Betalains. Other Miscellaneous Types of Alkaloids
Betalains
Betalains are a group of alkaloidal pigments restricted in distribution to about 10 families. They vary in color from red-violet to yellow. About 50 compounds are known. They belong to two main structural types. All are water soluble zwitterions.
The red color of beets is from betanin, a glycoside.
Proline appears to replace the cyclodopa-derived portion of betalains in betaxanthins.
Some betalain-like pigments are known from fungi. Muscaaurin II, muscaaurin I, and muscaflavin are examples of fungal pigments of this structural type.
The biosynthesis of cyclodopa from DOPA appears to occur via a monohydroxylated intermediate.
The second portion of the molecule of betalains, betalamic acid also is derived from DOPA, via cleavage of the aromatic ring. This compound is similar to stizolobic acid and muscaflavin. Most betalains and indicaxanthins have a 2S-configuration.
In the presence of weak base, betacyanin and betaxanthin pigments undergo exchange reactions in which the dihydropyridine portion is transferred from one amino acid to another.
Although many members of the order Caryophyllales contain betalain pigments, a number of families do not, or at least have very low levels of these compounds. The families Caryophyllaceae and Molluginaceae contain anthocyanins, but not betalains. To date, no plant is known to have both types of pigments.
The uv-visible spectra of betalains and anthocyanins are quite similar and in plants that have them, the betalains serve much the same role in pollination and seed and fruit dissemination.
Imidazole Alkaloids
Imidazole alkaloids are found in several families, but are probably best known from the genus Pilocarpus of the Rutaceae. Pilocarpine, derived from histidine, is a peripheral stimulant of the parasympathetic system and is used as a myotic to counteract the mydriatic effects of atropine and other parasympatholytic drugs. Pilocarpine is used in the treatment of glaucoma.
Other imidazole alkaloids are known from Alchornea, Euphorbiaceae, and a variety of
other plants. Because of the diverse taxonomic sources and the variation in struction, these may not
represent a homologous series of alkaloids.
Lythracaeae Alkaloids
About 45 alkaloids of this series are known. They coincidentally are quinolizidine types, but are derived from lysine and phenylalanine in a distinct manner./P>
Symmetrical intermediates are involved in their formation.
The Lythraceae alkaloids from Heimia spp. are hallucinogenic
Maytansinoids
This series of novel ansa-macrolides is best known from a series of genera of the family Celastraceae (Maytenus, Puterlickia). Several have pronounced antitumor activity, but are too toxic for practical use.
maytansine and maytanprine inhibit DNA synthesis, but not RNA or protein synthesis. They interact at the same binding sites as vincristine.
Muscarine alkaloids
The alkaloid muscarine is found in a number of fungi of the genera Amanita, Clitocybe, and Inocybe. The amount present is usually quite low. The best known species is Amanita muscaria, but much of the activity actually is due to the presence of ibotenic acid and muscimol.
Muscarine is derived from glutamic acid and pyruvic acid.
The action of muscarine and acetylcholine on smooth muscle is similar. Muscarine is a potent parasympathomimetic drug.
Naphthylisoquinoline alkaloids
This group of about 20 alkaloids is found in the families Ancistrocladaceae and Dioncophyllaceae.
Feeding studies with acetate suggest that these alkaloids are derived by a type of polyketide pathway.
Michellamine B, from Ancistrocladus korupensis has been shown to possess activity against
HIV-1 and HIV-2.
Peptide alkaloids
A series of more than 100 alkaloids with a peptide structure have been isolated and characterized.
A 10- or 12-membered (sometimes 13 or 14) spans the 1,3- or 1,4-positions of a benzene ring.
These alkaloids primarily are composed of simple amino acids. The basic properties are caused by an
N-methyl or an N,N-dimethyl group of an N-terminal amino acid. These compounds
have therapeutic and antibiotic properties. They may serve as ionophores in the plant. These compounds
are particularly common in the Rhamnaceae, but are found in a number of other families as well.
Purine or xanthine alkaloids
A number of free purine and pyrimidine bases have been isolated from plants. Most important of these are 1,3,7-trimethylxanthine (caffeine), 1,3-dimethylxanthine (theophylline), and 3,7-dimethylxanthine (theobromine).
Coffee, from Coffea spp., Rubiaceae, is native to East Africa, but widely cultivated. The seeds contain 1-2% caffeine complexed with chlorogenic acid. All parts of the plants contain caffeine. Caffeine also is found in cola fruits (Sterculiaceae) and in yerba mate (Ilex paraguayensis, Aquifoliaceae.
Tea, Camellia or Thea sinensis, Theaceae, is native to eastern Asia. Dried tea often contains 1-4% caffeine and smaller amounts of theobromine and theophylline.
Cacao, Theobroma cacao, Sterculiaceae, is native to the New World and is derived from the seed of the plant. These seeds contain 0.9-3% theobromine.
Other purine and xanthine alkaloids are known from a variety of plants.
Pyrazine alkaloids
Pyrazine alkaloids occur in both plants and insects. They serve as warning and alarm substances
in insects. 3-Isobutyl-2-methoxypyrazine is one of the compounds responsible for the odor properties
of Capsicum annuum.
Securinega alkaloids
Securinine is the major alkaloid of Securinega suffruticosa, Euphorbiaceae, and related
species. Securinine nitrate is a central nervous system stimulant. This compound raises blood pressure
stimulates respiration, and increases cardiac output.
Sesbania alkaloids
A number of unusual alkaloids have been isolated from the seeds of Sesbania drummondii , Fabaceae. Sesbanimide is the major antileukemic and cytotoxic compound in the plant.
Betalains and Related Compounds
Muscarine and Related Alkaloids
Lecture Slides
Plants with Miscellaneous
Alkaloids
© David S. Seigler, Plant Biology 363, Plant Secondary Metabolism, Department of Plant Biology,
265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
