The enzymatic basis for the rat liver 6-hydroxymelatonin sulfotransferase activity

Understanding 6-hydroxymelatonin (6HM) sulfation is deemed important to explaining normal and oncostatic actions of the pineal gland. Here we identify the enzymatic basis for this sulfation in rats. First, a quantitative assay was designed for measuring hepatic 6HM sulfotransferase (6HMST) activity. The assay was then used to identify a male dominant sexual dimorphism wherein liver from males contains double the 6HMST per g or per 100 g body weight seen in females. Examination of other rat tissues showed that most in vivo 6HM sulfation was likely to occur in liver. In addition, DEAE-Sephadex chromatography of liver cytosol indicated that 80-90% of the 6HMST activity in both sexes was due to an enzyme we named 6HMST II. A minor 6HM sulfotransferase (6HMST I) eluted from the columns prior to the main enzyme. 6HMST II, purified additionally, was shown to convert 6HM to a product that appeared to be 6HM sulfate (6-sulfa-toxymelatonin). The enzyme was inhibited by Na+, K+, Zn2+, and Cd2+. Its pH optimum was 7.80 +/- 0.30. Comparisons are made between 6HMST II, dopamine sulfotransferase II, and aryl sulfotransferase IV.