Hydroxyindole‐O‐methyltransferase activity in ocular and brain structures of rabbit and hen

Abstract

Relative activities of hydroxyindole-O-methyltransferase (HIOMT) of some brain and ocular structures of the rabbit and hen were analyzed using different 5-hydroxyindoles, i.e., N-acetylserotonin (NAS), 5-hydroxytryptophol (HTOL), 5-hydroxytryptophan (HTP), 5-hydroxytryptamine (HT), and 5-hydroxy-3-indoleacetic acid (HIAA), as enzyme substrates. Pineal glands of both species, as well as hen retina, are capable of producing, to varying degrees, melatonin, 5-methoxytryptophol, and 5-methoxytryptamine. Hen choroid and iris-ciliary body O-methylated NAS and HTOL, whereas rabbit choroid and, to a much lesser extent, hypothalamus and cerebral cortex all O-methylated only NAS. No measurable HIOMT activity 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 were equally good substrates for HIOMT. Other tested 5-hydroxyindoles, i.e., HTP, HT, and HIAA, were poor methyl acceptors. Of the tissues examined, the highest HIOMT activity was found in the hen pineal gland, followed by the rabbit pineal gland and hen retina. No significant differences between day and nighttime enzyme activities were observed in the pineal gland and retina of either species. The data suggest that in vertebrates some nervous and ocular tissues possess the potential to produce 5-methoxyindole compounds; however, the HIOMT-catalyzed process shows remarkable substrate-, tissue- and species-dependent variations.

Long‐term daily melatonin infusion induces a large increase in N‐acetyltransferase activity, hydroxyindole‐O‐methyltransferase activity, and melatonin content in the Harderian gland and eye of pinealectomized male Siberian hamsters (Phodopus sungorus)

“…This study firstly confirms the presence of the melatonin-synthesizing enzymes (NAT and HIOMT) in the mammalian Harderian gland and retina [Cardinali and Wurtman, 1972;Pévet et al, 1980;Menendez-Pelaez et al, 1993;Nowak et al, 1993;Niki et al, 1998], supporting the melatoninsynthesizing ability of these orbital structures [Pang et al, 1977;Skene et al, 1991;Steinlechner et al, 1995;Tosini and Menaker, 1996;Djeridane et al, 1998]. Secondly, this study demonstrates that long-term daily melatonin infusions to pinealectomized male Siberian hamsters significantly modify Harderian gland and ocular melatonin synthesis.…”

“…Day/night variations in ocular melatonin concentrations and enzyme activities may be revealed using more frequent sampling times. Moreover, besides the retina, the choroid also has the ability to synthesize melatonin [Nowak et al, 1993]. Thus, detection of day/night variations may have been masked by sampling all of these tissues together.…”

Season-Related Differences in the Biosynthetic Activity of the Neonatal Chicken Pineal Gland

“…In addition, HIOMT activity was 20-30 times lower than that of AA-NAT. However, it must be stressed that while HIOMT catalyzes the O-methylation of 5 different HO-indole compounds present in the pineal gland (N-acetylserotonin, 5-hydroxytryptophol, 5-hydroxytryptophan, 5-hydroxytryptamine and 5-hydroxy-3-indoleacetic acid) [26], in our experiment, HIOMT activity was assayed using only N-acetylserotonin as the substrate.…”

Daily variation in the concentration of 5‐methoxytryptophol and melatonin in the duck pineal gland and plasma

“…To measure HIOMT activity, aliquots of pineal supernatants were adjusted to pH 7.9 with 0.05 M sodium phosphate buffer (pH 9.0). HIOMT activity was measured using our routine radioisotopic procedure described previously in detail (Nowak et al., 1993), using N ‐acetylserotonin (1 m M ) and S ‐adenosyl‐ L ‐methionine (100 μ M ) containing 20 nCi of S ‐[methyl‐ 14 C]adenosyl‐ L ‐methionine as substrates.…”