Publications

• Hydroxyindole-O-methyltransferase activity in the pineal gland of the muskox (Ovibos moschatus).

  Tedesco S.C., Morton D.J., Reiter R.J.

  Characteristics of hydroxyindole-O-methyltransferase (HIOMT) activity were examined in pineal gland tissue from 10 muskoxen shot by native hunters in November, 1990. The enzyme preferentially methylated N-acetylserotonin, with other hydroxyindole compounds showing relatively low affinities; activi ... >> More

  Characteristics of hydroxyindole-O-methyltransferase (HIOMT) activity were examined in pineal gland tissue from 10 muskoxen shot by native hunters in November, 1990. The enzyme preferentially methylated N-acetylserotonin, with other hydroxyindole compounds showing relatively low affinities; activity peaked sharply at pH 8.2. HIOMT was noncompetitively inhibited by its substrate, N-acetylserotonin, and competitively inhibited by its product S-adenosylhomocysteine. The catalytic mechanism appeared to be ordered as described in previous studies: S-adenosylmethionine was the obligatory first substrate, followed by N-acetylserotonin; methyl transfer then occurred and the products, melatonin and S-adenosylhomocysteine, were released sequentially. Interestingly, the inhibition constant (Ki) for N-acetylserotonin was relatively close to the Michaelis-Menten constant (Km), which might allow physiological concentrations of N-acetylserotonin to inhibit HIOMT activity in vivo. This effect could be relevant to the ecology of free-living muskoxen during the dramatic seasonal fluctuations in dietary protein and daily photoperiod associated with their arctic habitat. << Less

  J Pineal Res 16:121-126(1994) [PubMed] [EuropePMC]

• Structural analysis of the human hydroxyindole-O-methyltransferase gene. Presence of two distinct promoters.

  Rodriguez I.R., Mazuruk K., Schoen T.J., Chader G.J.

  Hydroxyindole-O-methyltransferase (HIOMT) catalyzes the last step in the metabolic pathway that synthesizes the hormone melatonin. We have found HIOMT mRNA present in small amounts in human retina and in relatively high abundance in the pineal gland. Two distinct 5' ends were found in human retina ... >> More

  Hydroxyindole-O-methyltransferase (HIOMT) catalyzes the last step in the metabolic pathway that synthesizes the hormone melatonin. We have found HIOMT mRNA present in small amounts in human retina and in relatively high abundance in the pineal gland. Two distinct 5' ends were found in human retina using a solid-phase 5'-rapid amplification of cDNA ends technique. The two 5' regions appear to originate from two distinct putative promoters. Although many similarities exist between the two promoters, they contain distinctive elements. Putative promoter A, for example, contains a recently discovered photoreceptor-conserved element (PCE-1, CAATTAAG) at -27 not found in promoter B, while promoter B contains an Ap1 site (ATGAGTCAA) at -166 and an octamer site (ATGCAAT) at -59 not found in promoter A. The HIOMT messages are also alternatively spliced in between exons 6 and 8, generating three distinct messages. One of the alternatively spliced messages contains a line-1 repetitive element that is spliced into the mRNA precisely as exon 6. Importantly, the downstream open reading frame is not altered by any of these splicing combinations. The gene is approximately 35 kilobases long containing either 9 or 10 exons (including the line-1 element) depending on which promoter is active. All of the splice sites follow the GT/AG rule. The dual promoters and opportunities for alternative splicing suggest a variety of mechanisms for control of HIOMT expression and biological activity in different tissues not previously recognized. << Less

  J. Biol. Chem. 269:31969-31977(1994) [PubMed] [EuropePMC]

• Some properties of pineal gland hydroxyindole-O-methyltransferase from black rhinoceros (Diceros bicornis).

  Morton D.J., Kock N.

  Pineal glands were obtained from two young female black rhinoceri that had died as a result of postcapture trauma during a translocation exercise. Hydroxyindole-O-methyltransferase (HIOMT) from these pineal glands showed a peak activity at pH 8.2, although high activity extended over a fairly wide ... >> More

  Pineal glands were obtained from two young female black rhinoceri that had died as a result of postcapture trauma during a translocation exercise. Hydroxyindole-O-methyltransferase (HIOMT) from these pineal glands showed a peak activity at pH 8.2, although high activity extended over a fairly wide pH range (7.8-8.4). N-acetylserotonin was the best hydroxyindolic substrate for the enzyme, although other hydroxyindoles were methylated, the relative affinities being similar to values previously reported for bovine HIOMT. Kinetic analyses revealed that black rhinoceros HIOMT was subject to substrate inhibition by both substrates at high concentration; this observation is unlikely to have physiological significance. The catalytic mechanism was found to be ordered Bi-Bi, in which S-adenosylmethionine is the obligatory first substrate to bind to the enzyme, such binding allowing for binding of the hydroxyindolic substrate followed by catalysis, products again leaving the catalytic site in a sequential fashion. << Less

  J Pineal Res 8:35-40(1990) [PubMed] [EuropePMC]

• Hydroxyindole O-methyltransferase.

  Sugden D., Cena V., Klein D.C.

  Methods Enzymol 142:590-596(1987) [PubMed] [EuropePMC]

• Crystal structure and functional mapping of human ASMT, the last enzyme of the melatonin synthesis pathway.

  Botros H.G., Legrand P., Pagan C., Bondet V., Weber P., Ben-Abdallah M., Lemiere N., Huguet G., Bellalou J., Maronde E., Beguin P., Haouz A., Shepard W., Bourgeron T.

  Melatonin is a synchronizer of many physiological processes. Abnormal melatonin signaling is associated with human disorders related to sleep, metabolism, and neurodevelopment. Here, we present the X-ray crystal structure of human N-acetyl serotonin methyltransferase (ASMT), the last enzyme of the ... >> More

  Melatonin is a synchronizer of many physiological processes. Abnormal melatonin signaling is associated with human disorders related to sleep, metabolism, and neurodevelopment. Here, we present the X-ray crystal structure of human N-acetyl serotonin methyltransferase (ASMT), the last enzyme of the melatonin biosynthesis pathway. The polypeptide chain of ASMT consists of a C-terminal domain, which is typical of other SAM-dependent O-methyltransferases, and an N-terminal domain, which intertwines several helices with another monomer to form the physiologically active dimer. Using radioenzymology, we analyzed 20 nonsynonymous variants identified through the 1000 genomes project and in patients with neuropsychiatric disorders. We found that the majority of these mutations reduced or abolished ASMT activity including one relatively frequent polymorphism in the Han Chinese population (N17K, rs17149149). Overall, we estimate that the allelic frequency of ASMT deleterious mutations ranges from 0.66% in Europe to 2.97% in Asia. Mapping of the variants on to the 3-dimensional structure clarifies why some are harmful and provides a structural basis for understanding melatonin deficiency in humans. << Less

  J. Pineal Res. 54:46-57(2013) [PubMed] [EuropePMC]

• Cloning of Arabidopsis serotonin N-acetyltransferase and its role with caffeic acid O-methyltransferase in the biosynthesis of melatonin in vitro despite their different subcellular localizations.

  Lee H.Y., Byeon Y., Lee K., Lee H.-J., Back K.

  Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme in melatonin biosynthesis. We cloned SNAT from Arabidopsis thaliana (AtSNAT) and functionally characterized this enzyme for the first time from dicotyledonous plants. Similar to rice SNAT, AtSNAT was found to localize to chloroplasts w ... >> More

  Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme in melatonin biosynthesis. We cloned SNAT from Arabidopsis thaliana (AtSNAT) and functionally characterized this enzyme for the first time from dicotyledonous plants. Similar to rice SNAT, AtSNAT was found to localize to chloroplasts with peak enzyme activity at 45 °C (Km , 309 μm; Vmax , 1400 pmol/min/mg protein). AtSNAT also catalyzed 5-methoxytryptamine (5-MT) into melatonin with high catalytic activity (Km , 51 μm; Vmax , 5300 pmol/min/mg protein). In contrast, Arabidopsis caffeic acid O-methyltransferase (AtCOMT) localized to the cytoplasm. Interestingly, AtCOMT can methylate serotonin into 5-MT with low catalytic activity (Km , 3.396 mm; Vmax , 528 pmol/min/mg protein). These data suggest that serotonin can be converted into either N-acetylserotonin by SNAT or into 5-MT by COMT, after which it is metabolized into melatonin by COMT or SNAT, respectively. To support this hypothesis, serotonin was incubated in the presence of both AtSNAT and AtCOMT enzymes. In addition to melatonin production, the production of major intermediates depended on incubation temperatures; N-acetylserotonin was predominantly produced at high temperatures (45 °C), while low temperatures (37 °C) favored the production of 5-MT. Our results provide biochemical evidence for the presence of a serotonin O-methylation pathway in plant melatonin biosynthesis. << Less

  J. Pineal Res. 57:418-426(2014) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.