1. This study aimed to investigate the involvement of sulfation in the metabolism of
6-hydroxymelatonin (6-OH-Mel),
N-acetylserotonin (
NAS) and 4-hydroxyramelteon (4-OH-Ram), and to identify and characterize the human cytosolic
sulfotransferases (SULTs) capable of sulfating these drug compounds. 2. A systematic analysis using 13 known human SULTs revealed that SULT1A1 displayed the strongest activity in catalyzing the sulfation of 6-OH-Mel and 4-OH-Ram, whereas SULT1C4 exhibited the strongest sulfating-activity towards
NAS. pH-dependence and kinetic parameters of these SULT
enzymes in mediating the sulfation of respective drug compounds were determined. A metabolic labeling study showed the generation and release of [35S]sulfated 6-OH-Mel,
NAS and 4-OH-Ram by HepG2 human
hepatoma cells and Caco-2 human
colon adenocarcinoma cells labeled with [35S]
sulfate in the presence of these drug compounds. Cytosols of human lung, liver, kidney and small intestine were examined to verify the presence of 6-OH-Mel-,
NAS- and 4-OH-Ram-sulfating activity in vivo. Of the four human organ samples tested, small intestine and liver cytosols displayed considerably higher 6-OH-Mel-,
NAS- and 4-OH-Ram-sulfating activities than those of lung and kidney. 3. Collectively, these results provided a molecular basis for the metabolism of 6-OH-Mel,
NAS and 4-OH-Ram through sulfation.