Catalyzes the N-acetylation of serotonin into N-acetylserotonin, the penultimate step in the synthesis of melatonin (PubMed:27121038). Catalyzes in vitro the N-acetylation of tryptamine to produce N-acetyltryptamine, 5-methoxytryptamine to produce melatonin and tyramine to produce N-acetyltyramine (PubMed:27121038). acetyl-CoA + serotonin = CoA + H(+) + N-acetylserotonin acetyl-CoA + tyramine = CoA + H(+) + N-acetyltyramine acetyl-CoA + tryptamine = CoA + H(+) + N-acetyltryptamine 5-methoxytryptamine + acetyl-CoA = CoA + H(+) + melatonin Optimum pH is 8.8. Optimum temperature is 45 degrees Celsius. Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2. Down-regulated by cadmium.