HYDROXYINDOLE-O-METHYLTRANSFERASE ACTIVITY IN OCULAR AND BRAIN STRUCTURES OF RABBIT AND HEN

Relative activities of hydroxyindole-0-methyltransferase (HIOMT) of so me brain and ocular structures of the rabbit and hen were analyzed usi ng different 5-hydroxyindoles, i.e., N-acetylserotonin (NAS), 5-hydrox ytryptophol (HTOL), 5-hydroxytryptophan (HTP), 5-hydroxytryptamine (HT ), and 5-hydroxy-3-indoleacetic acid (HIAA), as enzyme substrates. Pin eal glands of both species, as well as hen retina, are capable of prod ucing, to varying degrees, melatonin, 5-methoxytryptophol, and 5-metho xytryptamine. Hen choroid and iris-ciliary body 0-methylated NAS and H TOL, whereas rabbit choroid and, to a much lesser extent, hypothalamus and cerebral cortex all 0-methylated only NAS. No measurable HIOMT ac tivity was found in hen brain. NAS was a preferred substrate for HIOMT in the hen tissues, whereas in the rabbit pineal gland NAS and HTOL w ere equally good substrates for HIOMT. Other tested 5-hydroxyindoles, i.e., HTP, HT, and HIAA, were poor methyl acceptors. Of the tissues ex amined, the highest HIOMT activity was found in the hen pineal gland, followed by the rabbit pineal gland and hen retina. No significant dif ferences between day and nighttime enzyme activities were observed in the pineal gland and retina of either species. The data suggest that i n vertebrates some nervous and ocular tissues possess the potential to produce 5-methoxyindole compounds; however, the HIOMT-catalyzed proce ss shows remarkable substrate-, tissue- and species-dependent variatio ns.