Evidence for existence of a serotonin N-acetyltransferase inactivating substance in rat pineal gland.

This study provides evidence for the existence of an inactivating substance in pineal glands, which may be responsible for the rapid inactivation of serotonin N-acetyltransferase seen in vivo and in vitro. This serotonin N-acetyltransferase inactivating substance enhances the thermal inactivation of the norepinephrine-stimulated serotonin N-acetyltransferase activity in rat pineal homogenate. Inactivation of serotonin N-acetyltransferase by the inactivating substance and the thermal inactivation of serotonin N-acetyltransferase at 37 degrees C exhibit the following identical properties. Both processes affect serotonin N-acetyltransferase without effect on other melatonin-related enzymes; can be blocked by addition of 0.5 mM [3H] acetyl CoA, but not coenzyme A in the preincubation mixture; and were unaffected by 0.1 M NaF or 4 mM beta-mercaptoethanol. These data are interpreted to suggest that protein dephosphorylation and disulfide exchange mechanisms are not involved in either inactivation processes. Unlike serotonin N-acetyltransferase, which is highly thermo labile, the inactivating substance is thermo stable at 37 degrees C for 40 minutes. In rat, the inactivating substance was found only in the pineal gland and was undetectable in other tissues. The inactivating substance is protein in nature, since it is not dialyzable but is inactivated by boiling or treatment with trypsin. The substance, which was able to inactivate serotonin N-acetyltransferase isolated from rate liver, exhibited no diurnal variation and its activity in rat pineal gland in culture was not influenced by norepinephrine. It is postulated that the interaction among acetyl coenzyme A, serotonin N-acetyltransferase and serotonin N-acetyltransferase inactivating substance may collectively regulate the synthesis of melatonin in pineal gland.