Pineal hydroxyindole-O-methyltransferase: mechanism, and inhibition by scotophobin A.
We had shown that the behaviorally active peptide, scotophobin A, a synthetic analogue of native scotophobin, acted to increase dark avoidance in goldfish by inhibiting pineal hydroxyindole-O-methyltransferase (HIOMT), the enzyme which converts N-acetylserotonin (NAS) to melatonin (MEL). Here we determine the reaction sequence of bovine pineal HIOMT and the mechanism whereby scotophobin A inhibits this enzyme. Initial rate studies in which the substrates NAS and the methyl donor, S-adenosylmethionine SAM), were independently varied indicated the enzyme reacted by a sequential mechanism. With the product, S-adenosylhomocysteine (SAH), included in the reaction mixtures, data were obtained consistent with the following order of substrate addition and product discharge: (see text). The turnover number was 5.7 moles melatonin formed/mole HIOMT/min. The substrate KMs were 4.2 x 10(-4) M for NAS and 4.9 x 10(-5) M for SAM. Further studies showed that scotophobin A is an inhibitor (KI = 7 x 10(-7) M) competitive with NAS, indicating that this peptide combines with the enzyme-SAM complex. The structural similarity of the tyrosinamide end of scotophobin A to NAS and several other HIOMT inhibitors, including two antischizophrenic drugs, is consistent with these observations.
D000121:Acetylserotonin O-Methyltransferase / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D007700:Kinetics; D008780:Methyltransferases / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology*; D010870:Pineal Gland / Q000201:enzymology*
D010455:Peptides; D008780:Methyltransferases; D000121:Acetylserotonin O-Methyltransferase