Melatonin formation in pineal gland from rats with hexachlorobenzene experimental porphyria

Hexachlorobenzene produces an experimental hepatic porphyria in rats, which is similar to human porphyria cutanea tarda, with hyperpigmentation as one of its characteristic features. Alterations in tryptophan metabolism have been previously observed in this chronic porphyria. Melatonin formation from tryptophan via serotonin shows diurnal rhythmicity in the pineal gland, and higher values are observed during the dark phase of an imposed light-dark cycle. The purpose of this study was to determine the contents of tryptophan and its metabolites in pineal gland of normal and hexachlorobenzene-treated rats in order to find alterations potentially related to porphyria cutanea tarda. Results show that in animals with this experimental porphyria some tryptophan metabolite levels (serotonin and 5-hydroxyindoleacetic acid) increase only during the light period, whereas tryptophan content remained equal to the controls. Hydroxyindole-O-methyltransferase activity also increases by light in pineal gland from hexachlorobenzene-treated rats. On the other hand, tryptophan is converted to melatonin in the dark period, but this route is not exacerbated in hexachlorobenzene porphyria. The relevance of these alterations is discussed in relation to hyperpigmentation, neoplastic and oxidative stress processes associated with this porphyria.