 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.^[ref]

For an idealized isolated forward reaction, $3\mu \mathrm{mol}$ acetyl-CoA and $5\mu \mathrm{mol}$ serotonin permit at most $3\mu \mathrm{mol}$ N-acetylserotonin, leaving at least $2\mu \mathrm{mol}$ 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.^[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.