Hydroxyindole‐O‐methyltransferase activity in the pineal gland of the muskox (Ovibos moschatus)

Tedesco SC, Morton DJ, Reiter RJ. Hydroxyindole‐O‐methyltransferase activity in the pineal gland of the muskox (Ovibos moschatus). J. Pineal Res. 1994: 16: 121–126. 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 (K) for N‐acetylserotonin was relatively close to the Michaelis‐Menten constant (K_m), 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.