The catecholamine neurotransmitter dopamine (DA) is predominantly (>95%) conjugated with sulfate (dopamine 3-O-sulfate, DAOS) in human blood circulation. Human SULT1A3 and SULT1A4 are the major sulfotransferases that sulfonate DA (as well as other catecholamines and phenols) (Reiter et al. 1983, Johnson et al. 1980, Thomae et al. 2003, Hildebrandt et al. 2004).