Hydroxyindole-O-methyltransferase activity in the pineal gland of the muskox (Ovibos moschatus).
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.
Tedesco
S C
SC
Department of Veterinary Anatomy, University of Saskatchewan, Saskatoon, Canada.
Morton
D J
DJ
Reiter
R J
RJ
D000121:Acetylserotonin O-Methyltransferase / Q000378:metabolism*; D000818:Animals; D007700:Kinetics; D017440:Photoperiod; D010870:Pineal Gland / Q000201:enzymology*; D012418:Ruminants / Q000378:metabolism*; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D012701:Serotonin / Q000031:analogs & derivatives / Q000494:pharmacology
D012701:Serotonin; D012435:S-Adenosylhomocysteine; D000121:Acetylserotonin O-Methyltransferase; C006389:N-acetylserotonin