Tedesco S C, Morton D J, Reiter R J
Department of Veterinary Anatomy, University of Saskatchewan, Saskatoon, Canada.
J Pineal Res. 1994 Apr;16(3):121-6. doi: 10.1111/j.1600-079x.1994.tb00091.x.
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.
1990年11月，对10头被当地猎人射杀的麝牛的松果体组织中羟吲哚 - O - 甲基转移酶（HIOMT）的活性特征进行了检测。该酶优先甲基化N - 乙酰血清素，其他羟吲哚化合物的亲和力相对较低；活性在pH 8.2时急剧达到峰值。HIOMT受到其底物N - 乙酰血清素的非竞争性抑制，以及其产物S - 腺苷同型半胱氨酸的竞争性抑制。催化机制似乎如先前研究中所述是有序的：S - 腺苷甲硫氨酸是必需的第一个底物，其次是N - 乙酰血清素；然后发生甲基转移，产物褪黑素和S - 腺苷同型半胱氨酸依次释放。有趣的是，N - 乙酰血清素的抑制常数（Ki）相对接近米氏常数（Km），这可能使N - 乙酰血清素的生理浓度在体内抑制HIOMT活性。这种效应可能与自由生活的麝牛在与其北极栖息地相关的饮食蛋白质和每日光周期的剧烈季节性波动期间的生态有关。
