ANTIOXIDATIVE PROTECTION IN A HIGH-MELATONIN ORGANISM - THE DINOFLAGELLATE GONYAULAX-POLYEDRA IS RESCUED FROM LETHAL OXIDATIVE STRESS BY STRONGLY ELEVATED, BUT PHYSIOLOGICALLY POSSIBLE CONCENTRATIONS OF MELATONIN

Lethal oxidative stress was investigated in the dinoflagellate Gonyaul ax polyedra by measuring the dying-peak of bioluminescence during circ adian phases of low physiological light emission, low bioluminescence capacity, and low sensitivity to stimulatory agents. Measurements were carried out in constant darkness after transfer of cells from light a t CT 6 (circadian time, 0600 hr). H2O2 (0.08 mM), when administered 1 hr after transfer of cells, led to a multifold, long-lasting enhanceme nt of light emission, which is typical for lethal cell damage. At the circadian phases of investigation, melatonin did not substantially sti mulate bioluminescence up to concentrations of 0.5 mM. At this concent ration, addition of melatonin prevented the dying-peak and reduced bio luminescence to almost basal values. The high concentration of melaton in applied is not unphysiological in Gonyaulax, because the indoleamin e can increase to levels of several millimolar, e.g., in response to t emperature signals. These protective effects of melatonin seem to be c aused mainly by the direct action of melatonin as an antioxidant, beca use the major enzymes of antioxidative protection were not stimulated by melatonin, although some of them responded to H2O2. The activities of neither superoxide dismutase, hemoperoxidase/catalase, glutathione peroxidase, nor haloperoxidase were enhanced under the influence of me latonin; glutathione S-transferase activity increased only slightly.