181
12
What You Will Learn in This Chapter
Alkaloids are characterized by containing a nitrogen atom, usually within a heterocyclic
ring. Many are derived from aromatic amino acids, but for some also other amino acids,
ornithine, spermidine, xanthosine and the terpenoids are the basic building blocks. Many
well-known compounds such as caffeine, nicotine and atropine fall into this group. The
major groups are terpenoid indole alkaloids and benzylisoquinoline alkaloids, but also in
many other pathways, toxic or psychedelic alkaloids can be synthesized. The biosynthesis
pathway is often spread over different cell types and is very compartmentalized within the
cells to avoid toxicity for the plant.
12.1 Introduction
Alkaloids are a diverse group of nitrogen-containing compounds (Facchini 2001; Herbert
2003). Usually this nitrogen atom is in an oxidative state within a heterocyclic ring.
Precursors are usually amino acids, and their entire carbon skeleton, except the carboxylic
acid of the amino acid, is incorporated into the final alkaloid in the case of quinoline alkaloids, isoquinoline alkaloids and tropane alkaloids (. Fig. 12.1). Other alkaloids incorporate the nitrogen atom from the amino acid via transamination and the remaining carbon
skeleton is derived from a non-amino acid source (such as acetate, shikimate, terpenoid
or steroid origin). This is the case for purine alkaloids, terpenoid alkaloids and steroidal
alkaloids. Pseudo alkaloids or not heterocyclic alkaloids are alkaloids in which the nitrogen is positioned outside of the ring system.
Alkaloid biosynthesis requires highly stereo-, substrate- and cell-specific enzymes
only present in about 20% of plant species. Among them are Apocynaceae, Fabaceae,
Papaveraceae, Rubiaceae and Solanaceae. Alkaloids are also found in bacteria and fungi
(e.g. psilocin, muscarine, ergot). Several animals contain alkaloids, which they accumulate most often through their diet. Among them are insects and frogs, feeding on insects,
especially the poisonous frogs from the group of poison arrow frog (Dendrobatidae). Over
12,000 alkaloids are estimated to exist, many of them are known for their pharmaceutical
actions. Alkaloids are often cytotoxic even for the plant that produces them; therefore,
they often accumulate in specific cell types such as idioblasts and laticifers. In contrast to
terpenes and phenylpropanoids, alkaloids are usually not found in secretory glands and
have not shown to be synthetized in glands.
Several classification of alkaloids exist, usually, as here presented, they are organized
into groups according their biosynthetic precursor or carbon skeletal structure.
12.2 Terpenoid Indole Alkaloids (Derived from Tryptophan,
e.g. Ibogaine, Reserpine, Strychnine, Physostigmine,
Dimethyltryptamine, Psilocybin and Ergotamine)
Terpenoid indole alkaloids can found mainly in Apocynaceae, Loganiaceae, Rubiaceae
and Nyssaceae. Especially Catharanthus roseus, Rauwolfia serpentine and Camptotheca
acuminate have been analysed according to the secondary metabolites they produce.
Tryptophan forms the basis for indole alkaloids. The initial steps are the decarboxylation
of tryptophan to tryptamine by the tryptophan decarboxylase, a pyridoxal dependent
enzyme, followed by the condensation of tryptamine with secologanin, an iridoid terpene
12.2 · Terpenoid Indole Alkaloids
12
What You Will Learn in This Chapter
Alkaloids are characterized by containing a nitrogen atom, usually within a heterocyclic
ring. Many are derived from aromatic amino acids, but for some also other amino acids,
ornithine, spermidine, xanthosine and the terpenoids are the basic building blocks. Many
well-known compounds such as caffeine, nicotine and atropine fall into this group. The
major groups are terpenoid indole alkaloids and benzylisoquinoline alkaloids, but also in
many other pathways, toxic or psychedelic alkaloids can be synthesized. The biosynthesis
pathway is often spread over different cell types and is very compartmentalized within the
cells to avoid toxicity for the plant.
12.1 Introduction
Alkaloids are a diverse group of nitrogen-containing compounds (Facchini 2001; Herbert
2003). Usually this nitrogen atom is in an oxidative state within a heterocyclic ring.
Precursors are usually amino acids, and their entire carbon skeleton, except the carboxylic
acid of the amino acid, is incorporated into the final alkaloid in the case of quinoline alkaloids, isoquinoline alkaloids and tropane alkaloids (. Fig. 12.1). Other alkaloids incorporate the nitrogen atom from the amino acid via transamination and the remaining carbon
skeleton is derived from a non-amino acid source (such as acetate, shikimate, terpenoid
or steroid origin). This is the case for purine alkaloids, terpenoid alkaloids and steroidal
alkaloids. Pseudo alkaloids or not heterocyclic alkaloids are alkaloids in which the nitrogen is positioned outside of the ring system.
Alkaloid biosynthesis requires highly stereo-, substrate- and cell-specific enzymes
only present in about 20% of plant species. Among them are Apocynaceae, Fabaceae,
Papaveraceae, Rubiaceae and Solanaceae. Alkaloids are also found in bacteria and fungi
(e.g. psilocin, muscarine, ergot). Several animals contain alkaloids, which they accumulate most often through their diet. Among them are insects and frogs, feeding on insects,
especially the poisonous frogs from the group of poison arrow frog (Dendrobatidae). Over
12,000 alkaloids are estimated to exist, many of them are known for their pharmaceutical
actions. Alkaloids are often cytotoxic even for the plant that produces them; therefore,
they often accumulate in specific cell types such as idioblasts and laticifers. In contrast to
terpenes and phenylpropanoids, alkaloids are usually not found in secretory glands and
have not shown to be synthetized in glands.
Several classification of alkaloids exist, usually, as here presented, they are organized
into groups according their biosynthetic precursor or carbon skeletal structure.
12.2 Terpenoid Indole Alkaloids (Derived from Tryptophan,
e.g. Ibogaine, Reserpine, Strychnine, Physostigmine,
Dimethyltryptamine, Psilocybin and Ergotamine)
Terpenoid indole alkaloids can found mainly in Apocynaceae, Loganiaceae, Rubiaceae
and Nyssaceae. Especially Catharanthus roseus, Rauwolfia serpentine and Camptotheca
acuminate have been analysed according to the secondary metabolites they produce.
Tryptophan forms the basis for indole alkaloids. The initial steps are the decarboxylation
of tryptophan to tryptamine by the tryptophan decarboxylase, a pyridoxal dependent
enzyme, followed by the condensation of tryptamine with secologanin, an iridoid terpene
12.2 · Terpenoid Indole Alkaloids