Book Alkaloids

8/3/2019 Book Alkaloids

1/26

Chapter 12Modes of Action of Alkaloids

Michael Wink

I. INTRODUCTION

We can safely assume that most alkaloids play an important role in the ecology of plants.In general, alkaloids serve as defense chemicals against herbivores and to a lesser degreeagainst bacteria, fungi. and viruses or provide a means of interaction with other plants (seeChapters 13 and 14). A protective function has also been attributed to those alkaloids thatare produced or sequestered by animals (see Chapters 15 and 16). In order to fulfill thisfunction, alkaloids must closely interact with specific targets in herbivores, predators.microorganisms, or competing plants, i.e., they must either inhibit or otherwise deregulateimportant processes that are vital for these organisms. A thorough understanding of howthese capabilities are effected is important for a comprehension of the evolutionary andecological implications of alkaloids and their rational use in medicine or as naturalpesticides in agriculture.

Whereas we know the structures of more than 12,000 individual alkaloids. ourknowledge of their biological activities and functions is still rather limited. In this chapterI have tried to summarize and discuss the modes of action of the better known alkaloids.considering molecular targets first and then more complex interactions with organs orcomplete organisms. For overviews see Habermehl (1983), Harbome (1993), Luckncr( 1990), Mann (1992), Mothes et al. (1985), Mutschler (1981), Rimpler (1990), Robinson( 1981), Rosenthal and Berenbaum (1991, 1992). Roth et al. (1994), Teuscher and Linde-quist (1994), Wagner (1993), Waller (1987). and Wink (J992a,b, 1993a).

2. MOLECULAR TARGETS OF ALKALOIDSIn the following I have identified a number of important cellular molecular targets

that are often affected by allclochemicals.Michael Willk Institute for Pharmaceutical Biology. University of Heidelberg. D-69 I 2 0 . Heidelberg.Germany.Alkaloids: Biochemistrv. Ecologv. and Medicinal Applications. edited by Roberts and Wink. Plenum Press. NewYork. 1998.

301


2/26

3 02 Michael Wink2.1. Blomembranes

Cells can only operate if they are enclosed by an intact biomembrane and eukaryoticcells must be sub structured by an internal membrane system providing a complex com-partmentation, i.e., separated reaction chambers. As biomembranes are almost imperme-able for ions and polar molecules, cells can prevent the uncontrolled flux or migration ofessential metabolites. The controlled flux of these compounds across biomembranes isachieved by specific transport proteins, which can be ion channels, pores, or carrierproteins. If the transport of metabolites has to proceed against a concentration gradient.transport processes must be either directly or indirectly energized (Alberts et al.. 1994;Lodish et al .. 1995). These complex transport systems are targets of many natural prod-ucts.

Steroidal alkaloids, such as solanine and tomatine which are present in many mem-bers of the Solanaceae, can form complexes with the cholesterol present in biomembranes.Important for this interaction is the presence of a lipophilic portion of the molecule (givenby the steroidal moiety) and a hydrophilic portion (provided by the sugar side chain).Whereas the lipophilic moiety "dives" into the lipophilic interior of the membrane andinteracts with the structurally similar cholesterol, the hydrophilic side chain remainsoutside and binds to external sugar receptors. As phospholipids are in continuous motion,a tension easily builds up which leads to membrane disruption, i.e., transient "holes" occurin the biomembrane rendering the cell leaky. A similar mechanism has been postulated forsaponins, a widely distributed group of natural products, to which the steroidal alkaloidsmay be assigned (Fig. I). Steroidal alkaloids can also interact with other targets. such asreceptors.

The alkaloids tetrandrine. berbamine, and cepharanthine have also been reported tointerfere with membrane integrity (Wink, 1993a).2.2. Signal Transduction at Biomembranes

Cells carefully control the homeostasis of their ion concentrations through the actionof specific ion channels (e.g . Na ", K+, Ca2+, and CI- channels) and of active Na ", K-.or Ca2+ pumps. such as Na+ IK +-ATPase and Ca2+-ATPase (for an overview see Albertset al .. 1994). Ion gradients and ion fluxes mediated by these channels and pumps are themain elements in active transport processes, and in neuronal and neuromuscular signaling.2.2.1. Na + IK"'- -ATPase

Cardiac glycosidcs, found in plants. some insects. and in the skin of toads (Buf-onidae). are potent and well-known inhibitors of Na+ IK "'--ATPase. A few alkaloids-harmaline. nitidine, sanguinarine. capsaicin, cassaine, and solenopsine (from ants)-ex-hibit Na + IK +-ATPase inhibition.2.2.2. NEUROTRANSMITTER RECEPTORS


3/26

of Alkaloidsodes of Action

:i= " _...Y.~ OO-?.[- HI :1:-0O-O-j V

o 0-0G I

o

303

..:


4/26

nerve terminal (presynapse)

+-- voltale-gatedCal+ channel

acetylcholine estrase

vesicle with acetylcholine

AChR

voltage-IatedNa+ channels

postsynaptic target cell

10%

Figure 2. Signal transduction in excitable synapses. (Bottom) Nicotinic ACh receptor; (top ) schematic vic" ofa chemical synapse. Events during ncurotrunsmission: When a nerve impulse arrives und dcpolarizcs the plasmamembrane, voltage-gated Ca2' channels "pen and Ca2 enters the presynapse. Increased Ca2 concentrationsactivate the synaptic vesicles (hy a complicated interaction of several proteins) so that they can fuse with thepresynaptic membrane by cxocytosi. Thus. acetylcholine is released into the synaptic deft and hinus to AChR


5/26

Modes of Action of Alkaloids 305coupled with ion channels. The neurotransmitters involved include. among others. nor-adrenaline (NA). serotonin, dopamine. histamine, glycine, GABA, and acetylcholine(ACh). The following mechanisms have been deduced for the direct and indirect channelsystems, which are very similar across a wide range of animals (Fig. 2):

Type I is a ligand-gated channel, i.e., a receptor that is part of an ion-channelcomplex. The well-studied nicotinic ACh receptor consists of five subunits. two of thembinding ACh. When the neurotransmitter binds, a conformational change induces theopening of aNa +IK + channel for microseconds. allowing Na + ions (the external concen-tration is about 145 rnlvl) to enter the cell following a concentration gradient (the internalNa+ concentration is between 5 and 15 mM). The ligand quickly dissociates from thereceptor and in the case of ACh is hydrolyzed by ACh esterase.

In Type II the muscarinic ACh receptor is an integral protein. When ACh binds, thereceptor changes its conformation, inducing a conformational change in an adjacentG-protein molecule. consisting of three subunits. The a-subunit dissociates and thenactivates the enzyme adenylyl cyclase. which in tum produces cAMP from ATP. ThecAMP molecule. a second messenger. activates protein kinases or Ca2+ channels directly.Whereas ACh is degraded in the synaptic gap. the biogenic amines are taken up by thepresynaptic membrane and consequently by the synaptic vesicles. These uptake processesinvolve transport proteins.

Nicotinic ACh receptors belong to Type I (Fig. 2), while the muscarinic ACh recep-tor. noradrenaline, serotonin. and dopamine receptors belong to Type II. The family ofGABA receptors, some of which are important for memory storage in the brain. resembleType I systems.Quite a number of alkaloids are known whose structures are more or less similar tothose of endogenous neurotransmitters and can function as structural analogues (review inWink, 1993a). In addition. several plants produce compounds that are identical to animalneurotransmitters. such as acetylcholine and histamine in stinging hairs of Urtica. orserotonin and dopamine in several species. Targets can be:

I. The receptor itself (Table I) through inhibition or overstimulation., The enzymes that deactivate neurotransmitters after they have bound to a recep-

tor (Table II)3. Transport processes (Table III), which are important for the storage of the neuro-transmitters in synaptic vesicles

4. Enzymes involved in the biosynthesis of a neurotransmitter

~.~.J. VOLTAGE-GATED Na ~/K - CHANNELSThe stimulation of neurotransmitter-activated ion channels leads to a rapid intlux of

:\a - ions which in tum activates voltage-gated Na - and K ~ channels essential for furthersignal transduction. These Nu - and K - channels constitute another important target foralkaloids (Table IV).

~.~.4. KEY ENZYMES OF SIGNAL PATHWAYS


6/26

306

~E~.cr-1 > 0

' S:c.a.E. . .01 > 0e

o~

;>" E(UJ

Michael Wink

.,.s~ce"0~,.s~c~"0~Z

"0c :,. ., .,: _ j .S .S"0 "0. . c : . . c :'-' u;;, ;;," "-' u- e - e'- '-E. :: g _, . . E. , . .'" '-'~ < .I l: !-'


7/26

Modes of Actio n o f A lk alo id s 307

~ '"cc C C ~c ' s ';;j ~0 0 c2 ' " c o c '1 3 0c. . ~ >. "0~ 0 "0 6 cC I ' l 0 0 - c (.lJ . .. . . . . . . ,. . . 0 0 g : :0 a '" a . . .'5. . . . ~ 0 . . .~ 0 0~ u '5. u '5. . .u ~ '5. c~ ~ ~~ u ~. . u ~ . ' s . . " : : : I. . . "0' l . J 6I' l 0 0 0 l!Q


8/26

308 Michael Wink

Table IIAlkaloids as Inhibitors of Neurotransmitter-Degrading Enzymes

Enzyme Natural substrate Alkaloid OccurrenceAcetylcholine esterase Acetylcholine Physostigmine (eserine)

BerberineCoptisineCoralyneGalanthamineChaconineDemissineSolarmargineSolanineSolanidineHuperzine A

Monoamine oxidase(MAO)

NA, dopamine,serotonin, histamine Harmaline

HarmineTetrahydro-j3-carbolineSalsolinolEphedrine

Catechol-O-methyltransferase

NA, adrenaline,dopamine Coralyne

Tetrahydroisoquinoline"More details in Wink (1993).

Table IIIAlkaloids as Inhibitors of Neurotransmitter Uptake

(Transport into Vesicles)

Physostigma venenosumSeveral PapaveraceaeSeveral PapaveraceaeSeveral AmaryllidaceaeSolanumSolanumSolanumSolanumSolanumHuperzia serrataPeganumPeganumPeganumEphedra

Transporter Alkaloid OccurrenceNoradrenaline Reserpine

EphedrineTetrahydro-j3-carbolineSalsolinolStepholidineTetrahydroisoquinolineTetrahydropalmatineCocaine

Biogenic amines

Dopamine

RauwolfiaEphedraPeganumSalsola

Ervthroxylum


9/26

Modes of Action of Alkaloids 3 0 9

Table IVAlkaloids as Modulators of Na ". K ". and Ca2+ Channels"

Alkaloid Occurrence (genera) ActionNa + and K + channelsAconitine> Aconitum ActivationSparteine" Cytisus. Lupinus. Genista InhibitionQuinine Cinchona InhibitionQuinidine" Cinchona InhibitionAjmaline" Rauwolfia InhibitionHarmaline Peganum InhibitionProtoveratrine A, Bh Veratrum ActivationVeratridine" Veratrum ActivationBatrachotoxin" Frogs (Dendrobatidae) ActivationSaxitoxin= Protogonvaulax (algae) InhibitionTetrodotoxin" Algae/fish Inhibition

Ca2 + channelsRyanodine Ryania speciosa InhibitionBastadin 5 lanthella basta Inhibition

"More details in Wink ( 1993a).bN a +channel.

signal can be important targets further down the pathway. These enzymes include(Fig. 3):

Adenylyl cyclase (making cAMP)Phosphodiesterase (inactivating cAMP)Phospholipase (releasing arachidonic acid or inositol phosphates)Several protein kinases, such as protein kinase C (which is activated by phorbolesters and the alkaloid chelerythrine) or tyrosine kinase (activating other regula-tory proteins or ion channels)

Table V lists some alkaloids that interfere with these targets.

2.3. CytoskeletonMany cellular activities, such as motility, endo- and exocytosis, and cell division, are

mediated through elements of the cytoskeleton, including microfilaments and micro-tubules (for an overview see Alberts et al .. 1994). A number of alkaloids identified inplants and fungi can interfere with them (among others: colchicine, Vinca alkaloids.rnaytansine. rnaytansinine, and taxol), Any alkaloid that impairs the function of micro-tubules or microfilaments is likely to be toxic, and from the point of view of defense, awell-working and well-shaped molecule.2.3.1. MICROTUBULES

Microtubules. which are important for cellular movements, vesicle transport in neu-rons, or the separation of chromosomes during cell division, are composed of tubulin


10/26

310

sipal molecule~cell surface receptor

tG-proteintadenylate cyclase~ATP

1cAMP1 .arget protem(enzyme, ion channel)

Michael Wink

sipal moleculelcell surface receptorlG-proteinlphospholipase C/PIPl/~- ~ ~ r ~ HtarKet protein protein kinase Cactivation

Figure 3. Signal pathway in animal cells. (Left) cAMP pathway; (right) phosphoinositollCa2+ pathway. PIP~.phosphoinositol ~.5-bisphosphate; InsP,. inositol lA.5-trisphosphate.

subunits. Movements and some transport processes (e.g . that of vesicles) are mediatedthrough either the rapid assembly or disassembly of microtubules.

2.3.2. COLCHICINEColchicine. the major alkaloid of Colchicum autumnale (Liliaceae), binds tightly to

tubulin (I : I ratio) and thus inhibits the assembly of microtubules. As a consequence theTable V

Alkaloids That Modulate Enzymes Involved in Signal Transduction"Enzyme Function OccurrencelkaloitiAdenylyl cyclase cAMP formation

Phosphodiesterase cAMP inactivation

Protein kinases Protein phosphorylation

AnonaineJ3-Carboline-l-propionic acidIsoboldineTetrahydroberberinePapaverineCaffeine. theobromineTheophyllinel-Ethyl-J3-carboline

PeumusPapaverCoffea. Camellia. Theobromullex paruguarensis, Paullinia

AnisomycinChelerythrineL yngbya toxin ATelocidin

Streptomyces griseolusChelidonium majusMarine seaweedsStreptomyces blastmvceticum


11/26

Modes of Action of Alkaloids 311

mitotic spindle of dividing cells disappears rapidly after colchicine treatment and chroma-tids are no longer separated. Whereas animal cells die under these conditions. plant cellsbecome polyploid. a trait often used in plant breeding. because polyploidy leads to biggerplants.

Because of its antimitotic activity. colchicine has been tested as an anticancer drug.but has been abandoned because of its general toxicity; however. a derivative. colcemide,is less toxic and can be employed in the treatment of certain cancers. Cellular motility isimpaired by colchicine. This property is exploited in the treatment of acute gout, in orderto prevent the migration of macrophages to the joints (see Chapter 18). Colchicine isindeed a very toxic alkaloid which is easily resorbed because of its lipophilicity; therefore.it is not surprising that Colchicum plants are not attacked by herbivores to any substantialdegree.

2.3.3. DIMERIC INDOLE ALKALOIDSAnother group of alkaloids with antimitotic properties are the dimeric monoterpene-

indole alkaloids. such as vinblastine and vincristine. which have been isolated fromCatharanthus roseus (Apocynaceae). These alkaloids also bind to tubulin and induce theformation of paracrystalline protein aggregates leading to microtubule depolymerization.The inhibition of cell division is similar to that described for colchicine. Both alkaloids arerather toxic but are nevertheless important antimitotic drugs for the treatment of someleukaemias and carcinomas (see Chapter 18).

2.3.4. TAXOLFrom several Taxus species. such as T. baccata and T. brevifolia (Taxaceae), the

alkaloid taxol has been isolated which also affects the architecture of microtubules. but incontrast to the compounds mentioned previously. it stabilizes them. The polymerization oftubulin is enhanced by taxol and becomes independent of GTP and microtubule-associatedproteins (MAPs). The diameter of taxol-induced microtubules is 22 nm (in contrast to 24nm for "normal" microtubules) and consists of 12 instead of 13 protofilaments. Taxolremains bound to tubulin in a ratio of I: I. As a consequence. taxol-induced microtubulesare very stable which arrests dividing cells in mitosis (overview by Reynolds. 1993).Taxol is a new antimitotic drug used in the treatment of ovarian and breast cancer (seeChapter 18).

2.3.5. MICROFILAMENTSCell stability. phagocytosis. cell-cell interactions, and cell movements are also con-

trolled by actin filaments. which are rapidly assembled or disassembled from actin mono-mers. Cytochalasin B. an alkaloid produced by a number of molds. binds to the plus end ofa growing actin filament. preventing the addition of actin monomers there. Latrunculin B


12/26

312 Michael Wink

Table VIAlkaloids Interacting with DNA/RNA and Related Enzymes"

Target Occurrence

DNA

Function Alkaloid

Photoaddition DictamnineHarmanHarmine

Alkylation Pyrrolizidinealkaloids

Aristolochic acid AristolochiaCycasin Cycas

Intercalation ellipticine9-MethoxyellipticineQuinine CinchonaSkimmianine

DNA polymerase Inhibition

DNA topoisomerase I InhibitionReverse transcriptase Inhibition

RNA polymerase InhibitionTranscription Inhibition Colchicum. Gloriosa

Amanita"More details on Wink r 19Y3a,.

2.4. DNA/RNA

AvicineBerberineChelerythrineCoptisineCoralyneFagaronineNitidineSanguinarineOlivacineAvicineCoralyneFagaronineNitidineHippeastrineLycorineCarnptothecineBerberineChelidonineCoralyneVincristine.vinblastineColchicine

Amanitin

DictamnusPeganumPeganumSeveral Asteraceae, Boraginaceae

Berberis. Mahonia. Thalictrum, ChelidoniumChelidoniumSeveral Papaveraceae

Several Papaveraceae

Several AmaryllidaceaeCamptotheca acuminataSeveral Berberidaceae, PapaveraceaeChelidoniumCatharanthus roseus

The genetic information of most organisms is encrypted in DNA (some viruses haveRNA in their genome). As DNA encodes all RNAs. proteins. and enzymes that areimportant for metabolism and development of an organism. DNA is a highly vulnerabletarget. It is not surprising that a number of secondary metabolites have been selectedduring evolution which interact with DNA or DNA-processing enzymes. Some alkaloidsare known to bind or to intercalate with DNA (Table VI and Fig. 4). Many of thesemolecules are planar. hydrophobic molecules which fit between the planar stacks of AT


13/26

Modes of Action of Alkaloids 313'4

1212

1010

7 E 81 , !., 8) cc

0 040 45 50 55 60 65 70 75 80 40 45 50 55 60 65 70 75 30

Temperature ("C) Temperature ("C)

w i Ergotamine 121815 r 101;: 13

8'0 r

a50 55 60 65 70 75 i!O !! 5 40 45 50 55 60 65 70 7S 80 35

Temperature t"C) Temperature ("C)

20 Sanguinarine 2018 1 1 115 1 1 1

74 _'4E1 , 12 !z8'0

2a

45 50 55 50 55 70 75 80 85 40 45 s o 55 60 65 70 75 80 35Temperature ("C) Temperature ( "C)Figure 4. Intercalation of alkaloids with DNA (after Latz-Bruning and Wink. unpublished). DNA was mea-sured at 260 nm in a spectrophotometer with (e)or without (0)alkaloids. The temperature of the solution was


14/26

314 Michael WinkThe effects of DNA-binding or intercalating compounds can be mutations, which

may result in malformations of newborn animals or in the initiation of cancer. In thefollowing a few mutagenic alkaloids will be considered.

When anabasine, coniine, or anagyrine is administered to pregnant cows or sheep, alarge proportion of the offspring develop malformations of the legs, the so-called "crookedcalf disease" (reviewed in Wink, 1993a,b). Some alkaloids of the monocot Veratrum, suchas jervine and cyclopamine, cause the formation of a large central eye, the cyclopean eye,which was probably known to the ancient Greeks and thus led to the mythical figure of thecyclops (see Chapter 4)

Other alkaloids are known as carcinogens, such as aristolochic acid from Aris-tolochia and pyrrolizidine alkaloids (PA) which are produced by approximately 3% of thehigher plants, especially within the families of Asteraceae and Boraginaceae. Aristolochicacid has a nitro group which can be transformed into reactive intermediates in the intes-tine. If resorbed, these metabolites can alkylate DNA. PA are not carcinogenic in theirnative form, but become so when they are "detoxified" in the liver. As can be seen in Fig.5, PA are usually present in the plant as their N-oxides, which are polar compounds thatcannot pass biomembranes by simple diffusion. In the intestine, PA N-oxides are reducedby gut bacteria. The free base is readily taken up by the gut cells and transported to theliver. There, the PA are transformed into alkylating compounds, which covalently bind toDNA. As a result, mutations and cancer can be initiated. The PA story is very intriguing,as it shows how ingenious nature was in the "arms race": The herbivores inventeddetoxifying enzymes and the plants the compounds that are activated by this process. Aherbivore feeding on PA-containing plants will eventually die, usually without reproduc-ing properly. Only those individuals that carefully avoid the respective bitter-tasting plantsmaintain their fitness and will thus survive. The protection resulting from PA can easily beseen on meadows, where Senecio and other PA-containing plants are usually not taken bycows and sheep, at least, as long as other food is available.

2.5. Protein Biosynthesis

Protein biosynthesis is essential for all cells and thus provides another importanttarget. Indeed, a number of alkaloids have been detected (although only a few have beenstudied in this context) that inhibit protein biosynthesis in vitro, emetine from Cephaelisipecacuanha (Rubiaceae) is the most potent. Other alkaloids with the same ability includeharringtonine, homoharringtonine, cryptopleurine, tubulosine, hemanthamine, lycorine,narciclasine, pretazettine, pseudolycorine, tylocrepine, and tylopherine.

Quinolizidine alkaloids, such as sparteine, lupanine, and cytisine, are relatively weakinhibitors at this target (they strongly affect ACh receptors and Na + and K+ channels; seeabove). The stages that are inhibited are the loading of aminoacyl-tRNA with amino acidsand the elongation step. The inhibitory activity was visible in heterologous systems, butprotein biosynthesis in the producing plants (here lupins) was not affected. A number ofantibiotics (from Streptomyces and other bacteria or fungi) are known that inhibit proteinbiosynthesis at specific steps, such as ( l) initiation, (2) peptidyltransferase, or (3) elonga-tion (Table VII). Depending on their affinity for prokaryotic or eukaryotic ribosomes,some of the antibiotics selectively inhibit microbial systems. As mitochondria also contain


15/26


16/26

316

Table VIIIAlkaloids That Modulate Enzyme Activity=

Michael Wink

Alkaloid Enzyme ActivityBrucineStrychnineEllipticineBerberineCanadineChelerythrineCastanospermineDeoxynorjirimycinSwainsonineOchratoxinFolimycinCalyculin A

Lactate dehydrogenaseLactate dehydrogenaseCytochrome oxidaseSeveral enzymesAldose reductaseSeveral enzymesSeveral hydrolasesSeveral hydro lasesSeveral hydro lasesGlucose transportVacuolar H+-ATPasePhosphatase (PP-l)

InhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibitionInhibition

"More details in Wink (I993a).

ribosomes of prokaryotic origins. side effects can occur. Some of these compounds con-tain a nitrogen and could also be classified as alkaloids.2.6. Electron Chains

The respiratory chain and ATP synthesis in mitochondria or photophosphorylation inchloroplasts demand the controlled flux of electrons. These targets seem to be attacked bysanguinarine. ellipticine, gramine, alpinigenine, capsaicine, and a few other alkaloids. Butthis activity may have been overlooked because. as has been mentioned before. only a fewalkaloids have been checked in depth.2.7. Modulation of Enzyme Activity through Alkaloids

A multitude of enzymes exist in animal cells and several alkaloids have been reportedthat interfere with at least one of them. A small selection of interactions is illustrated inTable VIII (see also Tables II and V).2.8. Alkaloids Affecting More than One Target

In general. the interactions of a particular alkaloid with a molecular target (as de-scribed above) suggest a high degree of specificity. A closer look. however. shows thatmany alkaloids interfere with more than one target. The phenomenon will be explained fortwo groups of alkaloids:2.8.1. ERGOT ALKALOIDS

Ergot alkaloids. such as ergotamine. ergometrine. or ergoclavine, are produced byfungi of the genus Claviceps which lives in close contact with many grasses (family


17/26


OH OHserotonine dopamine noradrenaline

Figure 6. Structure-function relationships of ergot alkaloids with the neurotransmitters noradrenaline. dopa-mine. and serotonin.

Poaceae) such as the cereal Hordeum vulgare. These alkaloids can modulate severalreceptors of neurotransmitters, such as dopamine. serotonin. and noradrenaline. As aconsequence, the pharmacological action of ergot alkaloids is rather broad. ranging fromvasoconstriction and uterine contraction to hallucinations. We can explain these activitiesof alkaloids through structural similarities with the different neurotransmitters (Fig. 6). Asexplained in Chapter II, it has been suggested that the interactions between Claviceps andits host plant are of a symbiotic nature. i.e .. infected plants exploit the chemistry of thefungus for their own protection against herbivores (otherwise it would be difficult toexplain why a fungal metabolite should interfere with targets that are only present inanimals).2.8.2. QUINOLIZIDINE ALKALOIDS

Quinolizidine alkaloids (QA). such as lupanine, sparteine. or cytisine, are producedby many members of the Leguminosae. QA are bitter for many animals (and plantsproducing them are therefore avoided as food). If ingested. QA exhibit a broad level oftoxicity: They interact with ACh receptors as agonists. QA. like many other alkaloids.occur as complex mixtures in plants. We have shown recently (Schmeller et al .. 1994) thatsome QA preferentially bind to the nicotinic AChR. whereas others reveal a strongerbinding to the muscarinic AChR (Table IX). Some QA exhibit a prominent cross-reac-tivity. Additionally. QA such as lupanine and sparteine inhibit Na + and K+ channels. thusblocking the signal transduction in nerve cells at a second critical point. As mentionedabove. QA slightly interfere with protein biosynthesis. A few QA. such as anagyrine,cytisine, and the bipiperidine alkaloid ammodendrine (which co-occurs with QA in many


18/26

318 Michael WinkTable IX

Binding of Quinolizidine Alkaloids to Nicotinic and Muscarinic AChRaAlkaloid n-AChR m-AChR

13-Hydroxylupanine 467.:! 139.7OH


19/26

Modes of Action of Alkaloids 319Table IX(Continued)

Lupa nine 5.3 118.0

Lupinine >500 189.9

N-Methlcy tisine 0.051 416.7

01j5p Multiflorine >500 49.4Spa rte ine 330 .8 21.3

Tetrahydrorhom ib ifo line 347.6 128.8

"Ie", (in 11M) values indicate the concentration of a panicular alkaloid that displaces 50% of the specifically bound radiolabeledligand. After Schmelter et til. (1994).

If we accept the hypothesis that alkaloids were developed as chemical defensecompounds through a process of "evolutionary molecular modeling," the "cross-reac-tivity" described makes sense: Any compound that can interfere with more than one targetor with more than one group of adverse organisms is likely to be more toxic and thus has abetter survival value in general than a more selective allelochemical. In addition. her-bivores will try to develop tolerance or resistance against the dietary toxins. For example,in the monarch butterfly (D an au s pJex ip pu s) which sequesters cardiac glycosides, the


20/26

320 Michael Wink

we can say that nature has obviously tried "to catch as many flies with one clap aspossible" in the selection of alkaloids during evolution.

3. TARGETS AT THE ORGAN LEVEL

Whereas the activities mentioned previously were more or less directed to moleculartargets present in or on cells. we can also see some activities that are oriented againstorgan systems or complete organisms. although ultimately. they have molecular targets.too. In some cases. only the toxicity of an alkaloid has been reported (Table X) evidencingsubstantial interactions. but the exact mode of action has not yet been elucidated or israther complex. involving several targets and organs.

3.1. Central Nervous System and Neuromuscular JunctionA remarkable number of alkaloids interfere with the metabolism and activity of

neurotransmitters in the brain and nerve cells. a fact known to man for some thousands ofyears (see Chapter 4). The cellular interactions have been discussed in Section 2 above. A .disturbance of the metabolism and binding of neurotransmitters and related signal path-ways impairs learning and memory and sensory faculties (smell. vision. or hearing) orproduces euphoric or hallucinogenic effects.

An animal that is no longer able to control its movements and senses properly hasonly a small chance of survival in nature. because it will have accidents (falling from treesor rocks or into water) or be killed by predators. Thus. euphoric and hallucinogeniccompounds. which are present in a number of plants but also in fungi and the skin of toads.can be regarded as potent defense compounds. Homo sapiens has used and still uses thesedrugs for their hallucinogenic properties. but here. also. it is evident that long-term usereduces survival and fitness dramatically (see Chapter 4). Muscle activity (e.g . skeletal.heart) is controlled by ACh and NA. It is plausible that any inhibition or overstimulationof neurotransmitter-regulated ion channels wiII severely influence muscule activity andthus the mobility or organ function (heart. lungs. gut) of an animal. In the case ofinhibition. muscles will relax. and in the case of overstirnulation, muscles will be tense orin tetanus. leading to a general paralysis (which is the effect of many of the more toxicalkaloids; Table X and Chapter 4).

Alkaloids that activate (so-called parasympathomimetics) or inhibit (parasyrn-patholytics) neuromuscular action are shown in Table I. These compounds are usuallyconsidered to be strong poisons (Table X) and it is obvious that they serve as chemicaldefense compounds against herbivores. for a paralyzed or anesthetized animal is an easyprey for predators. If higher doses of these alkaloids are ingested. the animal will die as adirect result of the alkaloid (see LD50 values in Table X). Skeletal muscles. muscle-


21/26

Modes of Action of Alkaloids

Table XLDso Values of Some Alkaloids=

321

Alkaloid Test system LD~Oh mg/kgAlkaloids derived from tryptophanBrucineCinchonidineCinchonineEllipticineErgocryptineErgometrineErgotamineHarmanHarminePhysostigminePsilocybinQuinidineQuinineReserpineStrychnine

RatRatRatMouseRabbitMouseMouseMouseMouseMouseMouseRa tAgelaiusAgelaiusAgelaiusRa tMouseMouseMouse

VinblastineVincarnineVincristine

Alkaloids derived from phenylalanine/tyrosineAristolochic acidBerberineBulbocapnineCanadineChelerythrineChelidonineCodeineColchicine

MouseMouseMouseMouseMouseMouseMouseMouseManAgelaiusMouseMouseMouseMouseMouseMouseMouseMouse

EmetineGalanthamineMorphinePapaverineProtopineSanguinarineThebaineTubocurarine

Steroid alkaloidBatrachotoxinJervineProtoveratrineSamandarineSolanineVeratridine

Tropane alkaloidsAtropineCocaine

MouseMouseRabbitMouseMouseMouseRatRat

p.o. 1i.p.206i.p. 152i.v. 1.2i.v. 1.1i.v.O.15i.v.62Lp. 50i.v.38p.o. 4.5i.v, 285i.v. 30; p.o. 263p.o. 100p.o. 100p.o. 6i.v.0.9i.v.9.5i.v.75Lp.5.2

i.v. 38-70; p.o. 56-106i.p.23p.o. 413p.o. 940s.c. 95i.v.35s.c. 300i.v.4.1p.o. 0.1-0.3p.o. 32s.c. 32i.v. 8; p.o. 18.7i.v.226-318i. v. 27.5; s.c. 150i.p.36-102s.c. 102; i.v. 16i.p.20p.o. 33.2s.c. 0.002i .v. 9.3i.p.


22/26

322

Table X(Continued)

Michael Wink

Alkaloid Test system

Pyrrolizidine alkaloidsEchimidineHeliotrineacobineMonocrotalineSenecionineSeneciphylline

Quinolizidine alkaloidsCytisine13-HydroxylupanineLupanineN-MethylcytisineSparteine

Miscellaneous alkaloidsAconitinea-AmanitinArecolineCaffeineConiineCycloheximideDelphinineMaytansineMuscimolNicotine

RatRatRatRatRatRatMouseMouseMouseMouseMouseMouseMouseMouseMouseAgelaiusMouseRabbitRatRatAgelaiusMouseMouseetrodotoxin

i.p.200i.p.300i.p. 138i.p. 175; p.o. 71i.p.85i.p. 77i.v. 1.7i.p.l72i.p.80i.v. 21; i.p. 51i.p. 55-67; p.o. 350-510i.v.0.17;p.o. Ii.p.O.1s.c. 100p.o. 127-137p.o. 56i.v.150i.p. 1.5-3.0s.c. 0.48p.o. 45p.o. 17.8i.v. 0.3; p.o. 230i.p. om ; s.c. 0.008

"More details in Wink (I993a).hi.p., intraperitoneal; i.v., intravenous; p.o., oral; s.c., subcutaneous.

3.2. Inhibition of the Digestive Process

Food uptake can be reduced by pungent or bitter taste in the first instance. as wasmentioned in Chapter 11. The next step can be the induction of vomiting. which is acommon reaction to the ingestion of a number of alkaloids.

Causing diarrhea. or the opposite. constipation. would be another activity that nega-tively influences the digestive system. Many intoxications with alkaloid containing plantshave diarrhea as one of the symptoms (see Chapter 4).

Another way to interfere would be the inhibition of digestive enzymes or of transportproteins for amino acids. sugars. or lipids. A recently discovered group of alkaloids are thepolyhydroxy alkaloids. such as swainsonine or castanospermine, which inhibit hydrolyticenzymes. such as glucosidase. galactosidase. trehalase (trehalose is a sugar found in manyinsects and fungi which is hydrolyzed by trehalase). and mannosidase selectively (Table


23/26

odes of Action of Alkaloids 323.3. Modulation of Liver and Kidney Function

Nutrients and xenobiotics (such as secondary metabolites) are transported to the liverfter resorption in the intestine. In the liver the metabolism of carbohydrates, amino acids.nd lipids, and the subsequent synthesis of proteins and glycogen takes place. The liver islso the main site for the detoxification of xenobiotics. Lipophilic compounds, which areasily resorbed from the diet, are often hydroxylated and then conjugated with a polar,ydrophilic molecule, such as glucuronic acid, sulfate, or an amino acid. These conju-ates. which are more water soluble, are exported via the blood to the kidney, where theyre transported into the urine for elimination. Other compounds are eliminated via the bileucts into the intestine.

Both organ systems are affected by a variety of secondary metabolites: The pyr-olizidine alkaloids have been discussed earlier. They are activated during the detoxifica-ion process and are converted into potent carcinogens, causing liver cancer. Many otheretabolic inhibitors. discussed previously, are also liver toxins.

Many alkaloids and other allelochemicals are known for their diuretic activity. For annimal, increased diuresis would also mean an increased elimination of water and essen-ial ions. As Na " ions are already limited in plant food (an antiherbivore device"), long-erm exposure to diuresis-inducing compounds would reduce the fitness of a herbivore

.4. Disturbance of Reproduction

Quite a number of allelochemicals are known to influence the reproductive system ofnimals, which will ultimately reduce their numbers (and fitness as a species). Antihormo-al effects could be achieved by mimicking the structure of sexual hormones. Theseffects are not known for alkaloids yet, but for other natural products: Estrogenic proper-ies have been reported for coumarins which dimerize to dicoumarols, or isoflavones. Thensect molting hormones, 0:- and l3-ecdysone, are mimicked by many plant sterols (ec-ysone itself is one of these) from the fern Polypodium vulgare and several Ajuga speciesr azadirachtin from the neem tree. Juvenile hormone is mimicked by a number oferpenes present in some Coniferae. Spermatogenesis is reduced by gossypol from cotton-eed oil.

The next target is the gestation process itself. As outlined above, a number oflkaloids are mutagenic (see Section 2.4) and lead to malformation of the offspring orirectly to the death of the embryo.

The last step would be a premature abortion of the embryo. This dramatic activity haseen reported for a number of allelochcmicals, including many mono- and sesquiterpenesnd alkaloids. Some alkaloids achieve this by the induction of uterine contraction. as dohe ergot and lupin alkaloids. .


24/26

324 Michael Wink3.5. Blood and Circulatory System

All animals need to transport nutrients, hormones, ions, signal compounds, O2 andCO2 between the different organs of the body. This is achieved in higher animals throughblood in the circulatory system. Inhibitors of its motor. the heart, were discussed earlier.But the synthesis of red blood cells is also vulnerable and can be inhibited by antimitoticalkaloids. such as vinblastine or colchicine (see Section 2.3). Some allelochemicals havehemolytic properties, such as saponins and steroidal alkaloids. If resorbed, these com-pounds complex membrane sterols and make the cells leaky (see Section 2.1).

3.6. Allergenic EffectsA number of secondary metabolites influence the immune system of animals, such as

coumarins. furanocoumarins, hypercin, helenalin, and others. Common to these com-pounds is a strong allergenic effect on those parts of the skin or mucosa that have comeinto contact with the compounds. Activation or repression of the immune response arecertainly targets that were selected during evolution as an antiherbivore strategy. A func-tion of alkaloids in this context is hardly known.

4. MECHANISMS OF ALLELOCHEMICAL ACTIVITIES IN ANTIVIRAL,ANTIMICROBIAL, AND ALLELOPATHIC INTERACTIONS

We have circumstantial evidence that some alkaloids protect the producing plantagainst viruses. bacteria (see Chapter 17), fungi, and other plants. Relative to alkaloid-animal interactions. these modes of action have been studied less well or hardly at all.

A number of antimicrobial alkaloids such as sanguinarine, quinine. or berberine(Table VI) intercalate with viral and microbial DNA or bind to it. These compounds maythus inhibit processes such as DNA replication and RNA transcription which are vital forthe microorganisms. Protein biosynthesis in ribosomes is another vulnerable target, at-tacked by emetine and several antibiotics (Table VII). The stability of biornernbranes canbe disturbed by steroidal alkaloids and tetrandine (as described in Section 2.1). Othertargets may be electron chains or just metabolically important enzymes. Antibiotics ofmicrobial origin (many of which could be classified as alkaloids from the chemical pointof view) have similar targets. although some of them interfere with specific bacterialtargets such as the biosynthesis and assembly of the bacterial cell wall.

Herbicidal properties or germination inhibition which can be observed in plant-plantinteractions. can also proceed via the above-mentioned mechanisms (Wink and Latz-Bri.ining. 1995: Waller. 1987: Chapter 14). But interactions with growth hormones andtheir metabolism must also be' considered.

5. CONCLUSIONS


25/26


this compilation only a limited number of structures have been discussed. In many in-stances. plants contain mixtures of related alkaloids, which only differ for particularsubstitution patterns. Very often these derivatives have properties similar to the better-known alkaloids listed in Tables I-X; therefore. with some caution their activity can beguessed. These allelochemical properties are requisite for a chemical defense compoundin an ecological context but also constitute the base for their exploitation in medicine oragriculture (Wink. 1993a.b).ACKNOWLEDGMENT

T. SchmelIer kindly helped prepare some of the figures.

REFERENCES

Major Reviews

Alberts. B .. Bray. D .. Lewis. 1.. Raff. M . . Roberts. K .. and Watson. 1. D .. 1993. Molecular Biology of the Cell.3rd ed .. Garland. New York.

Habermehl. G. . 1983. Gifttiere und ihre IVtlffen. Springer. Berlin.Harborne. J. B. 1993. Introduction to Ecological Biochemistrv. -trh ed .. Academic Press. San Diego.Luckner, M. . 1990. Secondary Metabolism in Microorganisms. Plants and Animals. Springer. Berlin.Mann, 1.. 1992. Murder: Magic and Medicine. Oxford University Press. London.Mothes, K .. Schutte. H. R .. and Luckner. M .. 1985. Biochemistry of Alkaloids. VCH. Weinheim.Mutschler, E.. 1981. Armeimittelwirkungen. WVG. Stuttgart.Rimpler, H.. 1990. Pharmat.eutische Biologie II: Biogene Arzeneistoffe. Thieme. Stuttgart.Robinson. T . A.. 1981. The Biochemistrv of Alkaloids, Springer. Berlin.Rosenthal. G. A .. and Berenbaum, M. R .. 1991. Herbivores: Their Interactions with Secondary Plant Metabo-

lites. Vol. I. Academic Press. San Diego.Rosenthal. G. A .. and Berenbaurn, M. R .. 1992. Herbivores: Their Interactions with Secondary Plant Metabo-

lites. Vol. 2. Academic Press. San Diego.Roth. L.. Daunderer. M .. and Kormann. K .. 1994. Giftpflanten lind Pflunzengifte. -nh ed .. Ecomed, Landsberg.Teuscher. E .. and Lindequist. U .. 1994. Biogene Gifte. Fischer. Stuttgart.Wagner. H.. 1993. Pharmat.eutische Biologie. 2. Drogen Wid ihre Inhaltsstoffe, Fischer. Stuttgart.Waller. G. . 1987. Allelochemicals: Roles in Agriculture lind Forestrv, ACS Syrnp. Ser, 330.Wink. M .. I 992a. Die chernische Verteidigung der Pflanzen und die Anpassungen der Pflanzenfresser. in:

"Lupinen 1991-ForIcllllllg. Anbau IlIId Vt'nn'rtrlll~ (\1. Wink. ed.), Uruversity of Heidelberg Press.Heidelberg. pp. 130-156.

Wink. M. . 1992b. The role of quinolizidine alkaloids in plant insect interactions. in: Insect-Plant interactions.Vol. IV (E. A. Bcrnays. ed.), CRC Press. Boca Raton. pp. 133-169.

Wink. M.. 19


26/26

3 2 6 Michae l Wink

Kebabian, J. Wooand Neumeyer. J. L.. 1994. The RBI Handbook of Receptor Classification. RBI. Natick.Lodish, H. Baltimore. D; Berk, A . Zipursky, S. L; Matsudaira, P. and Darnell. J.. 1995. Molculear Cell

Biologv. 3rd Ed .. Scientific American Books. Inc .. New York.Reynolds. J. E. F . (ed.), 1993. Martindale-The Extra Pharmacopoeia. The Pharmaceutical Press. London.Schrneller, TooSauerwein. M . Sporer. F.. Muller. W. Eooand Wink. M. 1994. Binding of quinolizidine alkaloids

to nicotinic and muscarinic receptors. J. Nat. Prod. 57:1316-1319.Schmelter, T . Sporer. FooSauerwein. M .. and Wink. M.. 1995. Binding of tropane alkaloids to nicotinic and

muscarinic receptors. Pharmazie 50:493-495.Wink. Mooand Latz-Bruning, Boo 1995. Allelopathic properties of alkaloids and other natural products: Possible

modes of action. in: Allelopathy: Organisms. Processesand Applications (Inderjit, Dakshini, K. M. M.. andEinhellig, F . A.. eds.). ACS Symp. Ser. 582. pp, 117-126.

Wink. Mooand Twardowski.T; 1992. Allelochemical properties of alkaloids. Effects on plants. bacteria andprotein biosynthesis. in: Allelopathv: Basic and Applied Aspects (S. J. H. Rizvi and V. Rizvi, eds.),Chapman & Hall. London. pp. 129-150.

Documents

Book Alkaloids