine, vobasinol, tabernaemontaninol, tabernae-montanine, isovoacangine, dregamine, dregaminol, dregaminol-methylether, 3-R/S-hydroxytabernaelegantine B, 3-methoxy-tabernaelegantine C, 3-R/S-hydroxy-conodurine, tabernaelegantine A, B, C, and D63. Alstoniaplants produce menilamine, which is known as a new anti-malarial alkaloid isolated from alstonia trees growing in the Philippines, where this plant is common64. These plants are known as prospective medicinal plants and they are well distributed throughout tropical America, India and Malaysia as evergreen trees and shrubs. Many prospective liana plants from this family grow particularly in Amazonian America, tropical Africa and Madagascar. From Alstonia macrophylla Wall. Ex G. Don growing in Thailand, talcarpine, pleiocarpamine, alstoumerine, 20-Epi-antirhine, alstonerine, alstophylline, macralstonine, villalstonine, alstomacroline and macrocarpamine were isolated65. All these alkaloids display strong bioactiv-ity and are considered to be of potential use in medicine. Moreover, two other Thai Alstonia species, Alstonia glaucescens and Alstonia scholaris were also found to be indentical or similar to alkaloids such as O-methylmacralstonine6465. It should be noted that more than 180 biologically active alkaloids have been isolated from the genus Alstonia. This makes this genus one of the most important in terms of potential alkaloid use. The Alstonia, Devil's pepper and Milkwood genera are endemic only in Asia and Australia, but they are distributed around the Globe in the tropics and subtropics. Ajmalicine, catharanthine, leurosine, vindoline, vindolinine, vinblastine, vincristine, vindesine and alioline are present in the periwinkle (e.g., Catharanthus roseus and Vinca spp.). From the leaves of Vinca difformis Pourr, vincamajine, vincamedine, vincadifformine, akuammidine, vellosimine, vincadiffine, difforlemenine, difforine and norma-cusine have been isolated66. From Aspidosperma megalocarpon Müll. Arg., growing in Colombia, three alkaloids were extracted - fendlerine, aspidoalbine and aspidolimidine67. All display bioactivity and the potential for applications in medicine. Jokela and Lounasmaa68 have presented 1H and 13C-NMR exact spectral data for seven types of ajmaline-type alkaloids from various species of the Dogbane family. These alkaloids are as follows: ajmaline, 17-O-acetyl-ajmaline, isoajmaline, isosandwichine, rauflorine, vincamajine and vincamedine. Eleven indole alkaloids were isolated from the stem bark of Kopsia hainanensis Tsiang, which is one of for species of Kopsia, endemic in China69. They are (—)-kopsinine, (—)-kopsinnic acid, (—)-kopsinoline, kopsinilam, kopsanome, (+)-5,22-dioxokopsane, eburnamenine, (+)-eburnamine, (—)-isoeburnamine, (+)-tubotaiwine and (+)-kopsoffine. Kopsia officinalis Tsiang seems to be very similar with respect to alkaloid content. In both species (—)-kopsinine is the principal alkaloids69. Moreover, in the Dogbane plant family are also phenylalanine-derived alkaloids, such as ^-skytanthine in the Skythantus species (Figure 10, Table 2 and 10). All alkaloids from the Dogbane family have a strong biological and medicinal effect. Many of them are used in cancer chemotherapy.
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