Plant toxins Diversity

Scott, J.G., Liu, N.A., and Wen, Z. (1998). Insect cytochromes P450 diversity, insect resistance and tolerance to plant toxins. In D.R. Livingstone and J.J. Stegeman (Eds.) Forms and Function of Cytochrome P450, 147-156. 

Many volumes have been written on the natural toxins of plants. While the negative elfects of plant toxins on people and the impact of plant toxins on livestock producers have been the most publicized, the diversity of these toxins and their potential as new pharmaceutical agents for the treatment of diseases in people and animals has received widespread interest in modern society. Scientists are actively screening plants from all regions of the world for bioactivity and potential pharmaceuticals for the treatment or prevention of many diseases. 

The demonstration that eclectic mechanisms have been evolved by insects for coping with the potentially toxic concomitants of their ingested nutrients necessitates careful analyses of the processing of each of these compounds by each adapted herbivore. Furthermore, it is important to realize that in Itself, sequestration is nothing more than an end product of a series of reactions that may reflect selective absorption, metabolism of specific compounds, and excretion of selected allelochemics ). In the present review, these diverse processing strategies will be explored in order to illustrate the various ways in which a multitude of herbivores accommodate potential plant toxins. IVo lepidopterous species will be used as models which, hopefully, will emphasize both the elegance and complexity identified with insects as processors of plant-derived compounds. 

A very large number of plant toxins are classified in a diverse group of natural products. What is this group What are some of the toxic effects of substances belonging to it ... 

The principle is fairly plain, even an old one. In 1939, the active ingredient of curare—an ancient plant toxin weapon still in use by Indians—was isolated for the first time. In 1943, it was introduced successfully into anesthesiology. Curare provided adequate muscle relaxation without the depressant effect of deep anesthesia induced by ether or chloroform. Over the last 20 years, physicians have used curare to ease the stiffened muscles caused by polio and to treat such diverse conditions as lockjaw, epilepsy, and cholea (a nervous disorder characterized by uncontrollable muscle movements). Eventually, more effective treatments were found for these illnesses, but the active ingredient of curare, d-tubocurarine, led to the skeletal muscle relaxant Intocostrin, which has been used in surgery ever since. Synthetic analogs of d-tubocurarine are used tens of thousands of times per day in the operating room [309]. 

The breadth of the categorization of the plant toxins speaks to the diverse nature of these compounds. Examples representative of these categories will be included in the following discussion of the interaction of plant toxins with plants, insects and mammals. 

Nature has created a diverse array of plant and animal toxins that act at mammalian muscle and ganglionic nAChRs or invertebrate nAChRs because the critical physiological functions of these receptors make them prime targets for defensive or predatory strategies. More recently, the perceived validity of neuronal nAChR as therapeutic targets has prompted the generation of new synthetic ligands. Examples are listed in Table 1. 

While poisonous plants on grazing lands have a significant impact on livestock production throughout the world, the natural toxins (secondary metabolites) in the plant may have multiple and diverse functions, not only for the plant world but also for the benefit of mankind. Many current pharmaceuticals have been chemically optimized from natural toxins of plant origin. New plant compounds and familiar compounds with renewed interest, e.g., nutraceuticals, herbal preparations, nutritional supplements, etc, are increasingly finding their value in human nutrition and health. 

Numerous nitrogen-free toxins occur in plants. As discussed in the introduction to this Chapter, many of these compounds are believed to be for the protection of the plant from herbivory. However, because there is such a diversity in plant compounds, there are other functions they serve, e.g., insect attractants for pollination, and protection against environmental factors, such as UV light, low or high temperatures, drought, etc. 

Due to their feeding habits, insects may potentially encounter a diversity of toxins. Both plants ( ) and microorganisms such as fungi (2) inake a diversity of chemicals that act as agents to defend against insects and other predators, including plant polymers such... 

Cyanobacterial toxins (both marine and freshwater) are functionally and chemically a diverse group of secondary chemicals. They show structure and function similarities to higher plant and algal toxins. Of particular importance to this publication is the production of toxins which appear to be identical with saxitoxin and neosaxitoxin. Since these are the primary toxins involved in cases of paralytic shellfish poisons, these aphantoxins could be a source of PSP standards and the study of their production by Aphanizomenon can provide information on the biosynthesis of PSP s. The cyanobacteria toxins have not received extensive attention since they have fewer vectors by which they come in contact with humans. As freshwater supplies become more eutrophicated and as cyanobacteria are increasingly used as a source of single cell protein toxic cyanobacteria will have increased importance (39). The study of these cyanobacterial toxins can contribute to a better understanding of seafood poisons. 

Immunoaffinity columns are extremely versatile and have been used for the isolation and concentration of a diverse number of analytes from a wide array of matrices (2). Analytes may include macromolecules such as proteins and receptors or small molecules such as environmental toxins, antibiotics, or pesticides. Matrices may include animal tissues or excreta, plant extracts, cell culture medium, or virtually any milieu encountered in biological work. Because of its value as a research tool, immunoaffinity chromatography has found extensive use by the pharmaceutical industry to purify therapeutic proteins, the food safety community to purify small amounts of toxins from food and as a general tool for analytical chemists to purify analytes for subsequent instrumental analysis. 

Fungal toxins are structurally more diverse and, correspondingly, involve a wide variety of biogenetic pathways. Some toxins are activated in the host by hydrolytic cleavage whether this is commonplace or not is unknown. Even though these toxins have no plant selectivity, they are are highly selective in how they act i. e., inhibit specific enzymes). 

Gunatilaka AA. Natural products from plant-associated microorganisms distribution, structural diversity, bioactivity, and imph-cations of their occurrence. J. Nat. Prod. 2006 69 509-526. Porter JK. Ergot alkaloids and alkaloids from other endophytes, responsible for causing toxic syndrome in cattle after eating contaminated grass. Prikl. Biokhim. Mikrobiol. 1993 29 51-55. Porter JK. Analysis of endophyte toxins fescue and other grasses toxic to livestock. J. Anim. Sci. 1995 73 871-880. 

Over 20,000 terpenoids have been identihed (1), and more are being discovered continuously. Plant terpenoids are important in both primary and secondary (speciahzed) metabolism. Their importance in primary metabolism includes physiological, metabolic, and stmctural roles such as plant hormones, chloro-plast pigments, roles in electron transport systems, and roles in the posttranslational modihcation of proteins. In secondary metabolism, the roles of plant terpenoids are incredibly diverse but are associated most often with defense and communication of sessile plants interacting with other organisms. Examples include terpenoid chemicals that form physical and chemical barriers, antibiotics, phytoalexins, repellents and antifeedants against insects and other herbivores, toxins, attractants for pollinators or fruit-dispersing animals, host/nonhost selection cues for herbivores, and mediators of plant-plant and mycorrhiza interactions (2, 3). 

Plants produce a large diversity of natural products, the so-called secondary metabolites. These are of great importance for the plant for its interaction with the environment due to their roles as pollinator attractants, for symbiosis and for defence against attacks by microorganisms, other plants or animals. Moreover, they are economically important to man as a source of pharmaceuticals, flavours, fragrances, insecticides, dyes, food additives, toxins, etc. Structures of an estimated 200,000 natural products have been elucidated [1] and each year approximately 4,000-5,000 novel compounds are characterised. 

A wide variety of natural toxins, from small heterocyclic molecules to large proteins, occurs in marine organisms. The phyletic diversity of plants in the ocean is far less than on land, while the number of marine animal phyla significantly exceeds that on land. Thus, it is not surprising that many of the known marine toxins are of animal origin. In this article, we will not only focus upon the toxins of unicellular organisms and marine animals, but also consider a few seaweed toxins. 

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