ABSTRACT: A novel sulfotransferase (O-ST), which transferred the sulfate group of 3′-phosphoadenosine 5′-phosphosulfate (PAPS) to O-22 of 11-α,β-hydroxy saxitoxin (STX) and produced GTX2 + 3, was purified to homogeneity from the cytosolic fraction of clonal-axenic vegetative cells of the toxic dinoflagellate Gymnodinium catenatum GC21V. After four purification steps, including affinity chromatography and anion exchange chromatography, the enzyme was purified 500-fold and the yield was 4%. On affinity chromatography with a PAP-agarose column, O-ST was observed in the bound fraction, and N-ST specific to N-21 of STX and GTX2 + 3 was found in the unbound fraction. The molecular mass of the purified enzyme was determined by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) to be 65 kDa. Gel filtration chromatography showed a native molecular mass of 67 kDa, indicating that O-ST is a monomeric enzyme. The enzyme was optimally active at pH 6.0 and 35°C. O-ST did not require metal cations for its activity. O-ST required PAPS as the sole source of sulfate. O-ST transferred a sulfate group from PAPS to only O-22 of 11-α,β-hydroxy STX and not to N-21 of these toxins. These observations suggested that two ST, N-ST and O-ST, participate in the sulfation of PSP toxins.