Characteristics and non-rhythmicity of retinal hydroxyindole-O-methyltransferase activity in trout (Salmo gairdneri)

The validation and optimization of an hydroxyindole-O-methyltransferase (HIOMT) activity assay system for trout ocular enzyme was undertaken. High-performance liquid chromatographic analysis, cross-validated with a standard thin-layer chromatographic technique verified that HIOMT activity was present in trout ocular tissue by identifying the 3H-melatonin end product. This activity was observed in the neural retina and was restricted to those fractions containing the cytosolic component. The enzyme displayed bimodal pH optima, one at pH 7.6, the other at 8.4. N-Acetylserotonin was O-methylated preferentially and a Km of 4.5 X 10(-7) M was calculated for this substrate. The S-[3H]adenosylmethionine Km was 4.6 X 10(-6) M. Time of incubation and enzyme concentration studies were conducted. The analysis of retinal HIOMT activity in groups of trout killed over a day--night period did not detect a rhythm in enzyme activity. Results of these investigations are discussed in terms of the evolution of melatonin synthesis and functions.