HYDROXYINDOLE-O-METHYLTRANSFERASE ACTIVITY IN THE PINEAL-GLAND OF THEMUSKOX (OVIBOS-MOSCHATUS)

Characteristics of hydroxyindole-O-methyltransferase (HIOMT) activity were examined in pineal gland tissue from 10 muskoxen shot by native h unters in November, 1990. The enzyme preferentially methylated N-acety lserotonin, with other hydroxyindole compounds showing relatively low affinities; activity peaked sharply at pH 8.2. HIOMT was noncompetitiv ely inhibited by its substrate, N-acetylserotonin, and competitively i nhibited by its product S-adenosylhomocysteine. The catalytic mechanis m appeared to be ordered as described in previous studies: S-adenosylm ethionine was the obligatory first substrate, followed by N-acetylsero tonin; methyl transfer then occurred and the products, melatonin and S -adenosylhomocysteine, were released sequentially. Interestingly, the inhibition constant (K-i) for N-acetylserotonin was relatively close t o 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 dail y photoperiod associated with their arctic habitat.