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Alkaloids – Natural nitrogenous secondary metabolites from plants and microbes. Some important classes of alkaloids Class Precursors Examples _____________ Piperidine alkaloids L-lysine (C 5 N) Piperine Pyrrolidine / tropane alkaloids L- ornithine (C 4 N) Cocaine , scopolamine
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Piperidine alkaloids L-lysine (C5N) Piperine
alkaloids L-ornithine(C4N) Cocaine, scopolamine
Pyridine alkaloids L-Trp or L-Asp Niacin (Vit. B3), nicotine
Catecholamines L-Tyr (C6C2N) Dopamine, adrenaline, mescaline
Opiates 2 L-Tyr units Morphine, tubocurarine
Phenylalanine-derived L-Phe(C6C3N) Capsaicin, ephedrine
Indole alkaloids L-TrpSerotonin, ergotamine, LSD, vinblastine
Quinoline alkaloids “ quinine, camptothecin
Purine alkaloids L-Gly, L-Gln, L-Asp Caffeine, theobromine
Building blocks from the acetate, shikimate, or deoxyxylulose phosphate pathways are also frequently incorporated into the alkaloid structures.
Many alkaloids acquire their N via transamination reactions (catalyzed by Vitamin B6).
Most alkaloids are quite toxic and produced by the plant as a defense against herbivores.
(an indole, aka 5-hydroxytryptamine or 5-HT)
Parkinson’s disease – dopamine transmission is inadequate, can be improved by L-DOPA
Depression – treatment with SSRIs increases dopamine in the synaptic cleft
throughout the body
a quarternary ammonium salt
Norepinephrine malfunction is implicated in anxiety and mood disorders, bipolar disorder
Acetylcholine: low levels may cause memory lapses associated with Alzheimers’, can be improved with cholinesterase inhibitors
Formation of the 6-membered ring
(also see Lobelia)
Piperine: from black pepper (Piper nigrum)
Spicy flavor and CYP3A4 inhibitor
The pungency of piperine is caused by activation of the heat and acidity sensing TRPV ion channel found on nociceptors (pain sensing nerve cells)[
Has insecticidal properties
Produced from three different building blocks
Formation of first ring
tropane skeleton found
in cocaine, hyoscyamine
From leaves of Erythoxylum coca. Well-known CNS
stimulant, also anaesthetic
Extracts of coca leaves
used in original Coca-cola
recipe (cola provided the caffeine) but in 1906 the Coca was eliminated.
Structural similarity to acetylcholine allows tropanes to block muscarinicACh receptors, providing anaesthetic effect.
Found in belladonna
henbane and Datura species
Can be used as sedative
or external pain relief, but
they are toxic and have psychotropic effects
From European Food Safety Authority website
From European Food Safety Authority website
Related to the tomato (also a member of Solanaceae)
Native to: Europe, North Africa, Naturalized in North America
Morphology: Branching herbaceous perennial, berry bearing, ~1.5m tall
It’s highly toxic! All parts of the plan contain tropanealkaloids atropine and hyoscyamine. Berries are sweet, but toxic
Historical use: Ladies “at court” in Europe applied to the eyes to dilate pupils, “enhance beauty”
Anticholinergic agent: Causes ataxia (loss of coordination)
Also causes pupil dilation, double vision, increased body temperature, confusion. Competes with ACh, reversibly inhibits neuron activityBackground and Occurrence
Henbane (Hyosycamusniger) contains primarily hyoscyamine, a stereoisomer of atropine, and scopolamine (-)-hyoscine.
Henbane was thought to induce hallucinations & visions of flight, associated with witches. Could be used to treat GI spasms if it wasn’t so toxic.
Jimson weed (Datura) grows wild throughout Europe, Americas.
Thieves used it in the Middle Ages to drug & rob people.
Some South American tribes used it to calm down unruly kids due to mild sedative effect at low doses.
Synthetic analogues of cocaine used as local anaesthetics have muscarine structural motif:
Local anaesthetic used to numb skin, rapidly absorbed.
Also used as anti-arrhythmia agent after a heart attack
Alters signal transduction in nerve membranes by blocking fast voltage-gated sodium channels, stopping pain signals
Used mainly as a
Found in anbesol,
Both are sodium channel
blockers, used mainly in
Tobacco - the cured and dried leaves of Nicotianatabacum(Solanaceae)
an annual herb indigenous to tropical America, cultivated widely for smoking
Tobacco leaves may contain from 0.6–9% of (−)-nicotine (an oily, volatile liquid) together with smaller amounts of structurally related alkaloids. Nicotine in small doses can act as a respiratory stimulant, but in larger doses it causes respiratory depression.
Arecoline is the major alkaloid in Areca nuts (betel nuts) - the seeds of Areca catechu (Palmae/Arecaceae), a tall palm cultivated in India & other parts of Asia. Nuts are mixed with lime, wrapped in leaves of the betel pepper and then chewed for their stimulant effect, and subsequent feeling of well-being and mild intoxication.
Lobelia inflataor “Indian tobacco” contains lobeline, which has a similar but weaker stimulant effect than nicotine
Used in smoking deterrent preparations
The dried sliced tops of Lophophorawilliamsii, found in Mexico & Southwestern US is used to prepare peyote, used by native Americans in religious ceremonies to induce hallucinations. The major psychoactive alkaloid in peyote is mescaline, named after the Mescaleros Apaches. Mechanism of action may involve blocking reuptake of norepinephrine at receptors, increased serotonin release.
(El-Seedi, et al, J. Ethnopharmacology (2005)101: 238-242.)
Many cactus alkaloids have a heterocyclic
6-membered ring (isoquinoline structure):
Block b-adrenoreceptors that regulate cardiac muscle contractions (b1) and smooth muscle relaxation in the lungs (b2)“Beta blockers”: synthetic antagonists of b-adrenergic receptorsand some agonists
vasoconstrictor, used in Afrin
nasal spray (natural)
b1 antagonist controls force of heart contractions, lowers bp
b2 agonist relaxes
The major stimulant in Ephedra is ephedrine, a sympathomimetic amine (similar effects to noradrenaline)
Unlike actual catecholamines, ephedrine only acts weakly on receptors, but it can displace noradrenaline from nerve storage vesicles which then stimulates receptors.
Ephedrine is orally active and lasts longer than noradrenaline.
“Khat” Catha edulis (Celastraceae): In African and Arabian countries, the leaves of this tree are chewed as a stimulant to alleviate hunger and fatigue, while also giving a sensation of general well-being. Users become cheerful and talkative, so khat has become a social drug and use may lead to psychological dependence.
Young, fresh leaves are most effective. The principal CNS stimulant is contain (−)cathinone ,with similar pharmacological properties as the synthetic CNS stimulant (+)-amphetamine.
Both compounds act by inducing release of catecholamines.
Other amphetamine-like derivatives include MMDA
(methoxymethylenedioxyamphetamine) and MDMA (methylene-dioxymethamphetamine)
MMDA may be formed in the body after ingestion of nutmeg (Myristicafragrans) by an amination process on myristicin, which may account for reported euphoric and hallucinogenic effects of consuming nutmeg.