An antiserum against chicken hydroxyindole- O-methyltransferase reacts with the enzyme from pineal gland and retina and labels pineal modified photoreceptors

Biosynthesis of the indolic hormone melatonin has been reported in the pineal gland and retina. The terminal step of melatonin synthesis is catalysed by hydroxyindole- O-methyltransferase (HIOMT), an enzyme displaying highest levels of activity in the pineal gland and retina. Several laboratories have suggested that melatonin synthesis might take place in retinal photoreceptors and in photoreceptor-derived cells of the pineal gland. Experimental support to this hypothesis is progressively building up with the immunocytochemical identification of HIOMT-containing cells in various animal species. In the present report, HIOMT was purified from the chicken pineal gland using a one-step chromatographic procedure and an antiserum against the enzyme was obtained in the rabbit. The antiserum was further purified by immunoadsorption chromatography on chicken brain proteins. Using electroblots immunochemical labeling, HIOMT from chicken pineal gland and retina was identified as a 38-kDa protein. Pineal HIOMT was further resolved into components of different pHi-values (5.4–5.7 and 6.8), using two-dimensional gel electrophoresis. Immunoprecipitation of HIOMT activity was observed in pineal homogenates and, for the first time, in homogenates of the retina. Immunofluorescence microscopy provided the first evidence that HIOMT is contained in modified photoreceptors of the chicken pineal gland. No immunofluorescence could be observed in the retina, maybe due to the lower level of HIOMT activity in this tissue. Together, the data indicate that the antiserum may be a useful tool to study the regulation of HIOMT synthesis in the pineal gland and in the retina. Further work is required to identify HIOMT-containing cells in the retina.