Plant Alkaloids

Abstract

Alkaloids are traditionally defined as basic (alkali‐like), nitrogen‐containing organic constituents that occur mainly in plants. The nitrogen in the alkaloid molecule is derived from amino acid metabolism. Since the amino acid skeleton is often largely retained in the alkaloid structure, alkaloids originating from the same amino acid show similar structural features and can be classified according to their biosynthetic origin. The biosynthesis of alkaloids often follows complex pathways and includes stereospecific steps. Alkaloids often have pronounced bioactivities and are therefore thought to play an important role in the interaction of plants with their environment. Alkaloids and extracts of alkaloid‐containing plants have been used throughout human history as remedies, poisons and psychoactive drugs.

Key Concepts

  • Alkaloids are a heterogeneous class of secondary metabolites.

  • Alkaloids are defined as basic compounds derived from amino acids, often with heterocyclically bound nitrogen. Pseudoalkaloids are nitrogen‐containing natural products that do not arise from an amino acid core and acquire their nitrogen by transamination of a precursor derived from other biosynthetic pathways.

  • Alkaloids are biosynthesized via complex pathways that may comprise 20 or more enzymatic steps.

  • Many alkaloids possess pronounced biological activities, which are often associated with their amine function.

  • Several alkaloids are used medicinally or provide lead structures for novel synthetic drugs.

  • Owing to their bioactivities, alkaloids are often toxic to herbivoures. Nevertheless, they may be exploited by specialized species of herbivoures as defence compounds.

  • Alkaloid‐producing tissue cultures provide an alternative for exploiting slow growing or protected plants. Moreover, they can be utilized for biosynthetic studies and for the commercial production of valuable alkaloids.

Keywords: alkaloids; natural products; plant constituents; plant drugs; secondary metabolites

Figure 2.

Alkaloids from nonflowering plants.

Figure 3.

Two examples for alkaloids that have been isolated from animals: coccinelline [XIX] from the seven‐spot ladybird (Coccinella septempunctata) and epibatidine from the Ecuadoran tree frog Epipedobates tricolor [XX].

Figure 7.

Some alkaloids are sequestered and metabolized by insects. The pyrrolizidine alkaloid heliotrine [XLV] can be converted by the Asian arctiid moth Creatonotos transiens to the pheromone hydroxydanaidal [XLVI] (Schulz et al., ).

Figure 8.

Some alkaloids that cause poisoning of livestock.

Figure 1.

Classification of alkaloids.

Figure 4.

Some alkaloids are derived from the amino acid l‐tryptophan [XIII]. Labelled (*) tryptophan was employed in feeding experiments to determine the biosynthetic origin of the alkaloids. Although the indole ring of tryptophan is retained in reserpine [XXI] and ajmaline [XXII] from Rauvolfia serpentina as well as in vinblastine [XXIII] and vincristine [XXIV] from Catharanthus roseus, it is rearranged to a quinoline system for the biosynthesis of quinine [XXV] (Cinchona succirubra) and camptothecin [XXVI] (Camptotheca acuminata). The atoms derived from the precursor tryptophan are labelled red in the alkaloids.

Figure 5.

The biosynthetic origin of some alkaloids derived from the amino acid l‐tyrosine [XII]. The precursor tyrosine is labelled with isotopic carbon (*) and during biosynthesis the alkaloidal metabolites are specifically labelled at the sites shown (*). The atoms derived from tyrosine are marked red in the depicted alkaloids. The respective feeding experiments were carried out with Papaver somniferum (morphine [XXVII], codeine [XXVIII], papaverine [XXIX] and narcotine [XXX]), Hydrastis canadensis (berberine [XXXI]), Lophophora williamsii (mescaline [XXXII]) and Psychotria ipecacuanha (emetine [XXXIII]).

Figure 6.

Biosynthesis of the tropane alkaloids (S)‐hyoscyamine [XL] and (S)‐scopolamine [XLII] which are confined to the Solanaceae family, and of the more abundantly occurring calystegine nortropanes [XLIV]. Some enzymes involved in this biosynthetic pathway are putrescine methyltransferase (PMT), diamine oxidase (DAO), tropinone reductases I and II (TRI and TRII), and hyoscyamine 6‐hydroxylase (H6H). Dashed lines indicate reactions that remain to be elucidated and may require more than one enzyme.

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Further Reading

Cordell GA (1981) Introduction to Alkaloids. New York: Wiley.

Cordell GA (1998) The Alkaloids, vol. 52. New York: Academic Press. [a continuing series of expert reviews].

Dewick PM (2009) Medicinal Natural Products: A Biosynthetic Approach, 3rd edn. Chichester: Wiley.

Evans WC (1996) Pharmacognosy. London: WB Saunders.

Manske RFH and Holmes HL (1950) The Alkaloids, vol. 1. New York: Academic Press.

Osbourn AE and Lanzotti V (2009) Plant‐derived Natural Products. Synthesis, Function, and Application. New York: Springer.

Pelletier SW (1983) Alkaloids: Chemical and Biological Perspectives, vol. 1. Oxford: Pergamon Press.

Pelletier SW (1998) Alkaloids: Chemical and Biological Perspectives, vol. 12. Oxford: Pergamon Press. [a continuing series of expert reviews].

Roberts MF and Wink M (1999) Alkaloids: Biochemistry, Ecology, and Medicinal Applications. New York: Springer.

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Fester, Karin(Mar 2010) Plant Alkaloids. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001914.pub2]