The Ecological Role of Alkaloids

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Naturam mutare difficile est.

Seneca

Abstract: Alkaloids are a special group of secondary compounds and are part of an organism's adaptation mechanism to its living environment. They are not toxic when stored, but become toxic as a result of cell pH change. The defensive function of alkaloids is only secondary, and connected to internal immune and regulation processes. Animal responses to alkaloids are very diverse. Some animals can tolerate alkaloids relatively well, while others are harmed or even poisoned by them. Animal behaviour in relation to alkaloids depends on evolutionary and co-evolutionary factors. Sequestration of alkaloids is connected with these processes. Alkaloids are a part of plant-derived nutrition. A selective toxicity of these compounds in vertebrates is clearly observed. Vertebrates have the capacity to recognize alkaloids.

Alkaloids take part in the life processes of some invertebrates as pheromones, inducers of sexual behaviour, and in reproduction. A case study of quinolizidine alkaloids and population changes proved that these alkaloids occur in all legume species studied but not, however, in all individuals. The distribution and frequency changes of alkaloidal and non-alkaloidal plants in populations is a direct expression of natural selection; natural hybridization and micro-evolution can be considered as an evidence of current evolutionary responses by ecological and genetic systems.

Key words: alkaloids, attraction, case study, deterrence, ecology, food, MEC, micro-evolution, pheromones, quinolizidine alkaloids, selective toxicity, sexual life

Alkaloids have long represented a research subject in organic chemistry and pharmacology. The main object of research has been to recognize the profound chemical structure and the physical characteristics of these compounds. Advances in alkaloid chemistry also served to advance biological studies of alkaloids, since the chemical structure of these compounds defines their biological activity. As outlined in previous chapters, some areas remain unclear and under-researched; however, the knowledge of alkaloids is relatively large and very detailed in many areas. Moreover, this knowledge has been utilized in the development of many contemporary applications important to human life and society.

From the beginnings of alkaloid research (from the discovering of morphine) to today, one of the most interesting questions has been and remains the function of alkaloids. In particular, the external function of alkaloids has been a popular area of study from both ecological and evolutional points of view. The basis for ecological studies of alkaloids is adaptation theory, according to which all living organisms adapt to environmental change588'687'688'689. Adaptation is the ability of an organism to use newfound conditions to increase its own chances of survival and reproduction. Adaptation has genetic and chemical levels. The short-term (during the life of one generation or of one population) adaptation can be described as acclimatization and in longer timeframes as evolution. During the acclimatization processes, physiological and biochemical adaptations occur588'689 690 691692. The mechanism of adaptation influences both the primary metabolism (enzymes and proteins) and the secondary metabolism588. Alkaloids, as a special, genetically depended group of secondary compounds, function as a part of this mechanism. Moreover, plants and insects radiate and speciate in association with one another in a process referred to as co-evolution577 693 694 695. The basis for this co-evolution is the realization of the nutritional necessity of insects feeding on related species of plants696. This necessity leads to the co-evolution of interacting organisms: plants and insects. However, this co-evolution has also a reciprocal character. It is a consequence of the chemical interaction575'577'697'698'699. On the basis of these theories, secondary compounds have been recognized as plants' defence agents against herbivores. This also has an influence on studies on alkaloids. Many studies have mentioned the main role of alkaloids as defenders of organisms and mediators with the proximate environment323'325'588'700'701'702'703'704'705'706'707'708. However, it is still debatable whether this is the primary function of alkaloids in organisms. As mentioned above, alkaloids are not toxic when stored. They only become toxic as a result of plant cell pH change. This means that alkaloids have a primary role of non-toxic compounds in plants. The defence function is only secondary and related to internal immune and regulation processes. Although this topic needs more empirical research in cell biology, the connection between the physiological, inside-organism function of alkaloids and the outside, ecological function is evident. Alkaloids are produced by the organism primarily for its internal metabolism and activity purposes. Moreover, alkaloids represent only one group of the compounds needed in the defence process. Other compounds, such as non-alkaloid compounds, and phenolics709'710'711'712 specifically, also seem to be important in this function.

1. Animal sequestration of alkaloids

Animal responses to secondary compounds, including alkaloids, are as diverse as natural chemicals. In the case of alkaloids produced by plants, animal responses depend on evolutionary and co-evolutionary factors. Some animals tolerate alkaloids relatively well, while others have well-developed detoxification systems.

Some animals, especially mammals, can be harmed or even poisoned by these compounds. There are many known cases of symptoms of poisoning in cattle by pyrrolizidine alkaloids (senecionine) from the Senecio species475'493'588'713. Anagyrine, from the quinolizidine alkaloid group with pyridone nucleus, has been known to cause skeletal deformities in the foetuses of pregnant cows consuming toxic lupines7'236. Some animals, including dairy cows, have been shown to selectively feed on only alkaloid-poor green plants394. Similar results were observed in a field test trial in 1983 at the Central Finland Research Station in Torikka (Laukaa), where lupin green mass of three cultivars, two bitter and one sweet, were offered to outdoor-grazing dairy cows. One cow approached the sweet green mass of one cultivar, tasted it and continued to consume it for approximately 20 minutes. Afterwards, it grazed on grass. Two other cows tried the bitter mass, tasted it and in both cases spait it out. They were restless during the spitting and did not consume any more lupin mass. These behavioural responses were very important for researchers. The chemical analysis of the green matter clearly confirmed field observations and the animals' consumption behaviour. Therefore, this is also a proof that a tester method of alkaloid analysis can be useful in some cases, especially when it is necessary to do so quickly, as in the decision on green mass quality as fodder. This simple test has also provided interesting for discussion. One might ask what was the cow's mechanism for recognizing the alkaloids in the green matter. Although there do not exist deep investigations into this question, the mechanism is most likely based on the recognition of bitterness by the animal's taste receptors. Moreover, the configuration of alkaloid skeletons might not be an adequate fit for the configuration possibilities of taste receptors, since the lupin juice from green matter started the bitter taste reaction mechanism of a cow.

Animal sequestration of alkaloids is connected not only with taste but also with the toxicity of these compounds. It has been stated that the toxicity of alkaloids is very selective. Aniszewski328 has published data with some LD50 coefficients for some alkaloids and some pesticides and compared their toxi-city from a selectivity point of view. There was clear evidence that alkaloids (sparteine and lupanine) are much more toxic for vertebrates than are some pesticides (e.g. malatione, phenitrothione, etc.). For invertebrates, pesticides were clearly more toxic than alkaloids. Selective toxicity coefficients (STC) were counted by dividing the LD50 for vertebrates by the LD50 for invertebrates. When the STC is 1.0 there is no selectivity; when STC is >1 there is invertebrate selectivity; and when <1 there is vertebrate selectivity. Selectivity simply means there exists more ability to toxify the organism.

Generally speaking, alkaloids are more toxic for vertebrates than for invertebrates328. The coefficients of the selective toxicity show that alkaloids are very dominantly selective toxins to vertebrates (Table 26). Vertebrate very strong selectivity (<0.01) is observed in such alkaloids as ajmalicine, brucine, ephedrine, ergometrine, harmaline, lupanine, lupinine, scopolamine and

Table 26 Selective Toxicity Coefficients (STC) of some alkaloids and selective toxicity in the ecosystem

Alkaloid

STC

Selective Toxicity in Ecosystem

Ajmalicine

0.0015M,S

vvss

Ajmaline

0.014m,s

vss

Anabasine

0.013M,S

vss

Arecoline

0.014m,s

vss

Atropine

0.75r,b

msv

Berberine

1 0M,BE

ns

Boldine

0.12m,s

vss

Brucine

0.005R,BE

vvss

Caffeine

0 7M,BE

msv

Castanospernine

0.1R,A

msv

Chaconine

0.15R,A

msv

Chelidonine

0.45R,S

msv

Cinchonidine

3.6r,be

msiv

Cinchonine

3 1M,BE

msiv

Codeine

5.0M,F

msiv

Colchicine

0.003ma,be

vvss

Coniine

0.112ag,p

msv

Cytisine

0.03M,F

vss

Emetine

0.044m,s

vss

Ephedrine

0.001M,BE

vvss

Ergometrine

0.0003M,S

vvss

Ergotamine

0.11M,S

msv

Gramine

0.1M,S

msv

Harmaline

0.006M,S

vvss

Harmine

0 7M,BE

msv

Heliotrine

0.45R,BE

msv

Hyoscyamine

0.3R,BE

msv

Jacobine

15.0R,L

msiv

Lobeline

2.5r,be

msiv

Lupanine

0.008m,a"

vvss

Lupinine

0.009m,a

vvss

Nicotine

0.08M,BE*"

vss

Papaverine

3.0m,f

msiv

Pilocarpine

0.9M,F

vsss

Quinine

0.01A,BE

vss

Reserpine

0.1A,B

msv

Sanguinarine

0.9m,s

vsss

Scopolamine

0.003M,BE

vvss

Senecionine

0.63m,be

msv

Solanidine

0.09m,c

vss

Solanine

0.06M,C

vss

Sparteine

0.01M,BE"

vss

Strychnine

0.005M,BE

vvss

The Ecological Role of Alkaloids Table 26 (Continued)

Alkaloid STC Selective Toxicity in

Ecosystem

Alkaloid STC Selective Toxicity in

Ecosystem

Tomatidine

0.08M'C

vss

Vinblastine

0.8m'be

msv

Vincamine

1 0M'BE

ns

Vincristine

0.9M'BE

vsms

Yohimbine

0.45m'be

msv

Abbreviations: R = rat; M = mouse; B = Bruchidius; BE = bee; MA = man; S = Syntomis; A = Aphids; F = Formia; AG = Agelaius; L = Locusta; C = Choristoneura; * = 0.02-0.007 for other animals 328; ** = 0.004-0.01 for other animals 328; *** = 0.04 for other animals 328 ; vvss = vertebrate very strong selectivity; vss = vertebrate strong selectivity; msv = more selectivity for vertebrate; ns = no selectivity; msiv = more selectivity for invertebrate; vsss = vertebrate slight selectivity.

Abbreviations: R = rat; M = mouse; B = Bruchidius; BE = bee; MA = man; S = Syntomis; A = Aphids; F = Formia; AG = Agelaius; L = Locusta; C = Choristoneura; * = 0.02-0.007 for other animals 328; ** = 0.004-0.01 for other animals 328; *** = 0.04 for other animals 328 ; vvss = vertebrate very strong selectivity; vss = vertebrate strong selectivity; msv = more selectivity for vertebrate; ns = no selectivity; msiv = more selectivity for invertebrate; vsss = vertebrate slight selectivity.

strychnine (Table 26). Vertebrate strong selectivity (<0.1) exists in the case of ajmaline, anabasine, arecoline, boldine, cytisine, emetine, nicotine, quinine, solanidine, solanine, sparteine and tomatidine. More selectivity to vertebrates (<0.9) is observed in alkaloids such as atropine, caffeine, castanospermine, chaconine, chelidonine, ergotamine, gramine, harmine, heliotrine, hyoscyamine, reserpine, senecionine and vinblastine. Vertebrate slight selectivity (0.9-0.95) has been observed in pilocarpine and sanguinarine. No selectivity (1.0) was observed in the case of berberine and vincamine (Table 26). There are only a small number of alkaloids which have more selectivity to invertebrates (>1.0). These are alkaloids such as cinchonidine, cinchonine, codeine, jacobine, lobeline and papaverine (Table 26).

The sequestration of alkaloids by insects is considered to be a form of defence. Insects sequester alkaloids and accumulate them for protection against their enemies701'702'703'704'714'715'727. Some examples of this kind of insect behaviour have been seen in Aphis cytisorum, Aphis genistae and Macrosiphum albifrons. Wink703 has mentioned that M. albifrons stores alkaloids in order to provide protection against carnivorous beetles, such as Carabus problematicus or Coc-cinella septempunctata. However, the protection provided by the sequestration of alkaloids seems to be still more conjecture than scientifically proven. Stermitz524 has stated that there exists no proof in field conditions that the sequestration of alkaloids by certain insect species provides them with any defensive purpose. More recently, Wackers595 underlined that insect behaviour and the function of secondary compounds should be proven only under field conditions. This is very important when analysing the STC data presented in Table 26. The alkaloids have, in general, no special selective toxicity to insects. Only a very small number of these compounds with reduced distribution in plants are more toxic for invertebrates than vertebrates. Therefore, alkaloids are not strong selective toxins against insects, and their defence ability after sequestration seems problematic. Moreover, the lack of toxin selectivity to insects suggests that between alkaloid-containing plants and insects a type of mutualism exists rather than an antagonist relationship595. By sequestering alkaloids through food, insects fulfil their physiological needs. Therefore, insects use alkaloids in their metabolism and life cycle more than in their direct defence. There exists no evidence in nature that insects sequester cinchonidine, cinchonine, codeine, jacobine, lobeline or papaverine, alkaloids that do have selective toxicity for insect enemies. Field studies on this topic are indispensable.

Alkaloids are a part of plant-based sustenance for herbivores, omnivores and according to the latest research, also for carnivores595'716'717'718'719'720'721'722'723'724'725. The literature mentions that alkaloids and other secondary compounds also occur in floral nectar, pollen, honeydew, leaves, stems and roots of plants592'593'595'703'707'726'728. Herbivores, omnivores and carnivores benefit from their interactions with plants, and alkaloids are a part of this benefit, having a valuable role in animal metabolism and behaviour. The sequestration and accumulation of these compounds in the liver strengthen this hypothesis. The possible protective role of alkaloids seems to be secondary, as previously stated. It is possible to see the interaction between plant and herbivore not only from the point of view of antagonistic theory (plant defends itself and the herbivore consumes it), but also from the point of view of a mutually beneficial relationship between plant and herbivore. In the case of insects, this relationship is more evident than an adversarial relationship. Alkaloids have an important role in this mutual relationship. As seen in the insect liver, alkaloids can be metabolized or accumulated (stored). Both can have the effect of mutualism between insects and plants. Although, more empirical studies are needed within this field, it can also be said that the present direction of ecological thinking is oriented more towards mutualism than towards older antagonistic approaches595.

There exists evidence that some insects store dietary alkaloids derived from natural sources. Figure 98 presents insect species that are known to accumulate pyrrolizidine alkaloids during different developmental stages. The larvae and adults of these insects can metabolize pyrrolizidine alkaloids in current physiological activities. These alkaloids are not toxic for these organisms. Moreover, there is observed trace accumulation of a portion of these compounds in the liver. There is no definitive purpose for these traces. Generally, the opinion presented in 1888 by Stahl in Germany that the accumulation of alkaloids is for defensive purposes703'731 has been most often cited in the research literature.

However, in the case of dietary alkaloids, it would seem that more than only traces of alkaloids, which do not exhibit selective toxicity to antagonist organisms, would be needed for defensive purposes. These trace alkaloids probably have a role in the organism's metabolism, development and behaviour. The traces of alkaloids in the eggs of Arctia caja also suggest a potential participation of these compounds in reproduction. Moreover, attention should be given to the fact that alkaloids are dietary sequestrations acquired from feeding on plants.

LARVAE.

HOMOPTERA

Aphis jacobaeae Aphis cacaliaster

COLEOPTERA

Coccinella spp. Oreina cacaliae

LEPIDOPTERA

Arctia caja Danaus plexippus Danaus chrysippus Idea leuconoe Tyria jacobaeae Utethesia ornatix Utethesia pulchelloides

PYRROLIZIDINE ALKALOIDS from natural sources

ADULTS

HOMOPTERA

Aphis jacobaeae Aphis cacaliaster

COLEOPTERA

Coccinella spp. Oreina cacaliae

LEPIDOPTERA

Arctia caja Danaus plexippus Danaus chrysippus

PYRROLIZIDINE ALKALOIDS from natural sources

LEPIDOPTERA

Arctia caja Danaus plexippus Danaus chrysippus Idea leuconoe Tyria jacobaeae Utethesia ornatix Utethesia pulchelloides

Figure 98. A diagram of the accumulation of pyrrolizidine alkaloids in some insect species during various developmental stages. It should be noted that it is not often that these alkaloids are present in the eggs, as in the case of the Arctia caja. Natural sources are pyrrolizidine alkaloid-rich plant species (Senecio spp., Homoptera; Senecio and Adenostyles spp., Coleoptera; and Senecio, Adenostyles, Petasites, Crotalaria and Heliotropium spp., Lep-idoptera).

Figure 98. A diagram of the accumulation of pyrrolizidine alkaloids in some insect species during various developmental stages. It should be noted that it is not often that these alkaloids are present in the eggs, as in the case of the Arctia caja. Natural sources are pyrrolizidine alkaloid-rich plant species (Senecio spp., Homoptera; Senecio and Adenostyles spp., Coleoptera; and Senecio, Adenostyles, Petasites, Crotalaria and Heliotropium spp., Lep-idoptera).

They are not, therefore, endogenous chemicals produced by the activity of the organism's genes or regulated by its metabolism. It could be then concluded that the pyrrolizidine alkaloids sequestered and accumulated in these cases are needed along with other nutrients, and therefore they function as a source of vitality and metabolic activity for these organisms. Future studies should strive to further elucidate this enduring secret of alkaloids.

There are two sources of alkaloids in animals, as already mentioned in the text. The first is the ability to synthesize them, and the second is through dietary intake. Food and the food chain is the most important factor in the development, growth and dynamics of populations in the ecosystem. Alkaloids are for some species very desirable, while for others they go unwanted. Therefore, there exist different means of animal behaviour in relation to the intake, metabolizing and accumulation of these compounds. Alkaloids, like other secondary compounds, can be avoided when they are undesirable to certain animals. As stated earlier in this chapter, cows are able to avoid ingesting these compounds by virtue of taste. Other animals also avoid alkaloid-rich plants by taste, by olfactory recognition or by the first effects of neurotransmission activity of these compounds. Vertebrates have the ability to recognize alkaloids and the compounds that have selective toxicity to these organisms. Ecological interaction between vertebrates and plants is fundamentally based on this ability.

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