Coelenterazine sulfotransferase from the luminous squid Watasenia scintillans

The squid Watasenia scintillans is a luminous marine organism that utilizes coelenterazine 2,6-disulfate (CTZ 2,6-SO₃H) as its luminous substrate. CTZ 2,6-SO₃H, a derivative of coelenterazine (CTZ), can be chemically synthesized by sulfating CTZ at its two phenolic hydroxyl groups. CTZ is produced by certain marine plankton and is widely distributed among marine organisms through the food web. However, CTZ 2,6-SO₃H has been identified only in W. scintillans, and it remains unclear whether the squid biosynthesizes CTZ 2,6-SO₃H from CTZ. In this study, we investigated the presence of a sulfotransferase (SULT) responsible for the sulfation of CTZ in W. scintillans. RNA-seq analysis of the squid's arm photophore revealed a putative SULT gene, which was successfully expressed in Escherichia coli. The recombinant protein exhibited sulfation activity toward CTZ in the presence of 3′-phosphoadenosine 5′-phosphosulfate (PAPS), as confirmed by ultra-performance liquid chromatography-photodiode array detection-mass spectrometry (UPLC-PDA-MS) analysis. The sulfated product was not CTZ 2,6-SO₃H but rather CTZ 2-monosulfate (CTZ 2-SO₃H). These findings suggest that W. scintillans possesses a biosynthetic pathway for CTZ 2,6-SO₃H from CTZ, which may involve additional SULT(s).