Enzymology of human cytosolic sulfotransferases

Conjugation of many xenobiotics, drugs, and endogenous compounds with a sulfonate moiety is an important reaction in their biotransformation. Sulfation of these compounds generally results in a decrease in biological activity and an increase in their urinary excretion. However, in certain instances, sulfation results in bioactivation to reactive electrophilic or therapeutically active forms. At least four cytosolic sulfotransferases (STs) have been identified and characterized from human tissues. These enzymes are two forms of phenol ST (PST), the phenol-sulfating and the monoamine-sulfating forms of PST (P-PST and M-PST, respectively), an estrogen sulfotransferase (EST), and a hydroxysteroid ST, dehydroepiandrosterone ST (DHEA-ST). Although four cytosolic STs have been well characterized in human tissues, evidence is accumulating for the presence of allelic forms or additional distinct forms of the STs in human tissues. The STs possess distinct but overlapping substrate specificities, and all of the STs are capable of conjugating both xenobiotic and endogenous compounds. The individual forms of ST may display distinct patterns of tissue specific expression and different mechanisms of regulation. Although the role of sulfation in drug metabolism is well recognized, an increased understanding of the biochemistry and molecular biology of the STs should also provide additional information as to their functions in many normal physiologic processes.