Some important classes of alkaloids

Class Precursors Examples _____________ Piperidine alkaloids L-lysine (C5N) PiperinePyrrolidine/tropane alkaloids L-ornithine (C4N) Cocaine, scopolamine

Pyridine alkaloids L-Trp or L-Asp Niacin (Vit. B3), nicotine

Catecholamines L-Tyr (C6C2N) Dopamine, adrenaline, mescaline and tetrahydroisoquinolines anhalamineOpiates 2 L-Tyr units Morphine, tubocurarine

Phenylalanine-derived L-Phe (C6C3N) Capsaicin, ephedrine

Indole alkaloids L-Trp Serotonin, ergotamine, LSD, vinblastineQuinoline alkaloids “ quinine, camptothecinPurine 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.

Opiates: morphine and related compounds

Precursor is formed by combining two C6C2 units, both originating from tyrosine

Focus: • pieces/precursors• key assembly steps• structure-activity• physiological interactions

First methylation

From (S)-N-methylcoclaurine

Further methylationis common

Two stereoisomers lead to different classes of alkaloids

Berberine is found in many members of the Berberidaceae, the Ranunculaceae, and other families used in traditional medicine. Berberine has antiamoebic, antibacterial, and anti-inflammatory properties.

Curare (Chondrodendron tomentosum)Arrow poison prepared in the rain forests by native South American tribes from bark and stems may contain >30 different plants, C. tomentosum is the main one.Radical dimerization of the two coclaurine isomers produces a dimer. Tubocurarine is an ACh agonist, once it gets into the bloodstream, it relaxes voluntary muscles to paralysis by blocking nerve impulses at neuromuscular junction.Synthetic derivatives used during surgery as muscle relaxant.

Oh oh…I think I just stuck myself… better

get out my cholinesterase inhibitor pen!

Making the morphine skeleton

Key steps: 1) oxidative coupling to

make the 4th ring2) reduction and acetylation

promotes formation of the furan ring

3) Demethylation then reduction of the ketone

“antitussive”

Opium

• 80% of the world’s illegal opium is grown in Afghanistan, supported by the Taliban.

• $ 4 billion/year industry• Northern alliance initiative to destroy poppy fields pits

farmers and corrupt local officials against military• U.S. troops are supporting efforts to provide farmers

with crop alternatives like pomegranates or grapes• However, many farmers told in 2007 that they will

receive assistance in establishing legal crops are still waiting for help

R. Draper, “Opium Wars”, National Geographic, Feb. 2011.

Properties of opiates

Brain has opioid receptors (d, k and m) that mediate adenylcyclase, lower cAMP levels,increase K+ conductance, close Ca2+ channels, decrease transmission of pain signalsMorphine’s use as analgesic involves overall CNS depression so side effects include drowsiness, lethargy, constipation, unconsciousness

Structural basis for bioactivity of opiates

“Morphine rule”: structure must contain an aromatic ring attached to a quarternary C (Cq) with a 2-C linker to a tertiary (3o) amine

Enkephalins and endorphins areendogenous opioid peptide ligands produced during labor, other times of stress or shock“Runner’s high” ACh inhibitors, rapidly degraded

Drugs derived from morphine semi-synthetically

“Demerol”

Similar activity, can beproduced from morphineby methylation

Effective analgesic andless addictive

Used in cough medicine Antitussive but not analgesic

Synthetic analogues

Heroin was introduced into cough syrup in 1898Acetylation decreases polarity, so itcrosses blood-brain barrier quickly.

More addictive though.

Orally active, produces lesseuphoria, less withdrawal

Fast, but short-acting analgesia

50-100X more active thanmorphine due to lipophilicity

From L-Tryptophan: Indole alkaloids

Produced from L-tryptophanplus an isoprene unit, the indole alkaloids have polycyclic ring structures

Fungi from Claviceps genus are best-known producers because they infect some grain crops, but the indoles are produced by other fungi including Aspergillus and Penicillium

Lysergic acid is probably the most well-known, as it is the precursor for hallucinogen LSD (lysergic acid diethylamide) andthe ergot alkaloids – mixed agonist/antagonist effects on 5-HT (serotonin) receptors leads to hallucinationsserotonin

skeleton

Ergot – not a fun guyErgot is the dried sclerotium of the fungus Claviceps purpurea

that develops on the ovary of rye and other grasses consumed by humans or animals.

The poisonous properties of ergots are caused by a group of indole alkaloids, the ergot alkaloids or ergolines. Consumption of ergot-infected rye produces a disease called ergotism. Ergot poisoning affects two types of receptors:

1) The a-adrenergic receptors for norepinephrine which causes• GI upsets, e.g. diarrhea, abdominal pains, and vomiting.• Circulatory changes, e.g. coldness of hands and feet due to vasoconstriction of the blood vessels to the extremities

Ergot – not a fun guy2) Ergot also acts on the serotonin (5-HT) receptors and the a-dopaminergic receptors causing neurological symptoms:Headache, vertigo, convulsions, psychotic disturbances, hallucinations

Vasoconstriction can cause restricted blood flow in small terminal arteries, death of the tissue, gangrene, and even the loss of hands, feet, or limbs.

Gangrenous ergotism was known as St. Anthony’s Fire because the Order of St. Anthony cared for the sufferers during the Middle Ages in Europe when outbreaks of the disease in humans and animals were relatively frequent.

Indole-isoprenoid is modified by attachment of a small peptide (Phe & Pro usually)Ergotamine’s vasoconstrictive activity has led to its use in treatment of migraines

For more information:A Trip Through LSD and the Ergot Alkaloids

Melissa MedeirosNatural Products

May 2006

Amaryllidaceae alkaloids

Compounds 1 – 6 are found in Narcissus pseudonarcissus (daffodils) and other plants from the Amaryllidaceae family.Pancratistatin (1) selectively induces apoptosis in cancer cells through a mitochondrial pathway, with little effect on normal cells. However, it’s not very abundant.Narciclasine (3) disrupts organization of the actin cytoskeleton of cancer cells at 30-90 nM.Compound (5) also has potent anticancer activity while its cis-isomer has none.Problems: low water-solubility, low natural abundance of (1) and (5), and toxicity of (3) ... Narciclasine inhibits CYP3A4 (major liver detoxifying cytochrome), which can damage the liver.

pancratistatin

narciclasine(abundant)

synthetic derivativetrans-dihydronarciclasine

Goal of study: prepare trans-dihydro derivative of narciclasine that has the anti-cancer activity without the liver toxicity.

SAR shows thispart of structureis key...

Protection of the OH groupsfollowed by hydrogenation usually produces a mixture of cis- and trans-isomers.Since the cis-isomer (7) has noanticancer activity, reaction conditions were modified until maximum yield of trans isomer was achieved.Compound 5 did not inhibitany of the cytochromes tested.

Result: an effective way to prepare a less toxic drug!

Complex polycyclic indole alkaloids are prevalent in species such as Uncaria tomentosa (Cat’s claw)see: Heitzman, M. E.; Neto, C. C.; Winiarz, E.; Vaisberg, A. J., Hammond, G. B.* Review: Ethnobotany, phytochemistry and pharmacology of Uncaria (Rubiaceae), Phytochemistry, 66: 5-29 (2005).

More indole alkaloids...from Uncaria tomentosa

Traditional uses of U. tomentosaAqueous extracts of bark or roots used by Ashaninka in

Peruvia rainforest:• Allergies, asthma• Wound healing, viral infections, fevers• Arthritis, gastric ulcers, rheumatism, inflammation• Contraception, menstrual irregularities, childbirth• Some of these activities are associated with alkaloids• Uncaria also contains flavonoids, catechins,

cinchonains, caffeic acid and ursolic acid – these may play a role in some of the bioactivities.

Pentacyclic oxindole alkaloids have immune-boosting effect, upregulate B & T cell production

Tetracyclic oxindole alkaloidsblock immune factor, may also inhibit central dopamine release, depressing movement

Weaker dopamine receptorantagonist – some locomotor depression observed

More indole alkaloids...terpene indole alkaloids with multicyclic structures

Coupling of tryptamine(decarboxylated Trp)with the secoiridoid secologanin, followed by a variety of cyclizationsforms terpene indolealkaloid skeletons with lots of structural diversity

Some interesting terpene indole alkaloids

Yohimbine• From Pausinystalia yohimbe• Folk use: Aphrodisiac,

weight loss• Pharmacology: dilates blood

vessels, binds a-adrenergic receptors

Reserpine, deserpidine• From Rauwolfia serpentina• Folk use: treat snake bite,

insanity• Pharmacology: lowers blood

pressure, tranquilizer, depletes stored catecholamines

Skeletal rearrangements and dimerization lead to vincristine/vinblastine structure

• Intermediates form through rearrangement of isoprenoid moieties

Anti-cancer agent, used to treatleukemia and lymphoma.

Strychnine

• From dried seeds of Strychnos nux-vomica

• Biosynthesis: Rearrangements in strictosidine

• Highly toxic convulsant• Binds to glycine binding sites in

spinal cord, causes asphyxiation• Has been used as a rat poison

Quinine – Antimalarial agent from bark of Cinchona trees

Discovered in 1600’s , cured a Peruvian countessDepolymerizes toxic byproducts of PlasmodiumQuinine was originally added to tonic water as aprophylactic against malaria – ginwas later added to mask the bitter taste

Some important classes of alkaloids

Class Precursors Examples _____________ Piperidine alkaloids L-lysine (C5N) PiperinePyrrolidine/tropane alkaloids L-ornithine (C4N) Cocaine, scopolamine

Pyridine alkaloids L-Trp or L-Asp Niacin (Vit. B3), nicotine

Catecholamines L-Tyr (C6C2N) Dopamine, adrenaline, mescaline and tetrahydroisoquinolines anhalamineOpiates 2 L-Tyr units Morphine, tubocurarine

Phenylalanine-derived L-Phe (C6C3N) Capsaicin, ephedrine

Indole alkaloids L-Trp Serotonin, ergotamine, LSD, vinblastineQuinoline alkaloids “ quinine, camptothecinPurine 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.

From amino acids to purines to caffeine and xanthine alkaloids

Purine heterocyclicring system is derivedfrom amino acids and various one C donors muscle relaxant

The xanthines

Caffeine, Theobromine, and Theophylline

• The purine alkaloids caffeine, theobromine, and theophylline are all methyl xanthines that commonly co-occur in plants. Major sources are stimulant beverages and foods such as tea, coffee, cocoa, and cola.

• Xanthines competitively inhibit phosphodiesterase, causing an increase in cyclic AMP and adrenaline release. This leads to CNS stimulation, relaxation of bronchial smooth muscle, and induction of diuresis. Inhibition of TNF-a and leukotriene synthesis is thought to occur, reducing inflammation and innate immunity.

• The effects vary among the three compounds. Caffeine is the best CNS stimulant. As a vasoconstrictor it can be combined with a therapeutic agent to increase effectiveness (e.g. compound analgesics). It has weaker diuretic action.

• Theobromine has little stimulant action, but has more diuretic activity and also muscle relaxant properties. Theophylline also has low stimulant action and is an effective diuretic, but it relaxes smooth muscle better than caffeine or theobromine and is frequently used in slow-release formulations.

Caffeine and adenosine• Caffeine readily crosses the blood-brain barrier, and once in the brain, the

principal mode of action is as a nonselective antagonist of adenosine receptors (competitive inhibition)

• Adenosine is found in every part of the body, but it has special functions in the brain. Concentrations of brain adenosine are thought to be increased by metabolic stresses such as anoxia or ischemia. It also may have a specific role in control of the sleep-wake cycle.

• Brain adenosine may also protect the brain by suppressing neural activity and increasing blood flow through A2A and A2B receptors located on vascular smooth muscle. By counteracting adenosine, caffeine reduces resting cerebral blood flow – it’s a vasoconstrictor.

• Adenosine is released in the brain through a complex mechanism. It is not likely that adenosine is the primary neurotransmitter for any group of neurons, but rather is released together with other transmitters by a number of neuron types.

So you can’t start the day without Joe?

You’re not alone!• Is caffeine addictive? Several classes of adenosine receptors

are known, and there is evidence that A 2A receptors interact with the dopamine system, which is involved in reward and arousal.

• Tolerance: because caffeine is primarily an antagonist of adenosine receptors, regular caffeine consumers may adapt to its continuous presence by increasing the number of adenosine receptors.

• This reduces the stimulatory effects (tolerance adaptation) and makes one much more sensitive to adenosine, so that reducing caffeine intake results in withdrawal symptoms.

From Drugs and the Human Body, 6th edition, K. Liska

Chocolate and pet poisoning• Dogs are most often affected, due to their ability to find chocolate

and the common 'sweet tooth' they develop, but cats and other mammals are susceptible to the toxic effects of chocolate, too.

• Theobromine is the major stimulant in chocolate. Its effects:– CNS and cardiovascular stimulant– Increases blood pressure (mild) – Nausea and vomiting

• Are some chocolates more toxic than others?Yes. Unsweetened chocolate contains 8-10 times the amount of Theobromine as milk chocolate. Semi-sweet or dark chocolate falls roughly in between the two.

• The toxic dose of Theobromine (or caffeine) for pets is 100-200mg/kg body weight. However, reports by the ASPCA have noted problems at doses much lower than this - i.e. 20mg/kg.

• Using the 20mg/kg as a measure of "problems can be seen” a 50 lb (23 kg) dog would have to consume 9 oz of milk chocolate. Some dogs won't see problems at this rate but others may.

http://vetmedicine.about.com/cs/nutritiondogs/a/chocolatetoxici.htm

Background• A Dutch cohort study in 2002 and others found that higher

coffee consumption correlates with lower risk of Type-2 diabetes.

• Coffee constituents that have been previously shown to affect glucose metabolism include caffeine, chlorogenic acid, lignans, trigonelline.

• Caffeine also reported to induce energy expenditure by boosting thermogenesis.

• However some cohort studies suggest that decaf also reduces risk.

• Is caffeine the active constituent against diabetes?

Mouse model

• KK-A3’ mice spontaneously develop type-2 diabetes by 6 weeks old

• Obesity, hyperglycemia, high triglycerides and fatty liver also develop

• Hyperinsulinemia is observed, suggesting that insulin resistance occurs in peripheral tissues.

• Altered signaling in adipose tissues: decreased adiponectin (a adipocytokine) and increased MCP-1 (monocyte chemoattractant protein) and TNF-a which induce insulin resistance.

• Study looked at mechanism of coffee’s action against type-2 diabetes and the effect of caffeine alone on development of hyperglycemia.

Experiments

• Expt 1: Diabetic mice were given diluted coffee vs. plain drinking water for 5 weeks ad libitum

• Blood drawn and blood glucose measured weekly• Insulin tolerance test given at 5 weeks• At study end, mice were sacrificed and tested for serum

insulin, triglycerides, total cholesterol, and pro-inflammatory markers MCP-1 and IL-6.

• Liver and adipose tissues were removed and examined for cholesterol, TG and phospholipid content.

• Expt 2: Same study was repeated with caffeine solution (250ug/mL) vs. plain drinking water

Coffee treatment lowered blood glucose and insulin tolerance in the mouse model

Coffee treatment significantly reduced liver weight, most types of body fat, liver lipid content, insulin and TAG levels, and inflammatory markers

Conclusions

• Coffee improved insulin sensitivity and decreased hyperglycemia (high blood sugar)

• Expression of inflammatory cytokines such as MCP-1, IL-6 and TNFa in adipose tissue was lower in the coffee treatment group.

• Coffee decreased adipose tissue inflammation, thus improving insulin resistance.

• Caffeine alone also reduced hyperglycemia, inflammation in some but not all fat tissues.

• Both coffee and caffeine improved fatty liver.