Résumé
The marine bioluminescent dinoflagellate Gonyaulax polyedra is capable of producing various indoleamines. The first enzyme in their formation, tryptophan hydroxylase, exhibits a high-amplitude circadian rhythm with a maximum during photophase. Hydroxyindole-O-methyltransferase shows a biphasic pattern with a major maximum during scotophase. 5-Methoxylated indoleamines, such as melatonin and 5-methoxytryptamine, peak at the beginning and in the second half of scotophase, respectively. A drop in temperature from 20 to 15 degrees Celsius leads to dramatic increases of melatonin, up to more than 50 ng/mg protein. This effect may explain why a lower temperature sensitizes this organism to photo-periodic, indoleamine-mediated induction of asexual cysts. Melatonin can be catabolized either enzymatically or non-enzymatically. The non-enzymatic pathway involves free radicals, e.g., photooxidant cation radicals, and leads to the formation of N1-acetyl-N2-formyl-5-methoxykynuramine. Enzymatic catabolism comprises deacetylation to 5-methoxytryptamine and formation of 5-methoxytryptophol. 5-Methoxytryptamine represents a key substance acting as a stimulator of bioluminescence and a mediator of the encystment response. It opens proton channels in the membrane of an intracellular acidic vacuole system which is loaded by the action of a V-type ATPase, as shown by experiments using bafilomycin A1.