Specialist toxic compounds

Nonapparent plants were fast growing, short-lived, and occurred unpredictably in space and/or time. Thus, many of these plants would escape detection by the majority of herbivores. Because these plants allocated most of their resources to rapid growth and reproduction, they were believed to be defended by relatively low concentrations of toxic compounds that were generally effective against many herbivores but that some specialist herbivores would be able to evolve detoxification mechanisms against (and may even use the qualitative defenses as cues to locate their host). These toxic compounds such as alkaloids and terpenes were termed qualitative defenses because their potency made them effective at low concentrations qualitative defenses of the PAM are analogous to mobile defenses of the RAM. 

Sequestration of plant natural products by herbivorous insects is widespread. This is not surprising, since most insects are herbivorous and it is estimated that there are between two and six million species. While these sequestrators obviously exploit their alkaloid-rich host plants, such specialists represent a very minor component of the total herbivorous population. In essence, these nitrogenous compounds are highly effective deterrents for most of the herbivorous species with which they share their world. On the other hand, a brief discussion of some of the alkaloids and their specialist herbivores illustrates the versatiliity of these insects as exploiters of toxic compounds This alkaloidal treatment has been derived from the tables of Brown and Trigo10 and emphasizes... 

For many toxic compounds, particularly the specialists that bind to very specific cell receptor sites, it would stand to reason that the three-dimensional structure of the toxic compound is responsible for its cellular impact. If the toxic compound is a key, then the receptor molecule is a lock, and for many toxic chemicals, differences in the chemical structure of the molecule (analogous to slight differences in the cutouts on the blade of a key) can lead to alterations in its toxicity. Furthermore, if the toxic mechanism by which a compound elicits a biological eflFect is understood, it stands to reason that molecules that share a similar chemical structure may cause similar effects when organisms are exposed to them. The structure-activity relationship, the correlation between the structure of a chemical compound and its biological activity, is the major corollary that can be derived from Fare s writings. 

In contrast, mono- and oligophagous species often select their host plants with respect to the composition of the nutrients and secondary metabolites present. For these specialists the originally noxious defense compounds are often attractive feeding and oviposition stimulants. These insects either tolerate the natural products or, more often, actively sequester and exploit them for their own defense against predators or for other purposes 1,4,10-12,14-17,28,31,33,494-496). These observations seem to contradict the first statement, that secondary metabolites are primarily defense compounds, and a number of renowned authors have fallen into this logical pit, such as Mothes 35) and Robinson 505). However, these specialized insects are exceptions to the general rule. For these specialists, the defense chemistry of the host plant is usually not toxic, but they are susceptible to the toxicity of natural toxins from non-host plants 32). As compared to the enormous number of potential herbivores, the number of adapted monophagous species is usually very small for a particular plant species. 

On the other hand, microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. In addition, many herbivores have potent mechanisms to detoxify xenobiotics, which allows the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids obviously function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. 

Crisp, D.J., Christie, A.O. and Ghobashy, A.F.A. Narcotic and toxic action of organic compounds on barnacle larvae. Comp. Biochem. Physiol. 22, 629-649 (1967). Lakshminarayanaiah, N. Membrane phenomena Ch. 5, p. 203-286, Hills, G.J., ed., "Electrochanistry a Specialist Periodical Report." Vol. 2. The Chemical Society, London, 1972. 

Ut. ApSimon (ed.). The Total Synthesis of Natural Products, vols. I-to. New York Wiley 1971-1997 Blum, The Toxic Action of Marine and Terrestrial Alkaloids, Fort Collins Alaken Inc. 1995 Blum, Chemistry and Toxicology of Diverse Classes of Alkaloids, Fort Collins Alaken Inc. 1996 Helv. Chim. Acta 75,647 - 688 (1992) (Review) Manske (53 vols. up to 2000) Molhes, SchUlle, and Luckner, Biochemistry of Alkaloids, Weinheim Verl. Chemie 1985 Pelletier, Alkaloids Biological Perspectives, vols. I -6, New York Wiley 1983-1988 vols. 7-9, Oxford Pergamon 1992-1995 Rodd s Chem. Carbon Compds. (2.) 4B (1997) (complete volume) Pharmazie 52, 546 (1997) (history) Saxton (ed. 1971-1975) and Grundon (ed. 1976-1983), The Alkaloids. Specialist Periodical Reports, 13 vols. up to 1983, London Royal Soc. Chem. 1971-1983 Southon and Buckingham (eds.). Dictionary of Alkaloids, London Chapman Hall 1989 UUmann (5.) A I, 353-407 Waterman (ed.). Alkaloids and Sulphur Compounds, London Academic Press 1993. -[HS293910-293990 ... 

Elemental fluorine Carbonyl compounds Aromatics C-H bonds Excellent atom economy Highly reactive, toxic Specialist handling needed Often generates HF by-products... 

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