Mechanism of inhibition of bovine pineal gland hydroxyindole-O-methyltransferase (EC 2.1.1.4) by divalent cations.

The effect of inhibitory cations on hydroxyindole-O-methyltransferase (HIOMT) was investigated in order to elucidate the mechanisms involved. Copper inhibition of HIOMT can be classified as partial, classic noncompetitive inhibition with respect to S-adenosyl methionine and as partial, mixed noncompetitive inhibition with respect to N-acetylserotonin. Binding of copper to the enzyme impaired but did not prevent catalysis, and the difference in inhibitory pattern observed with both substrates may be due to the obligatory order of substrate binding to HIOMT. Calcium and magnesium were uncompetitive inhibitors of HIOMT with respect to both substrates and binding of these cations to the enzyme prevents catalysis. The cations presumably bind to HIOMT in the vicinity of the catalytic site and in some way prevent or impair the catalytic process, possibly by interfering with substrate binding to the enzyme, interfering with methyl transfer or product release after catalysis, or a combination of these effects. The different inhibitory patterns observed may result from differences in molecular size or outer electron shell valence of the cations.