Animal defenses toxins

Animal Defense Mechanisms against Toxins Produced by Peants... 

Some species from practically all phyla of animals produce toxins for either offensive or defensive purposes. Some are passively venomous, often following inadvertent ingestion, whereas others are actively venomous, injecting poisons through specially... 

Some hormones, such as many sex hormones, are steroids that function to regulate metabolic processes. Cholesterol, another steroid, is an important structural component of cell membranes. Vitamin D also contains the four-ring steroid structure and plays a role in the formation of bones. The Giant Marine toad, Bufo marinus, shown in Figure 23.19 uses a steroid called bufotoxin as a defense mechanism. The toad secretes the toxin from warts on its back and from glands just behind the eye. The toxin is only an irritant for humans, but in small animals the toxin causes drooling, loss of coordination, convulsions, and death. 

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. 

Plant toxins have been known for many centuries. Rotenoids (rotenone), alkaloids (nicotine, coniine, strychnine), terpenoids (ovabin and hymenovin) are among the classes of natural products which provide numerous toxins (3-12). Toxins in plants often have the role of feeding repellents. They appear to be synthesized by plants as a defense against insects and other animals. 

Animal toxins are roughly divided into venoms and poisons. Venoms are offensive, used in the quest for food. Snakes produce toxins that can immobilize or kill prey for food. The venom of spiders paralyzes insects to allow the spider to feed on the victim s body fluids. While the venoms may also be used defensively, their primary purpose is in the quest for food. Most venom is delivered from the mouth, as in snakes and spiders, but there are exceptions like the scorpion that uses its tail. 

Poisons are primarily defensive, designed as protection against predators. Poisons are often sprayed or delivered with a stinger to penetrate the skin. Some fishes, for example, have poisonous spines. Toxins can also be on the skin or be part of the meat of the animal, thus making them poisonous to touch or eat. Some poisonous animals develop very colorful markings to advertise their undesirable qualities. 

Not surprisingly, although the evolution of sequestered microbial toxins appears to be rather widespread in marine environments, sequestration of defensive alkaloids in the apparent absence of microorganisms may generally characterize the chemical defenses of terrestrial animals. Careful searches for possible microbial syntheses of defensive compounds (allomones) have not been generally implemented, but recent studies in a few laboratories raise the possibility that microbial endosymbionts may be of major importance in the biogenesis of selected insect deterrents. 

Chemical Defense and Toxins of Lower Terrestrial and Freshwater Animals... 

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