Regulation of Rat Pineal Hydroxyindole-O-Methyltransferase: Evidence of S-Adenosylmethionine-Mediated Glucocorticoid Control

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Abstract
Rat pineal hydroxyindole-O-methyltransferase is controlled similarly to adrenal medullary phenylethanolamine N-methyltransferase. S-adenosylmethionine (SAM), the in vivo cofactor used by the enzyme to convert N-acetylserotonin to melatonin, protects this methyltransferase against tryptic proteolysis in vitro. In vivo studies suggest that the nucleoside itself is controlled by glucocorticoids. Hypophysectomy decreases hydroxyindole-O-methyltransferase levels as compared with control animals, while dexamethasone and SAM administration restore enzyme levels toward control values. In vitro proteolytic studies further demonstrate that although N-acetylserotonin does not stabilize the enzyme against trypsinization, this substrate acts synergistically with SAM to confer greater stabilization than observed with SAM alone.