= Serotonin N-acetyltransferase
<Serotonin N-acetyltransferase> transfers an <acetyl group> from <acetyl-CoA> to <serotonin>, making <N-acetylserotonin> and <coenzyme A>. The IUBMB accepted name aralkylamine N-acetyltransferase, EC 2.3.1.87, reflects its wider substrate range. This acetyl-transfer step precedes the final methyl-transfer step of <melatonin> synthesis.<Enzyme Nomenclature by the IUBMB>[EC 2.3.1.87, accepted name, Reaction and Comments]{ref}
For an idealized isolated forward reaction, $3\,\mathrm{\mu mol}$ acetyl-CoA and $5\,\mathrm{\mu mol}$ serotonin permit at most $3\,\mathrm{\mu mol}$ N-acetylserotonin, leaving at least $2\,\mathrm{\mu mol}$ serotonin. That calculation is a stoichiometric bound, not a measured rate or a prediction of complete conversion to melatonin.
In 1970, <D. C. Klein> and <J. L. Weller> measured a <circadian rhythm> in rat <pineal gland> N-acetyltransferase activity, with activity more than 15-fold higher at night than by day. The rhythm persisted in darkness and blinded animals and was suppressed by constant illumination. Their in-vitro comparisons implicated <norepinephrine> in activity regulation.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Abstract, activity rhythm and lighting comparisons]{ref}
Persistence in darkness makes a simple instantaneous light-off switch inadequate as a complete explanation. The measured endpoint was enzymatic activity in a specified animal preparation, rather than a universal ratio for circulating melatonin in every organism.
Back to article page
