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Introduction Among all the groups of natural products, alkaloids have the most colorful history, having achieved the most fame and notoriety as drugs. Where alkaloids occur, they tend to dominate the biological activity. Despite their relatively limited distribution, the alkaloids probably have the most significant impact in human history particularly in medicine, social issues, economics and politics. In 1819, Carl Friedrich Meissner, a pioneering German pharmacist, coined the term “alkaloid” which referred to any natural product with the characteristic presence of the basic nitrogen atom, excluding peptides. (Amides, however, are generally included.) 9.0 Alkaloids (Dayrit)
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9.0 Alkaloids RA Macahig FM Dayrit Introduction Among all the
groups of natural products, alkaloids havethe most colorful
history, having achieved the most fameand notoriety as drugs. Where
alkaloids occur, they tend todominate the biological activity.
Despite their relativelylimited distribution, the alkaloids
probably have the mostsignificant impact in human history
particularly inmedicine, social issues, economics and politics. In
1819, Carl Friedrich Meissner, a pioneering Germanpharmacist,
coined the term alkaloid which referred toany natural product with
the characteristic presence of thebasic nitrogen atom, excluding
peptides. (Amides, however, are generally included.) 9.0 Alkaloids
(Dayrit) Introduction Because many alkaloids can bepurified from
crude extracts byacid-base extraction andrecrystallization, these
were the firstnatural products to be purified,characterized and
commercialized. The powerful and immediate effects ofalkaloids are
thought to be due to thepresence of the cationizable N-atomwhich
gives it lipid- and water-solublecharacteristics, and enables it to
crossmembrane barriers more readily. 9.0 Alkaloids (Dayrit) A
number of fungi produce toxic alkaloids, notably Claviceps
purpurea.
Introduction Many of the earliest pure compounds to be used as
drugsdeveloped were alkaloids: Cocaine: anaesthetic, from South
American Erythroxylum coca Quinine: antimalarial, from the bark of
the Cinchona tree Morphine: anaesthetic, from opium (Papaver
somniferum) Emetine: for amoebiasis, from ipecac, the powdered
roots (Cephaelis species) Strychnine: poison, from the seeds of
Strychnos nux-vomica The alkaloids have a relatively limited
distribution in naturecompared with the other natural product
groups. Alkaloids wereoriginally thought to be uniquely plant
products until the 1950swhen several alkaloids were isolated from
bacteria, fungi andalgae, insects, and amphibians. A number of
fungi produce toxic alkaloids, notably Clavicepspurpurea. 9.0
Alkaloids (Dayrit) Introduction In the plant kingdom, the
angiosperms produce alkaloids:Apocynaceae, Papaveraceae, Rubiaceae,
Ranunculaceae,Solanaceae, and Berberidaceae. Among the monocots,
only theAmaryllidaeae and Liliaceae produce alkaloids. The
discovery of many alkaloids areassociated with
anthropologicalexplorations. Here, a Kams youth from aBrazilian
tribe holds a blossom of Culebraborrachera which is a toxic
psychoactiveplant. It contains tropane alkaloids thatcan induce a
frightening state of psychoticdelirium, and ultimately stupor and
death. (cms.herbalgram.org) 9.0 Alkaloids (Dayrit) Introduction
Some animals, notably some soft corals and frogs producehighly
bioactive alkaloids. In some cases, however, the alkaloidswere
found to have been ingested in the diet by the organism andthen
modified for use. Well-known examples are thepyrrolizidine
alkaloids in caterpillars and moths. Alkaloids aremuch less common
in mammals. Intermedine (I) and lycopsamine (II) ingested from the
leaves of Mikania scandens monocrotaline (III) 9.0 Alkaloids
(Dayrit) Introduction There are four major groups of
nitrogen-containing organiccompounds in biological systems: the
amino acids (peptidesand proteins), the nucleoside bases (DNA and
RNA), theporphyrins; and the alkaloids. The first 3 groups are
primarymetabolites; the alkaloids are secondary metabolites.
Alkaloids are defined simply as nitrogen-containing
naturalproducts. In terms of chemical structure, alkaloids can
beclassified into the following: 2, 3, and 4 alkyl amines;
andheterocyclic amines (e.g., pyrrolidine, pyridine,
indole,quinoline, and isoquinoline). 9.0 Alkaloids (Dayrit)
Caffeine, widely used central nervous system stimulant
Ajmaline, antiarrythmic thatfunctions by inhibition of
glucoseuptake by heart tissuemitochondria Atropine
(hyoscyamine),anticholinergic, antidote to nervegas poisoning
Caffeine, widely used centralnervous system stimulant Camptothecin,
potent anticanceragent Cocaine, topical anesthetic,potent central
nervous systemstimulant, and adrenergic blockingagent, drug of
abuse (Kutchan, The Plant Cell, 7, , July 1995) Codeine, relatively
nonaddictiveanalgesic and antitussive; Coniine,first alkaloid to be
synthesized,extremely toxic, causes paralysisof motor nerve
endings, used inhomeopathy Emetine, orally active
emetic,amoebicide; morphine, powerfulnarcotic analgesic, addictive
drugof abuse; Nicotine, highly toxic,causes respiratory
paralysis,horticultural insecticide Pilocarpine, peripheral
stimulantof the parasympathetic system,used to treat glaucoma
Quinine, traditional antimalarial,important in treating
Plasmodiumfalcipafum strains that are resistantto other
antimalarials Sanguinarine, antibacterialshowing antiplaque
activity, usedin toothpastes and oral rinses
Scopolamine, powerful narcotic,used as a sedative for
motionsickness Strychnine, violent tetanicpoison, rat poison, used
inhomeopathy; Taxol, antitumoragent
(+)-Tubocurarine,nondepolarizing muscle relaxantproducing
paralysis, adjuvant toanesthesia Vinblastine, antineoplastic that
isused to treat Hodgkins diseaseand other lymphomas. Survey of some
well-known alkaloids.
9.0 Alkaloids (Dayrit) 9.0 Alkaloids (Dayrit) Merck, one of the
worlds largest pharmaceutical companies, made its initial fortune
from cocaine, morphine, and codeine. 9.0 Alkaloids (Dayrit) 9.0
Alkaloids (Dayrit) Introduction Many alkaloids are toxic in
thecytoplasm, even in the plants that producethem.Their
accumulation, therefore,requires a well-regulated
andcompartmentalized system. The mostcommon storage organ is the
vacuole. In some plants, alkaloids are synthesizedin one part of
the plant and then aretransported to another part for storage.
Intobacco, for example, alkaloids (e.g.,nicotine) are synthesized
in the roots andare then transported via the xylem to theleaves
where they are accumulated. 9.0 Alkaloids (Dayrit) Many alkaloids
are mixed metabolites.
Introduction Among the natural products groups, the biochemistry of
alkaloids is the most complex. Many alkaloids are mixed
metabolites. Enzymes involved in alkaloid biosynthesis are
associated with diverse subcellular compartments including the
cytosol, vacuole, tonoplast membrane, endoplasmic reticulum,
chloroplast stroma, thylakoid membranes, and perhaps unique
biosynthetic or transport vesicles. Localization studies have shown
that sequential alkaloid biosynthetic enzymes can also occur in
distinct cell types, suggesting the intercellular transport of
pathway intermediates. (PJ Facchini, Ann. Rev. of Plant Physiol.
and Plant Mol. Bio., Vol. 52: 29-66) 9.0 Alkaloids (Dayrit)
Overview Despite the wide variety and complex structures
observedamong the alkaloids, most of the compounds from this
groupare formed from only a handful of starting
materials.Thenitrogen and the initial carbon skeleton are derived
from thefollowing: A. aliphatic amino acids: lysine, glutamic acid,
and ornithine; B. aromatic amino acids: phenylalanine, tyrosine,
andtryptophan; and C. others: secologanin: terpene-derived C10
unit. 9.0 Alkaloids (Dayrit) 9.0 Alkaloids (Dayrit) Overview The
major reactions in alkaloid biosynthesis are common biosynthetic
mechanisms: Oxidation: epoxidation, aromatic hydroxylation, etc.
Oxidation: dehydrogenation (-2[H]); in the case of thealkaloids,
this includes the conversion of amine imine. Reduction:
hydrogenation (+2[H]); in the case of thealkaloids, this includes
the conversion of imine amine. Phenolic dimerization by radical
coupling. Decarboxylation, in particular in the conversion of
aminoacid to amine. 9.0 Alkaloids (Dayrit) Reactions which are
particular to alkaloid biosynthesis.
9.0 Alkaloids (Dayrit) Reactions which are particular to alkaloid
biosynthesis.
tryptophan 9.0 Alkaloids (Dayrit) Aliphatic alkaloids The aliphatic
alkaloids can be classified into three main types, depending on its
biogenesis: the amino acid ornithine the amino acid lysine the
polyketide pathway with the nitrogen atom beingintroduced in a late
step. 9.0 Alkaloids (Dayrit) Aliphatic alkaloids from
ornithine
Glutamic is transformed into ornithine by addition of another CH2
unit. Therefore, in a way, glutamic acid is the original precursor
and ornithine is the immediate precursor. Decarboxylation and loss
of one nitrogen leads to formation of the pyrrolidine ring. There
are three main types: pyrrolidine (monocyclic), tropane (bicyclic)
and pyrrolizidine (fused). 9.0 Alkaloids (Dayrit) Ornithine comes
from glutamic acid.
9.0 Alkaloids (Dayrit) Pyrrolidine alkaloids from ornithine
Pyrrolidine alkaloids from ornithine. Labeling studies show that
although pyrrolidine itself is symmetric, the biosynthesis is
regiospecific.This suggests that once ornithine is held by the
enzyme, biosynthesis proceeds without release of any of the
intermediates. 9.0 Alkaloids (Dayrit) Tropane alkaloids from
ornithine
Tropane alkaloids from ornithine. Consistent with what has been
observed, labeling shows that the biosynthesis is regiospecific.
9.0 Alkaloids (Dayrit) Pyrrolizidine alkaloids from ornithine
Pyrrolizidine alkaloids from ornithine. Pyrrolizidine alkaloids are
common in the butterflies Senecio and Crotolaria species. 9.0
Alkaloids (Dayrit) Pyrrolizidine alkaloids are converted into
aphrodisiac substances which the male butterflies store in its wing
hair pencils. These compounds also protect the plants against
feeding by mammals because these compounds are converted in the
liver into toxic and carcinogenic compounds. 9.0 Alkaloids (Dayrit)
Polyhydroxylated cycloalkyl alkaloids found in the leaves, flowers
and seeds of Ipomoea carnea (Convolvulaceae) cause natural
intoxication of livestock by inhibiting key digestive enzymes.
Alkaloids 1 and 2 are powerful inhibitors of lysosomal
a-mannosidase; 3, 4, and 6 showed potent inhibitory activity toward
rat lysosomal b-glucosidase; and alkaloid 5 was a moderate
inhibitor of a- and b-mannosidases. (Haraguchi, et al., J. Agric.
Food Chem. 2003, 51, ) 9.0 Alkaloids (Dayrit) Aliphatic alkaloids
from lysine
Lysine is modified following an analogous pathway to ornithine.
There are many similarities between the ornithine-derived and
lysine-derived alkaloids. The alkaloids produced are the 6-membered
piperidine, homotropane and quinolizidine structures. 9.0 Alkaloids
(Dayrit) Alkaloids from lysine. 1-Piperidine and pellieterine are
key intermediates to this group of alkaloids. Quinolizidine
alkaloids have the characteristic fused 6,6-bicyclic group and are
derived from lysine.Lupinine is a dimeric metabolite while
sparteine is trimeric. 9.0 Alkaloids (Dayrit) Aliphatic alkaloids
from polyketides
Some aliphatic alkaloids are derived from the polyketide
pathway.The biogenesis of these alkaloids can be determined by
studies using labeled acetyl CoA. 9.0 Alkaloids (Dayrit)
Polyketide-derived piperidine alkaloids
Polyketide-derived piperidine alkaloids. Some alkaloids having the
piperidine-type structure are not derived from lysine. 9.0
Alkaloids (Dayrit) Alkaloids from phenylalanine and tyrosine
The aromatic alkaloids derived from phenylalanine and tyrosine form
a diverse and often structurally complex group of metabolites. By
tradition, these alkaloids are identified according to plant
family, of which the best known are: Papaveraceae, Morphinan,
Erythria, Berberidaceae, Amaryllidaceae. Structurally and
biosynthetically, there are six main groupings: 1.
Phenylethylamines 2. Phenylethylamine + alkyl aldehyde or ketone 3.
Phenylethylamine + benzaldehyde (C6-C1) 4. Phenylethylamine + C6-C2
5. Phenylethylamine + C6-C3 6. Phenylethylamine + secologanin 9.0
Alkaloids (Dayrit) Alkaloids from phenylalanine/tyrosine
Alkaloids from phenylalanine/tyrosine. Structurally, there are six
main types. 9.0 Alkaloids (Dayrit) Alkaloids from
phenylalanine/tyrosine
Alkaloids from phenylalanine/tyrosine. Structurally, there are six
main types. 9.0 Alkaloids (Dayrit) Alkaloids from phenylalanine/
tyrosine: Simple phenylethylamines
Alkaloids fromphenylalanine/tyrosine:
Simplephenylethylamines.Biosynthesis of thisgroup involvessimply
loss of thecarboxylic acidcarbon. Someimportant membersof this
group are thehallucinogeniccompound mescalineand the
drugepinephrine. 9.0 Alkaloids (Dayrit) Alkaloids from
phenylalanine/ tyrosine: Condensation of phenylethylamine with
alkyl aldehydes or ketones.This group of alkaloids is formed via a
Pictet-Springler or Bischler-Napieralski condensation. The
alkaloids shown here are found in the hallucinogenic peyote cactus
plant. 9.0 Alkaloids (Dayrit) Alkaloids from phenylalanine /
tyrosine + Ar-C1
Alkaloids from phenylalanine / tyrosine + Ar-C1.
Phenylethylaminecouples with Ar-C1 (benzaldehyde) via a
Pictet-Springler condensation.This is followed by oxidation of the
phenol. 9.0 Alkaloids (Dayrit) Alkaloids from phenylalanine /
tyrosine + Ar-C1
Alkaloids fromphenylalanine /tyrosine + Ar-C1.The two oxidizedrings
can couplevia differentfoldingconformationsleading topara- para,
para-orthoor ortho-paracoupling. 9.0 Alkaloids (Dayrit) Alkaloids
from phenylalanine / tyrosine + Ar-C1
Alkaloids fromphenylalanine / tyrosine +Ar-C1. (overview)
Couplingof the radical intermediatesin different
foldingconformations leads topara- para, para-ortho and ortho- para
couplings. This is atheme that is repeated forother alkaloids with
similarstructural characteristics. The alkaloid families
thatcomprise this group includethe Amaryllidaceae andMesembrine
species. 9.0 Alkaloids (Dayrit) Alkaloids from phenylalanine /
tyrosine + Ar-C2
Alkaloids from phenylalanine / tyrosine + Ar-C2. Condensation of
phenylethylamine with an Ar-C2 group, such as phenylpyruvic acid,
yields the benzyltetrahydro isoquinoline structure. These alkaloids
are characteristic of the Papaveraceae. Reticuline is a key
intermediate of this group. 9.0 Alkaloids (Dayrit) Alkaloids from
phenylalanine / tyrosine + Ar-C2
Alkaloids from phenylalanine / tyrosine + Ar-C2. The Aprophine
alkaloids are produced by oxidation of reticuline. Various isomeric
radical intermediates are formed. Alkaloids from phenylalanine /
tyrosine + Ar-C2
Alkaloids from phenylalanine / tyrosine + Ar-C2. Four regiochemical
couplings are observed: ortho-ortho, ortho-para, para-ortho, and
para-para. Alkaloids from phenylalanine / tyrosine + Ar-C2
Alkaloids from phenylalanine / tyrosine + Ar-C2. The Aprophine
alkaloids are produced by radical coupling of the
benzylisoquinoline system of reticuline. Schematic presentation of
the biosynthesis of codeine, laudanine, and (S)-scoulerine from
(S)-norcoclaurine in the opium poppy. The cellular localizations of
the enzymes indicated have been determined experimentally.
(Jorgensen et al., Curr Opinion in Plant Biol 2005, 8:280291) 9.0
Alkaloids (Dayrit) Alkaloids from phenylalanine/tyrosine +
Ar-C2
Alkaloids from phenylalanine/tyrosine + Ar-C2. The morphine
alkaloids are produced from thebaine. Note that the level of
methylation decreases towards the end of the biosynthetic sequence
from thebaine codeine morphine. 9.0 Alkaloids (Dayrit) Alkaloids
from phenylalanine / tyrosine + Ar-C2
Alkaloids fromphenylalanine /tyrosine + Ar-C2. Coupling using
theN-methyl group. 9.0 Alkaloids (Dayrit) Alkaloids from
phenylalanine / tyrosine + Ar-C3
Alkaloids fromphenylalanine /tyrosine + Ar-C3. This group
ishomologous to thebenzyltetrahydro- isoquinolines(dopamine +
C2).The biosyntheticsteps are assumed tobe similar. 9.0 Alkaloids
(Dayrit) Alkaloids from phenylalanine / tyrosine +
secologanin
Alkaloids from phenylalanine / tyrosine + secologanin. Secologanin
is an iridoid belonging to the monoterpene group. Condensation of
phenylethylamine with secologanin leads to a group of mixed
metabolites. 9.0 Alkaloids (Dayrit) Alkaloids from phenylalanine /
tyrosine + secologanin.
9.0 Alkaloids (Dayrit) Alkaloids from tryptophan: the indole
alkaloids
The indole alkaloids are derived from tryptophan and arefound in
both plants and microorganisms.They comprise thesingle largest
group of alkaloids, accounting for almost one- fourth of all
alkaloids isolated.Many of the members of thisgroup are
biologically active and some possess very importantmedicinal
properties. Among the best known sources are:Catharanthus, Curare,
Rauwolfia, and Vinca plant species andthe ergot fungi. The indole
alkaloids can be classified as follows: 1. Simple indole
alkylamines 2. Simple -carbolines 3. Tryptophan + C5 4. Tryptamine
+ secologanin 9.0 Alkaloids (Dayrit) Indole alkaloids. Structural
groups..
9.0 Alkaloids (Dayrit) Indole alkaloids. Structural groups..
9.0 Alkaloids (Dayrit) Simple indole alkaloids
Simple indole alkaloids. Decarboxylation of tryptophan
yieldsserotonin, a neurotransmitter; methylation yields bufotenin,
ahallucinogenic compound isolated from toadstools. 9.0 Alkaloids
(Dayrit) Simple indole alkaloids
Simple indole alkaloids. Bishler-Napieralski condensation
oftryptamine with simple alkyl groups yields the b-carbolines.
Theharmanes are CNS stimulants. 9.0 Alkaloids (Dayrit) Ergot
alkaloids. Condensation with C5 DMAPP
Ergot alkaloids. Condensation with C5 DMAPP. This group
ofmetabolites is produced by the fungus Claviceps purpurea and
includesthe hallucinogen lysergic acid. 9.0 Alkaloids (Dayrit)
Indole alkaloids: tryptophan + secologanin
Indolealkaloids:tryptophan +secologanin.More than
1,100compoundsfrom this groupof mixedmetaboliteshave
beenidentified.Theyoccurpredominantlyin Loganiaciae,Apocynaceae,and
Rubiaceae.This figuregives the majorskeletal types. Indole
alkaloids: tryptophan + secologanin
Indole alkaloids:tryptophan +secologanin. TheVincosan
alkaloidsyield straight- forwardincorporation ofsecologanin. Loss
of glucoseenables moreextensivestructuralchanges to occur.The
glycoside ishypothesized toact as aprotecting group. Many
alkaloidsare formed fromstrictosidine. Indole alkaloids: tryptophan
+ secologanin
Indolealkaloids:tryptophan +secologanin.Loss of glucoseinitiates
thechemicaltransformation.Reserpine is atranquilizer andsedative
isolatedfrom the roots ofRauwolfiaserpentina. 9.0 Alkaloids
(Dayrit) Indole alkaloids: tryptophan + secologanin
Indolealkaloids:tryptophan +secologanin.Strychnine(from theseeds
ofStrychnosnux-vomica)was used inmedicine as aCNSstimulant.
Athigher doses,it was used aspoison forhumans and rodents. 9.0
Alkaloids (Dayrit) Indole alkaloids: tryptophan + secologanin
Indole alkaloids:tryptophan +secologanin. Thecinchona
alkaloids(from the bark ofCinchona spp.)involve
extensiverearrangement.Quin ine was used as anantimalarial drug.
9.0 Alkaloids (Dayrit) Alkaloids from other pathways
Other groups of alkaloids arise from various pathways. A number of
them are metabolites from other biogenetic groups, but are
classified as alkaloids simply because they have an amine
functionality. This mixed group of alkaloids includes: the
quinoline alkaloids (from anthranilic acid, shikimates) terpene
alkaloids nicotine alkaloids xanthine alkaloids. 9.0 Alkaloids
(Dayrit) Quinolinealkaloids.Except for thecinchonaalkaloids,
thequinolinealkaloids aremixedmetabolitesbeing derivedfrom
anthranilicacid, whichbelongs to theshikimategroup, andpolyketides.
9.0 Alkaloids (Dayrit) The terpene alkaloids. The sequence of
addition of nitrogen into theterpene is suggested to be: R-CH2-OH
R-CHO R-CH2-NH2 (whereR is a terpene metabolite). 9.0 Alkaloids
(Dayrit) The terpene alkaloids. Steroidal alkaloids are formed from
completedsteroids. Solasidine and tomatidine occur in potatoes and
tomatoes,respectively. 9.0 Alkaloids (Dayrit) Nicotine alkaloids
Tobacco (Nicotiana tabacum) is another plant from which a large
commercial sector has formed. It is a practice that originated from
the American Indians. Nicotiana comes from the name of Jean Nicot,
a French diplomat who probably introduced the habit to Europe;
tabacum comes from the Indian name for the pipe that was used to
smoke it. Nicotine, the chief constituent of N. tabacum, is formed
from nicotinic acid. 9.0 Alkaloids (Dayrit) Nicotinic acid.In
plants, nicotinic acid is biosynthesized fromtryptophan.
9.0 Alkaloids (Dayrit) Nicotinic acid. In bacteria, nicotinic acid
is biosynthesized fromglyceraldehyde + aspartic acid.
9.0 Alkaloids (Dayrit) Nicotinic acid. Mimosine, which resembles
phenylalanine, is a toxinfound in grass which is used as animal
feed. 9.0 Alkaloids (Dayrit) Nicotinic acid. Biosynthesis of
nicotine
Nicotinic acid. Biosynthesis of nicotine. Nicotine is a
ganglioniccholinergic-receptor agonist. Chronic ingestion of
nicotine often results inpsychological addiction and physical
dependence. 9.0 Alkaloids (Dayrit) Xanthine alkaloids are important
components of a number of culturally, historically and commercially
important plants, in particular coffee, cola (kolanut), tea and
cacao (chocolate). The active constituents are methyl xanthines,
the best known of which are: caffeine, which occurs in coffee
(Coffea arabica); theophylline, which is found in tea (Camellia
sinensis); and theobromine, which is found in cacao (Theobroma
cacao). Note that theophylline and theobromine are isomers. 9.0
Alkaloids (Dayrit) The methylxanthines (caffeine, theophylline and
theobromine) are CNS stimulants and smooth muscle relaxants.
Research into their physiological mechanisms are continuing
research topics. The biogenetic origin of xanthine is complex and
arises from various primary metabolites. Carbons 2 and 8 come from
an active 1-carbon fragment (e.g, formate, methyl methionine,
etc.); carbon 6 comes form CO2; and carbon atoms 4 and 5 and
nitrogen 7 come from glycine. The nitrogen atom at 1 comes from
aspartic acid, while those at 3 and 9 come from the amide nitrogen
of glutamine. 9.0 Alkaloids (Dayrit) Summary Structurally, the
alkaloids are a very diverse group; the onlyunifying characteristic
is the presence of an amine. The origin of the carbons in alkaloids
include the aliphaticamino acids (ornithine and lysine), aromatic
amino acids(phenylalanine, tyrosine and tryptophan, which arises
fromshikimic acid via phenylpropanoids), anthranilic acid
(fromshikimic acid), polyketides, and terpenes. 9.0 Alkaloids
(Dayrit) Tropane alkaloids (e.g., Atropa) Pyrrolizidine
alkaloids
Summary The alkaloids are divided into characteristic structures,
which are also usually associated with specific plants or
organisms. Among the best known groups of alkaloids are: Tropane
alkaloids (e.g., Atropa) Pyrrolizidine alkaloids Phenylethylamines:
(e.g., Ephedra) Phenylalanine + C6-C2: (e.g., Aprophine, Papaver
andErythrina) Tryptophan + DMAPP: (e.g., ergot alkaloids)
Tryptophan + secologanin: (Vinca, Catharanthus, Strychnos,Cinchona)
Steroidal alkaloids Nicotinic acid: (Nicotiana) Xanthine alkaloids:
(Coffea, Camelia, Theobroma) 9.0 Alkaloids (Dayrit) Overview of
alkaloid biosynthesis
Overview of alkaloid biosynthesis. The biogenetic location of
thexanthines is diverse and not included here. 9.0 Alkaloids
(Dayrit)