Melatonin: Deacetylation to 5‐methoxytryptamine by Liver but Not Brain Aryl Acylamidase

Rat liver and brain slices were incubated in vitro with [ 3 H]melatonin. Liver slices synthesized small amounts of [ 3 H]5‐methoxyindoleacetic acid ([ 3 H]5‐MIAA) along with other melatonin metabolites including 6‐hydroxymelatonin. Pretreatment of animals prior to killing with the irreversible monoamine oxidase inhibitor pargyline allowed [ 3 H]5‐methoxytryptamine ([ 3 H]5‐MT) to be recovered from the incubation. No [ 3 H]5‐MIAA or [ 3 H]5‐MT could be detected in incubations with hypothalamic slices or following intraventrieular injection of [ 3 H]melatonin. The possibility that the deacetylase aryl acylamidase was in part responsible for the deacetylation occurring in liver slices was examined. Liver aryl acylamidase was able to utilize [ 3 H]melatonin as substrate to produce [ 3 H]5‐MT. Furthermore, the liver enzyme was inhibited by melatonin ( K i . 1 m m ) when tested with the alternate substrate o ‐nitroacetanalide. Brain aryl acylamidase did not generate any detectable [ 3 H]5‐MT nor was it inhibited by melatonin. These results suggest that 5‐MT is not formed in brain from melatonin although trace amounts of 5‐MT in the periphery could be derived from this precursor.