Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

of drinks and consumption data difficult, so assess-

ment and comparison of data from many countries

would be greatly facilitated by the uniform expres-

sion of alcohol intakes and by the adoption of

a uniform international definition of a standard

drink.

Alcohol Control Policies

0032 In order to reduce the prevalence of alcohol-related

problems, many countries apply control measures

that affect the availability and/or the price of alco-

holic drinks. In addition, governments may use

public health campaigns and alcohol advertising as

instruments to diminish the prevalence of alcohol-

related problems. Alcohol control policies may be

directed at reducing the average alcohol consumption

among the total population (public health approach)

or at reducing the number of heavy users or drinking

in high-risk situations (high-risk approach). Alcohol

control measures are more effective as public support

is higher, and so education of the public and possibly

self-regulation by the alcohol industry may be

important contributory measures.

Price

0033 Taxation of alcoholic beverages is essentially for

the purpose of raising revenue. It is not generally

regarded as a primary tool for controlling consump-

tion, although it is used as such in some countries. In

most countries, alcoholic drinks are perceived to be

luxury items and, as such, candidates for taxation.

Alcoholic drinks are price-elastic, which means that

the demand is sensitive to price changes. This is the

main reason for the argument that taxation should be

used to reduce consumption. However, it has been

found that elasticity values relating to different

regions, different periods, and different types of

alcohol vary considerably.

0034More important is the effect of price increases

on different types of consumers. Some claim that

heavy drinkers are at least as responsive to price

increases than light-to-moderate consumers. Others

claim that heavy drinkers reduce their consumption

much more than other drinkers after excise duties are

increased.

Advertising and Alcohol Availability

0035A commonly held assumption is that alcohol adver-

tising increases alcohol consumption. The main

objective of advertising is to persuade customers to

continue to enjoy, or to switch their preference to, the

brand being advertised. Several studies have dem-

onstrated that advertising does not increase con-

sumption. Also, restrictions on advertising do not

automatically result in a decline in total alcohol

consumption.

0036It has been suggested that there is an association

between hours of sale and outlet density, and the

consumption of alcohol, but some studies show no

tbl0004 Table 4 Alcohol content of a ‘standard drink’ as defined in various countries, and some daily consumption guidelines recommended

by governments or learned bodies

Country Size of standard drink

(grams of absolute alcohol)

Daily consumption guidelines

(grams of absolute alcohol)

Men Women

Australia 10 40 20

Austria 10 30 20

Canada 13.5 13.5 13.5

Denmark 12 36 27

Finland 11 41 27

France 12 60 36

Hungary 17

Iceland 9.5

Ireland 8

Italy 10

Japan 19.75 39.5

Netherlands 9.9 2–3 units a few times a week 2–3 units a few times a week

New Zealand 10 30 20

Portugal 14

Spain 10 30 20

UK 8 24–32 16–24

USA 14 28 14

Data adapted from ICAP (1998b) Safe Alcohol Consumption: A Comparison of Nutrition and Your Health: Dietary Guidelines for Americans and Sensible Drinking.

ICAP Report No. 1. Washington, DC: International Center for Alcohol Policies; ICAP (1998c) What is a ‘Standard Drink’? ICAP Report No. 5. Washington, DC:

International Center for Alcohol Policies.

124 ALCOHOL/Alcohol Consumption

such association. Prohibition of alcohol has been

shown to be an ineffective and even counterproduc-

tive measure. Although the alcoholic cirrhosis mor-

tality and other alcohol problems may decrease, it

might not produce positive results in public health

terms as, for example, organized crime increased in

the United States after complete alcohol ban.

Drinking Age Limits

0037 Drinking age laws cover a broad spectrum of behav-

iors concerned with where, when, and under what

circumstances alcoholic beverages can be purchased

and consumed. Some countries focus their legislation

on both a minimum legal drinking age and a

minimum age at which beverage alcohol can be

purchased, and these ages may be different. Other

countries do not address a minimum age for con-

sumption. Table 5 shows some minimum drinking

and purchasing age laws. Both consumption and

purchasing laws in most countries are set at 18. The

national laws generally apply to drinking age limits

for venues outside the home, such as taverns, bars,

restaurants, nightclubs, and similar establishments.

The UK is the only country that legislates a minimum

consumption age in the home (from age 5 with

parental consent).

Effectiveness of Alcohol Control Policies

0038 Price policy may be an effective means of reducing

alcohol consumption and alcohol-related problems as

long as increases in price keep up with or outstrip

rises in income. However, the effectiveness may

depend very much on the population group affected

and on socio-economic and demographic factors.

Also, stable political and economic conditions as

well as appropriate law enforcement and administra-

tive systems are prerequisites for success.

See also: Alcohol: Properties and Determination;

Metabolism, Beneficial Effects, and Toxicology; Body

Composition; Dietary Surveys: Measurement of Food

Intake; Surveys of National Food Intake; Surveys of Food

Intakes in Groups and Individuals

Further Reading

Boffetta P and Garfinkel L (1990) Alcohol drinking and

mortality among men enrolled in an American Cancer

Society prospective study. Epidemiology 1: 342–348.

Grant M and Litva J (eds) (1998) Drinking Patterns &

Their Consequences. London: Taylor & Francis.

ICAP (1998a) Drinking Age Limits. ICAP Report No. 4.

Washington, DC: International Center for Alcohol

Policies.

ICAP (1998b) Safe Alcohol Consumption: A Comparison

of Nutrition and Your Health: Dietary Guidelines for

Americans and Sensible Drinking. ICAP Report No. 1.

Washington, DC: International Center for Alcohol

Policies.

ICAP (1998c) What is a ‘Standard Drink’? ICAP Report

No. 5. Washington, DC: International Center for

Alcohol Policies.

MacDonald I (ed.) (1999) Health Issues Related to Alcohol

Consumption. Brussels: ILSI Europe.

tbl0005 Table 5 Minimum drinking and purchasing age laws in various countries

Country Minimum drinking age (MDA) Minimum purchasing age (MPA)

Australia 18 18

Austria 16 16

18 (spirits)

Canada In Alberta and Quebec: 19 All provinces have their own legal MPA

In all other provinces: 18

Denmark 18

Finland 18 18 (for all beverages up to 21% alcohol by volume)

France 16 16

Hungary 18

Iceland 20

Ireland 18 18

Italy 16 16

Japan 20 20

Netherlands 16 (beer and wine)

18 (spirits, 16 if accompanied by an adult)

New Zealand 20 20

Portugal No MDA No MPA

Spain 16 16

UK 18 18

USA 21 21

Data adapted from ICAP (1998) Drinking Age Limits. ICAP Report No. 4. Washington, DC: International Center for Alcohol Policies.

ALCOHOL/Alcohol Consumption 125

Pikaar NA, Wedel M and Hermus RJJ (1988) Influence of

several factors on blood alcohol concentrations after

drinking alcohol. Alcohol & Alcoholism 23: 289–297.

Van Tol A and Hendriks HFJ (2001) Moderate alcohol

consumption: Effects on lipids and cardiovascular dis-

ease risk. Current Opinion in Lipidology 12: 19–23.

World Drink Trends 2000 International Beverage Con-

sumption and Production Trends. Henley-on-Thames,

UK: NTC Publications, Commodity Board for the Dis-

tilled Spirits Industry.

Ales See Beers: History and Types; Wort Production; Raw Materials; Chemistry of Brewing; Biochemistry of

Fermentation; Microbreweries

Algae See Single-cell Protein: Algae; Yeasts and Bacteria; Marine Foods: Production and Uses of Marine

Algae; Edible Animals Found in the Sea; Marine Mammals as Meat Sources

ALKALOIDS

Contents

Properties and Determination

Toxicology

Properties and Determination

M Wink, University of Heidelberg, Heidelberg,

Germany

Background

0001 Plants produce a wide variety of secondary metabol-

ites, many of which function as allelochemicals or

signal compounds to attract pollinating insects and

seed- or fruit-dispersing animals. Their main function

is chemical defense against herbivores, microrganisms,

or competing plants. Over 13 000 nitrogen-containing

secondary metabolites have been described so far in

plants. Alkaloids contribute over 12 000 compounds,

followed by amines, nonprotein amino acids, cyano-

genic glycosides, and glucosinolates.

Definition

0002 The definition of alkaloids has changed throughout

the years. Formerly, this class of secondary compounds

was restricted to plant bases with a heterocyclic

nitrogen atom. Exocyclic nitrogen bases were termed

‘pseudoalkaloids.’ Other definitions demanded that

the skeleton of alkaloids should derive from amino

acids or that these bases have explicit pharmaco-

logical activities. At present, alkaloids are defined in

a more pragmatic way; they include all nitrogen-

containing natural products that are not otherwise

classified as peptides, nonprotein amino acids,

amines, cyanogenic glycosides, glucosinolates, cofac-

tors, phytohormones, or primary metabolites (such as

purine and pyrimidine bases). Even a number of anti-

biotics produced by bacteria or fungi are therefore

included in the group of alkaloids.

Occurrence

0003Alkaloids have been detected in about 15% of plants,

bacteria, fungi, and even in animals. Within the plant

kingdom, they occur in primitive groups such as

Lycopodium or Equisetum, in gymnosperms and

angiosperms. In higher plants (angiosperms) some

families contain more alkaloid-containing taxa than

126 ALKALOIDS/Properties and Determination

others. Such alkaloid-rich’ taxa include Papaveraceae,

Berberidaceae, Fabaceae, Boraginaceae, Apocyna-

ceae, Asclepiadaceae, Asteraceae, Liliaceae, Gne-

taceae, Ranunculaceae, Rubiaceae, Solanaceae, and

Rutaceae. Also, several food plants and food items

may contain alkaloids (Table 1).

0004 It has been speculated that alkaloids evolved early

in evolution and were already present at the time

(c. 200 million years ago) when the angiosperms

began to radiate. In general, specific alkaloid types

are restricted to particular systematic units and are

therefore of some importance for the systematics,

taxonomy, and phylogeny of plants. For example,

benzylisoquinoline alkaloids are typical for Papaver-

aceae, Berberidaceae, and Ranunculaceae, which

seem to be phylogenetically related. Other alkaloids

occur in phylogenetically unrelated plant families.

Ergot alkaloids occur in fungi (Claviceps) but also in

members of the Convolvulaceae; quinolizidine alka-

loids are typical for some Fabaceae, but have also

been detected in Berberidaceae (Caulophyllum, Leon-

tice). It has been speculated that the genes encoding

enzymes leading to the main quinolizidine alkaloid

skeleton are distributed much more widely in the

plant kingdom, but are normally switched off. Alter-

natively, convergent evolution and horizontal gene

transfer have been suggested.

Isolation and Detection

0005Alkaloids form free bases in alkaline solutions. The

free base is usually lipophilic and mostly insoluble

in water but soluble in organic solvents (ethanol,

methanol, diethylether, methylene chloride). At

higher hydrogen ion concentrations (i.e., pH < 7),

alkaloids occur in a protonated state and are usually

tbl0001 Table 1 Alkaloids present in food plants, beverages, stimulants, or hallucinogens

Substance Occurrence Activity

Alkaloids in food

Solanine and other steroid alkaloids Solanum (potato, tomato) Membrane disruption, Neuroreceptor

interaction, mutagenicity

Pyrrolizidine alkaloids Symphytum (comfrey); honey

(if bees have visited PA plants)

DNA and protein alkylation; mutagenicity,

cancer

Cycasin Cycas, other cycads Mutagenicity

Ergotalkaloids Claviceps purpurea Neuroreceptor interaction;

vasoconstriction; uterus contraction

neurotoxin (Na

channel)

Saxitoxin Algae; ends up in food chain (molluscs;

fish)

Lupanine and other quinolizidine alkaloids Lupinus; other genistoids Interaction with AChR

,Na

channels

Pelletierine Punica granatum Interaction with AChR

Alkaloids in beverages

Caffeine, theophylline, theobromine Coffea,Thea, Paullinia, Ilex paraguarensis Stimulant

Quinine Cinchona succirubra Bitter tonic; neuroreceptor and ion

channel interaction; central stimulant

Ephedrine and related compounds Ephedra; Catha edulis

Alkaloids as stimulants and hallucinogens

Morphine Papaver somniferum, opium Hallucinogen

N-Methyltryptamine Fungi, plants Central stimulant

N, N-dimethyltryptamine, 5-methoxy-N,

N-dimethyltryptamine

Mimosaceae: Anadenanthera

(syn. Piptadenia), Mimosa hostilis;

Myristicaceae: Virola; Malpighiaceae:

Banisteriopsis, Poaceae: Phalaris

Central stimulant, hallucinogen

Serotonin Fungi (Amanita); stinging hairs of Urtica,

Laportea, Jatropha urens, Mucuna

pruriens; seeds and fruits

Bufotenine Fungi (Amanita); animal poisons (Cnidaria,

spider, scorpions, wasps, toads)

Psilocybin, psilocin Fungi (Psilocybe, Stropharia, Conocybe,

Panaeolus, Gymnophilus, Psathyrella,

etc.)

Mescalin Lophophora williamsii; other cacti Hallucinogen

Harmalin and other b-carboline alkaloids Peganum harmala, Banisteriopsis caapi Hallucinogen

Cocaine Hallucinogen

Arecoline Nuts of Areca palm Stimulant

AChR, acetylcholine receptor.

ALKALOIDS/Properties and Determination 127

soluble in water but insoluble in apolar organic solv-

ents. The different solubilities are useful to isolate and

purify alkaloids. In the laboratory, alkaloids are often

solubilized from plant material by dissolving them in

acidic solutions (e.g., 0.5 M HCl). Treatment of this

solution with organic solvents will remove nonalka-

loidal substances. The solutions are brought to pH

> 12 in the next step and extracted with CH

ethylacetate, or ethylether to yield free alkaloids.

0006 Plant extracts are often analyzed by thin-layer

chromatography (TLC) as a first screening to find

out whether alkaloids are present or not. A number

of reagents give typical color reactions, such as Dra-

gendorff’s, Mayer’s reagent, etc. Because alkaloids are

typically present in complex mixtures consisting of

two to five main and up to 20–50 minor alkaloids,

TLC does not have sufficient separation capacities.

Better methods are high-performance liquid chroma-

tography (HPLC) and capillary gas–liquid chro-

matography (GLC). The latter method is extremely

useful because it has a strong separation power and is

very sensitive and selective if a nitrogen-specific de-

tector is used. Furthermore, GLC can be directly

coupled with a mass spectrometer, allowing mass

spectra to be obtained, even from very minor com-

ponents. Since many alkaloids have been analyzed by

mass spectrometry, already a large collection of mass

spectra is available, making it possible to identify

many of the known alkaloids unambiguously. Usu-

ally, mass spectra are recorded in the electron impact

(EI) mode, which promotes fragmentation. If infor-

mation on the molecular ions is needed (which can be

elusive in EI-MS), other MS techniques, such as

chemical ionization (CI), field desorption (FD), or

fast-atom bombardment (FAB) are the methods of

choice.

0007 HPLC tends to be less sensitive and of lower separ-

ation capacity than capillary GLC. However, modern

photodiode array detectors are very helpful to iden-

tify known metabolites by UV-VIS spectroscopy.

Today, also, HPLC and capillary electrophoresis can

be coupled to a mass spectrometer, thus widening the

use of MS for the analysis of natural products.

0008 In addition, HPLC has a major advantage in that it

is possible to isolate a compound in milligram quan-

tities that allow structural elucidation by nuclear

magnetic resonance (

C). Nuclear magnetic res-

onance is the method of choice if unknown structures

are to be elucidated, whereas mass spectrometry is

extremely useful for identifying previously described

substances or substances that are slightly different to

known compounds.

0009 If small quantities (femtogram or nanogram

amounts) of known alkaloids need to be detected

routinely, immunological procedures such as

radioimmunoassays (RIA) or enzyme immunoassays

(EIA, ELISA) should be appropriate. In order to

obtain specific antibodies, the alkaloid in question

has to be coupled chemically to a large protein, such

as albumin, usually by some sort of spacers (e.g.,

succinic acid).

Biosynthesis

0010The skeleton of most alkaloids is derived from amino

acids (Figure 1, Table 2), although moieties from

other pathways, such as terpenoids, are often

combined. In addition, in a number of alkaloids

(e.g., steroid alkaloids), the nitrogen (deriving from

glutamine or other NH

sources) is added near the

end of a biosynthetic pathway, i.e., the alkaloid skel-

eton does not stem from amino acids. Biosynthetic

pathways have been worked out in detail for a few

alkaloids.

Accumulation and Storage

0011Although the exact site of alkaloid formation in a

plant cell has been elucidated for a few species only,

it is certainly correct to assume that most compounds

are synthesized in the cytoplasm. Membrane-enclosed

vesicles have been implied in the biosynthesis of ber-

berine. Not only is the chloroplast the site of photo-

synthesis and related processes, but it also harbors a

number of biosynthetic pathways, such as those for

fatty acids and amino acids. In addition, quinolizidine

alkaloids (QA) are synthesized in the chloroplast

stroma; thus, both the alkaloid and its amino acid

precursor, l-lysine, share the same compartment.

QA formation is regulated by light and displays a

diurnal rhythm. Light regulation seems to be trig-

gered by (1) lysine availability (it is also made during

the day), (2) the change in stromal hydrogen ion

concentration to pH 8 (enzymes of QA formation

have a pH optimum at pH 8), and (3) the reduction

of QA enzymes by thioredoxin (reduced thioredoxin

is generated under illumination).

0012Alkaloids are not formed in the extracellular space

or in the vacuole. The storage of high concentrations

of alkaloids is a prerequisite for their allelochemical

roles as defense compounds. Since these concentra-

tions would interfere with the normal metabolisms,

the allelochemicals are safely stored in the vacuole

(Table 3) of often specialized cells or tissues (such

as epidermis). The vacuole of these alkaloid-

accumulating cells has been termed the ‘toxic’ or

‘defense’ compartment. A number of plants produce

latex, which, in addition to its gluing properties

(think of insect mandibles!), often contains defense

chemicals, such as alkaloids (morphine and related

128 ALKALOIDS/Properties and Determination

Photosynthesis

Glucose

GAP

Pyruvate

Oxalacetate

Malate

Succinate

Oxoglutarate

Citrate

AcCoA

IPP

DMAPP

Fatty acids

Waxes

Polyketides

Aspartate

Lysine

Krebs Cycle

Glutamate

Glutamine

Ornithine

Arginine

Malonyl-CoA

Erythrose-4-P Phosphoenolpyruvate

Gallic acid

Shikimate

Chorismate

L-Tryptophan

Anthranilate

Prephenate

Arogenate

L-Tyrosine L-Phenylalanine

Tetrahydroisoquinoline alkaloids

Benzylisoquinoline alkaloids

Benzophenanthridine alkaloids

Morphinane alkaloids

Aporphine alkaloids

Protoberberine alkaloids

Indol alkaloids

Monoterpene indol alkaloids

β-carboline alkaloids

Ergot alkaloids

Quinoline alkaloids

Acridone alkaloids

Piperidine alkaloids

Quinolizidine alkaloids

Sedum alkaloids

Pyrimidines

Purines

Pyrrolizidine alkaloids

Tropane alkaloids

Coca alkaloids

Nicotiana alkaloids

Steroidal alkaloids

Diterpen alkaloids

Conium alkaloids

fig0001 Figure 1 Schematic overview of biosynthetic pathways leading to different groups of alkaloids.

ALKALOIDS/Properties and Determination 129

benzylisoquinoline alkaloids in Papaveraceae; proto-

berberine and benzophenanthridine alkaloids in Che-

lidonium; lobeline and other piperidine alkaloids in

Lobelia) and terpenoids (e.g., phorbole esters). The

alkaloids are selectively sequestered in small (diam-

eter < 1 mm) latex vesicles and reach local concentra-

tions of up to 1 M.

0013 Storage in the vacuole or in vesicles demands that

alkaloids pass the tonoplast and accumulate within

the vacuole against a concentration gradient. For the

passage across the tonoplast, three mechanisms are

plausible:

0014simple diffusion (takes place in case of lipophilic

alkaloids, e.g., nicotine, ajmalicine, vinblastine,

colchicine);

0015carrier-mediated transport (in case of polar and

charged alkaloids, which is the rule for most alka-

loids under physiological conditions, e.g., hyoscya-

mine, lupanine, reticuline, scoulerine, senecionine);

0016membrane fusion (in case of alkaloids that are

formed in vesicle-enclosed compartments, e.g.,

berberine).

Because alkaloids are sequestered against a concen-

tration gradient in the vacuole or in latex vesicles, the

driving force of uphill accumulation needs to be de-

termined. In some instances, vesicles or vacuoles con-

tain alkaloid binding or complexing compounds. For

example, latex vesicles of Chelidonium majus contain

between 500 and 1200 mM chelidonic acid, which

binds or complexes berberine and benzophenanthri-

dine alkaloids. It has been shown experimentally that

chelidonic acid acts as a trapping agent that causes

the apparent uphill transport resulting in alkaloid

concentrations in vesicles of 500–1200 mM. Protona-

tion, organic acids, and specific peptides may

constitute other trapping mechanisms. There is some

evidence that amino acids and some ions are trans-

ported into the vacuole with aid transporters that

are fueled by proton–substrate antiport mechanisms.

tbl0003 Table 3 Storage of alkaloids in the vacuole

Alkaloid Species

Leaf vacuoles

Lupanine Lupinus

Sparteine Cytisus, Lupinus

Hyoscyamine Atropa

Nicotine Nicotiana

S-Coulerine Fumaria

S-Reticuline Fumaria

Ajmalicine Catharanthus

Serpentine Catharanthus

Catharanthine Catharanthus

Betalaines Beta, Chenopodium

Senecionine-N-oxide Senecio

Capsaicin Capsicum

Latex vesicles

Sanguinarine Chelidonium

Berberine Chelidonium

Morphine and other morphinane

alkaloids

Papaver

tbl0002 Table 2 Examples for important alkaloids and their biosynthetic precursors

Amino acid Alkaloid Main occurrences Example structure

Lysine Quinolizidine alkaloids Fabaceae Lupanine, sparteine, cytisine

Lycopodium alkaloids Lycopodiaceae Lycopodine

Piperidine alkaloids Punicaceae, Crassulaceae Pelletierine, sedamine

Ornithine Tropane alkaloids Solanaceae, Erythroxylaceae Hyoscyamine, cocaine

Pyrrolizidine alkaloids Asteraceae, Boraginaceae, Fabaceae Senecionine, heliotrine

Nicotiana alkaloids Solanaceae Nicotine, anabasine

Tryptophan Monoterpene indol alkaloids Apocynaceae, Aslepiadaceae . . . Strychnine, vincamine,

Yohimbine, ajmalicine . . .

Simple indole alkaloids Fabaceae Physostigmine

Quinoline alkaloids Rubiaceae, Cornaceae Quinine, cinchonine,

camptothecine

Ergot alkaloids Claviceps, Convolvulaceae Ergotamine, lysergic acid

b-carboline alkaloids Loganiaceae, Zygophyllaceae Harman, harmaline

Phenylalanine/tyrosine Ephedra alkaloids Ephedraceae Ephedrine

Tetrahydro-isoquinoline alkaloids Rubiaceae Emetine

Benzylisoquinoline alkaloids Papaveraceae, Berberidaceae Papaverine

Benzophenanthridine alkaloids Papaveraceae Sanguinarine

Protoberberine alkaloids Berberidaceae, Papaveraceae Berberine

Morphinane alkaloids Papaveraceae Morphine, codeine

Aporphine alkaloids Monimiaceae Boldine

Phenylethyl-isoquinoline alkaloids Colchicaceae Colchicine

Aristolochia alkaloids Aristolochiaceae Aristolochic acid

Anthranilic acid Ruta alkaloids Rutaceae Skimmianine, dictamine

130 ALKALOIDS/Properties and Determination

Protons are enriched in the vacuole, which generally

has hydrogen ion concentrations of 0.001–1mM

(¼pH 6–3), by proton-translocating ATPases and

pyrophosphatases. In analogy, it has been assumed

(supported by experimental evidence) that carrier

transport of alkaloids is achieved by a proton–alkaloid

antiport mechanism or with the aid of ABC trans-

porters.

0017 Whereas a few alkaloids are formed ubiquitously in

all plant organs, organ- or even tissue-specific forma-

tion seems to be more common (Table 4). Since most

eukaryotic genes are regulated in a cell-, tissue-, and

organ-specific manner, genes of alkaloid formation

seem to be no exception. The corresponding promo-

tors are presumed to be regulated by respective regu-

latory proteins. This conclusion is important for the

interpretation of results obtained with cell suspension

cultures. Whereas alkaloid formation is active in

differentiated systems (root or shoot cultures), it is

usually reduced, or even absent, in undifferentiated

suspension cultured cells. It is likely that the corres-

ponding alkaloid genes are just ‘switched off.’ The

production of berberine and sanguinarine in cell cul-

tures seems to be more the exception than the rule.

0018 Alkaloids are stored predominantly in tissues that

are important for survival and reproduction, which

include actively growing young tissues, root and stem

bark, flowers (especially seeds), seedlings, and photo-

synthetically active tissues. Alkaloid contents in stor-

ing organs can be quite high, reaching up to 10% of

dry weight in some instances, which is important if

the alkaloids are to function as effective defense com-

pounds. In several herbaceous plants, alkaloids are

stored in epidermal and subepidermal tissues (e.g.

cocaine, colchicine, aconitine, steroidal alkaloids,

nicotine, veratrine, buxine, and coniine), which have

to ward off small enemies (insects, microorganisms)

in the first place. In lupins and broom, quinolizidine

alkaloid concentrations are up to 200 mM in epider-

mal tissue, whereas mesophyl tissue has values below

5 mM. Some plants possess typical alkaloid-storing

cells, called ‘idioblasts.’ These idioblasts have been

detected in Corydalis (for corydaline), Sanguinaria

(for sanguinarine), Ruta (for rutacridones), Cathar-

anthus (for indole alkaloids), and Macleaya (for pro-

topine). Short- and long-distance transport are often

required to reach these sites of accumulation.

0019Alkaloid patterns usually vary between the site of

synthesis and the sites of accumulation, since a

number of secondary substitutions may take place in

the latter tissues, or transport may be selective, dis-

tributing differing cocktails. In addition, alkaloid

profiles of seeds and seedlings often differ from

those of the mature plant. Both patterns and concen-

trations usually change during the development of

plants and the annual cycle. In general, alkaloid levels

are markedly reduced in senescing tissues, so that

shedded leaves are often nearly alkaloid-free. Alkal-

oid formation and storage may be influenced by

environmental stress, such as wounding or infection.

Transport and Turnover

0020A number of alkaloids are synthesized and stored in

all parts of plants, whereas others are restricted to a

particular organ. Alternatively, alkaloids are synthe-

sized but accumulated in many organs, which usually

demands long-distance transport. Theoretically, this

transport could procede sym- or apoplastically. How-

ever, utilization of established transport routes, such

as xylem or phloem, seems to be more common.

Because it is technically difficult to sample and ana-

lyze xylem and phloem sap, only a limited number

of appropriate data are available (Table 5). Besides

long-distance transport, short-distance and intra-

cellular transport need to be reviewed. In general,

alkaloids are synthesized in the cytosol or in mem-

brane-enclosed vesicles (endoplasmic reticulum,

mitochondria, chloroplasts), but are accumulated

and sequestered in the vacuole.

0021In general, alkaloids are not end products of

metabolism but can be degraded, which seems to be

tbl0004 Table 4 Examples for organ-specific biosynthesis of alkaloids

Alkaloid Organ Species

Tropane alkaloids Roots Atropa, Datura, Hyoscyamus, Mandragora

Nicotine Roots Nicotiana

Senecionine and other PA Roots Senecio

Emetine Roots Cephaelis

Sanguinarine Roots Sanguinaria

Betalaines Roots, shoots Beta

Quinine Stem bark Cinchona

Berberine Stem and root bark Berberis, Mahonia

Caffeine Green tissue Coffea

Quinolizidine alkaloids Leaves and other photosynthetic tissues Lupinus, Cytisus, Laburnum, Baptisia

Steroid alkaloids Roots, tubers, leaves Solanum

ALKALOIDS/Properties and Determination 131

plausible because nitrogen is a limited nutrient for

plants. As discussed previously, alkaloids stored in

seeds are partly degraded during germination and

seedling development, and their nitrogen is probably

used for the synthesis of amino acids. Degradative

pathways have not been worked out yet.

0022 In addition to this developmentally specific recyc-

ling, there is evidence that a number of alkaloids are

turned over all the time, with half-lives of between

2 and 48 h. Examples are nicotine, QAs, and tropane

alkaloids. Alkaloid turnover is often quite substantial

in cell cultures: Lupinus callus cultures are even able

to live on the QA sparteine as a sole nitrogen source

for more than 6 months.

0023 How can we explain this phenomenon? A number

of alkaloids are allelochemicals and affect molecular

targets such as receptors of neurotransmitters (tro-

panes, nicotine, etc.). For this interaction, a correct

stereochemical configuration is required. Because

alkaloids may oxidize or give rise to racemic mixtures

spontaneously, a continuous turnover would make

sure that a sufficient concentration of active com-

pounds is always available, similar to the situation

of protein turnover.

Functions

0024 Although several alkaloids and other secondary me-

tabolites have been used by mankind for thousands

of years as dyes (e.g., indigo, shikonine), flavors

(e.g., vanillin, capsaicin, mustard oils), fragrances

(e.g., rose oil, lavender oil, and other essential oils),

stimulants (e.g., caffeine, nicotine, ephedrine), hal-

lucinogens (e.g., morphine, cocaine, mescaline,

hyoscyamine, scopolamine, tetrahydrocannabinol),

insecticides (e.g., nicotine, piperine, pyrethrin), verte-

brate and human poisons (e.g., coniine, strychnine,

aconitine) and even therapeutic agents (e.g., atropine,

quinine, codeine, cardenolides, etc.), their putative

functions have been discussed controversially.

0025Alkaloid biology is tightly connected with the basic

physiology of plants. Many of the features described

before would make no sense if these compounds

did not have a vital function for the producer. As a

common theme, it has been observed that plants that

produce seeds rich in energy supplies (carbohydrates,

lipids, proteins) concomitantly accumulate potent

chemical defense compounds, often alkaloids, non-

protein amino acids, cyanogenic glycosides, gluco-

sinolates, protease inhibitors, lectins, or other

toxalbumins. Their presence in seeds can be mutually

exclusive, i.e., legume seeds store either alkaloids

(e.g., quinolizidines, pyrrolizidines) or nonprotein

amino acids, but not both at the same time. During

germination, the breakdown of nutrient reserves

is a general procedure and usually includes the

nitrogenous defense compounds. They serve a double

purpose, i.e., that of N-storage and protection.

They are thus degradable and toxic N-storage

compounds.

0026The main function is obviously that of chemical

defense against herbivores (insects, other arthropods,

and vertebrates), which can be deduced from the fact

that many alkaloid have a high affinity for receptors

of neurotransmitters that are present only in animals.

In some instances, alkaloids play a role (additionally)

in the antimicrobial defense (against bacteria, fungi,

viruses) and even in the interaction between plants

(allelopathy).

0027Alkaloids are certainly multipurpose compounds

that, depending on the situation, may be active

in more than one environmental interaction. For

example: QAs are certainly the most important de-

fense chemical in Fabaceae against insects and other

herbivores, but they also influence bacteria, fungi,

viruses, and even the germination of other plants. In

addition, they are employed as degradable N-trans-

port and N-storage compounds.

0028Alkaloids repel or deter the feeding of many animals

(many have a bitter or pungent taste to humans and

other vertebrates) or are toxic if ingested. In micro-

organisms and competing plants, a reduction of

growth and antibiosis are usually the visible effects

of alkaloid intoxication. How are these diverse effects

being achieved? Although most compounds have not

been studied in full detail, an impressive number of

cellular and molecular targets have been identified

that are selectively inhibited or modulated by

alkaloids. As a consequence of such interactions,

organ malfunctions (heart, lung, liver, kidney, CNS

disorders) result that may impair reproduction and

fertility in animals and other organisms, or simply kill

them.

0029Because many alkaloids have been shaped during

evolution by ‘molecular modeling,’ many of them are

tbl0005 Table 5 Long-distance transport of alkaloids by phloem or

xylem

Alkaloid Xylem Phloem Occurrence

Lupanine, sparteine X Lupinus, Cytisus

Cytisine X Laburnum, Petteria,

Genista, Spartium

Matrin X Sophora

Senecionine (N-oxide) X Senecio

Aconitine ? X Aconitum

Nicotine X Nicotiana

Hyoscyamine X Atropa

Scopolamine X Datura, Hyosycamus

Swainsonine X Astragalus

132 ALKALOIDS/Properties and Determination

used by humans as medicinal compounds; allelo-

chemicals may have positive effects if used at non-

toxic concentrations.

Presence at the Right Concentration at

the Right Time and Place

0030 To be effective, alkaloids need to be present at the

right time, site, and concentration. Alkaloid metabol-

ism and biochemistry seem to have been optimized

and coordinated in most systems to fulfill this pre-

requisite. An interesting variation can be seen in some

plants, where alkaloid formation is enhanced by

wounding or microbial attack, i.e., in case of

emergency, the production of defense compounds is

stimulated.

0031 How effective are alkaloidal defenses? In lupins,

which normally produce high amounts of QAs,

mutants have been selected (so-called sweet lupins)

that accumulate only trace amounts of alkaloids.

When both alkaloid-rich and low-alkaloid lupins are

planted in the field without any fences and phytopro-

tectives, a dramatic effect can often be seen (Figure 2).

Whereas the low-alkaloid lupins were selectively

grazed or infested by herbivores, their alkaloid-rich

counterparts remained almost undisturbed. More

experimental data are certainly needed, but the

defensive role of alkaloids seems to be beyond doubt.

0032Similar to the situation in sweet lupins, in which

plant breeders have eliminated the alkaloid trait, also

in other crop plants, secondary metabolites, which

had evolved to serve as defense compounds, have

been bred away or strongly decreased. As a conse-

quence, many crop plants have lost their original

resistance to pests and herbivores. Man-made chem-

istry (i.e., synthetic pesticides) has to be used if these

crop plants have to be cultured.

Exceptions to the Rule – Adaptations of

Specialists

0033No defense is absolute. Whereas chemical defense

works against the majority of potential enemies, usu-

ally a few specialists have evolved during evolution

that have become specialized in the toxin-protected

ecological niche. This phenomenon can be clearly

seen in many insects, which are often highly host-

plant-specific. Some of these insects take up the

dietary alkaloids, store, and exploit them for their

own chemical defense or that of their offspring.

123456 123456

Alkaloid content (µg g

−1

FW)

Percentage completely eaten lupins

500

1000

100

Alkaloid content (µg g

−1

FW)

Percentage lupins with Agromyzidae

500

1000

100

Selective herbivory by rabbits

Selective herbivory by mining flies

1−4 = Lupinus albus

5 = L. mutabilis

6 = L. polyphyllus

1−3 = Su lupinen

Sweet lupins

fig0002 Figure 2 Selective advantage of alkaloids in lupins against herbivores (rabbits, mining flies). Lupins without alkaloids suffer heavily

from herbivory, whereas alkaloid-rich lupins are widely protected.

ALKALOIDS/Properties and Determination 133