Alkaloid Sequestration In Vertebrates And Invertebrates

In some cases very toxic alkaloids are not produced de novo by their hosts but rather represent sequestered products that have been synthesized by microorganisms present in the host animals. This is the case for tetrodotoxin, a steroidal alkaloid with great mammalian toxicity whether administered intraperitoneally ( LD50 in mice about 0.2 ug.) or orally in the case of human beings

(Japan) who have died after eating a portion of the "specialty of the house," the tetrodotoxin-containing Japanese puffer fish, Fuga rub/pes.14 This compound has also been identified in the venom gland of the octopus Hapalochlaena maculosa where it presumably augments the toxicity of the other venomous constituents.34

This guanidinium alkaloid has a very wide distribution, especially among marine organisms. This compound has been identified in genera of newts and salamanders in the family Salamandridae as well as frogs and toads (Anura) in the families Bufonidae, Hylidae, Ranidae, and Pipidae.8 In addition to the puffer fish, tetrodotoxin has been identfied in goby fish in the family Gobiidae. Significantly,

this steroidal alkaloid is synthesized by bacteria in the genera Alteromonas, Vibrio, and Pseudomonas, all of which had been isolated from a red alga in the genus Jania.2 Although complementary in vivo studies are generally lacking, it appears that a remarkable diversity of animals has been able to safely sequester tetrodotoxin-producing microorganisms and, in so doing, possess a powerful defensive system. Other highly toxic microbial toxins (e.g., saxitoxin) mirror tetrodotoxin in being present in a variety of marine animals25 and in some cases these alkaloids, after sequestration from prey, have been demonstrated to be potent deterrents.

Sea slugs (nudibranchs), Tambja abere and T. eliora, feed on the bryozoan ectoproct Sessibugula translucens, and sequester several tambjamines (bipyrrolic alkaloids), probably of bacterial origin, from their nudibranch prey.11 A more ferocious nudibranch predator, Roboastra tigris, is deterred by the sequestered bipyrroles in the the mucus eliminated by the Tambja species. While low concentrations of tambjamines are used by the sea slugs such as R. tigris to locate prey, higher concentrations of the pyrroles are toxic to R. tigris and inhibit feeding by the spotted kelpfish Gibbonsia elegans. Similarly, three tambjamines and a blue tetrapyrrole have been demonstrated to be feeding deterrents for seven genera of coral reef fishes.11

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.

0 0

Post a comment