A modified enzymatic-isotopic microassay for serotonin (5HT) using 5HT-N-acetyltransferase partially purified from...

Serotonin-N-acetyltransferase (EC 2.3.1.5), partially purified from Drosophila melanogaster (DNAT) was substituted for rat liver enzyme (RNAT) in a previously published radioenzymatic assay for serotonin (5HT). The purpose was to determine if this modification would increase the sensitivity and reliability of the original assay. Compared with RNAT, DNAT is 3-4 times more active; it lacks secondary 5HTP decarboxylase activity; and it is more stable. Under conditions of the modified assay, the only radioactive product formed from hypothalamic tissue extracts is 3H-melatonin; which can be measured from as little as 15 pg of serotonin substrate. Thus, substitution of DNAT for RNAT improves the original radioenzymatic assay allowing measurement of endogenous 5HT in hypothalamic nuclei from individual animals.