Signal transduction by xenobiotics in fish

Signal transduction by xenobiotics in fish has recently gained much attention. The better known transduction mechanisms are those elicited by organochlorines, organophosphates, carbamates and heavy metals. Organochlorines specifically bind to the membrane bound ouabain sensitive Na+-K+-ATPase affecting neural transmission while the organophosphates and carbamates bind specifically to the membrane bound enzyme acetylcholinesterase again affecting neural transmission. Since the nervous system is one of the important integrative and interactive physiological systems in animals, hypofunction of the nervous system leads to secondary effects in the endocrine system including thyroidal, gonadal, interrenal, pituitary and hypothalamic functions. Even low levels of xenobiotics are efficient enough to bring about remarkable changes in the functional physiology of the non target animals. Heavy metals such as cadmium or mercury belonging to the same group II B in the periodic table probably have a similar mechanism of action. Avidity of these metals to SH-radicals allow them to bind indiscriminately to SH groups in proteins. One pathway of interaction by inorganic mercury with the membrane bound ouabain sensitive Na+-K+-ATPase has been clearly established in fish liver and ovary. Binding of inorganic mercury to the membrane bound enzyme is through sulfhydryl group which inactivates the sodium pump leading to accumulation of the cation in the cytosol. The inorganic mercury is next conjugated by the cytosolar nucleophile, glutathione, and is transported to the nucleus where dissociation occurs and the free metal binds to the metal regulatory element to initiate gene expression. The inducible proteins are 3beta-hydroxysteroid dehydrogenase in the oocyte and metallothionein and C-reactive protein in the liver. The present review deals with the role of xenobiotic as a stress factor.