RAT-LIVER N-ACETYLTRANSFERASE - INHIBITION BY MELATONIN

Melatonin and N-acetylserotonin, potent inhibitors of rat liver N-acetyltransferase (EC 2.3.1.5) activity, were studied. Rat liver N-acetyltransferase had an apparent Km for tryptamine of 6 .times. 10-5 M and a Ki for melatonin of 1 .times. 10-6 M at an AcCoA [acetyl CoA] concentration of 3.5 .times. 10-5 M. With tryptamine near saturation (10-4 M), 14C-acetyltryptamine formed about 3 pmol/min mg-1 liver. Inhibition by melatonin of N-acetyltransferase activity was competitive with tryptamine and noncompetitive with AcCoA. Acetylserotonin and melatonin equally inhibited the enzyme. The liver enzyme acetylated serotonin more slowly than it did tryptamine, and melatonin was a better inhibitor: 50% inhibition acetylation was obtained with 10 ng (43 pmol) of melatonin with tryptamine as substrate compared to 5 ng (22 pmol) with serotonin as substrate. Rat pineal N-acetyltransferase, at pH 6.5 with 3.5 .times. 10-5 M AcCoA, had an apparent Km for tryptamine of 6 .times. 10-4 M. Melatonin [up to 10-4 M] did not significantly inhibit this enzyme. With tryptamine as substrate, in a concentration near saturation (10-3 M), the rate of 14C-acetyltryptamine formation was about 60 p/mol min-1 mg-1 pineal. Bovine pineal extract N-acetyltransferase at pH 6.5, with 3.5 .times. 10-5 M AcCoA had a Km for tryptamine of 10-5 M. Melatonin and acetylserotonin competitively inhibited this enzyme (Ki about 10-6 M). Bovine pineal enzyme activity, with tryptamine concentration near saturation (10-4 M), was about 0.03 pmol/min mg-1 pineal. Rat liver N-acetyltransferase and bovine pineal N-acetyltransferase are sensitive to inhibition by melatonin and acetylserotonin; while the N-acetyltransferase activity in pineals of rats killed at night appears insensitive to those indoles. Rat pineal melatonin concentrations of approximately 5 .times. 10-6 M and 3 .times. 10-5 M did not affect rat pineal N-acetyltransferase activity. Inhibition of rat liver-N-acetyltransferase by melatonin allows an enzyme-inhibition assay for melatonin with a sensitivity of less than 2 ng. More sensitive assays are available, but these involve bioassay or immunochemical techniques. An assay based on enzyme inhibition might have use where bioassay or immunoassay facilities are unavailable.