Reciprocal relationship between the concentrations of serotonin and the activity of serotonin-N-acetyltransferase in rat pineal glands in culture.

Abstract

Serotonin is a co-substrate for catabolism of acetyl CoA by serotonin N-acetyltransferase (NAT), and acetyl CoA is the most potent known stabilizer of NAT activity against rapid thermal inactivation in pineal homogenate. The possibility that serotonin may modulate NAT activity was considered and subjected to experimentation. Chlorophenylalanine (4 mM) and R04–4602 (100 μM) enhanced the nor- epinephrine (NE)-induced stimulation of NAT activity in rat pineal glands in culture; whereas, 5-hydroxytryptophan (1 mM) attenuated it. Chlorophenylalanine (4 mM) decreased the fall of NAT activity induced by Z-propranolol; whereas, reserpine (50 μM) and serotonin (0.5 mM) had no effect. In contrast, 5-hydroxytryptophan (1 mM) accelerated the fall of NAT activity induced by Z-propranolol. Consequently, a reciprocal relationship appears to exist between the serotonin concentrations and NAT activity in pineal gland. Comparing the kinetics of native and NE-stimulated NAT, there occurred changes in Vmax with regard to both substrates - tryptamine (from 1.2 to 51.28 pmoles/mg protein/min) and acetyl CoA (from 21 to 2000 pmoles/mg protein/hr) without any changes in KM with respect to both tryptamine (KM = 714 μM) or acetyl CoA (KM = 40 μM). In addition, the NAT activity of unstimulated NAT in pineal homogenates could not be activated by incubation with NE-stimulated NAT in pineal homogenates, by cyclic AMP or by trypsin. These data are interpreted to indicate that the NE-induced stimulation of NAT results from increases in NAT molecules, and not activation of pre- existent enzyme. Furthermore, these data propose a role for serotonin in modulating the activity of NAT in pineal glands. This modulation is mediated via serotonin-induced alteration in catabolism of acetyl CoA, which in turn may stabilize the newly synthesized NAT.