Expression characteristics of sulfur dioxygenase and its function adaption to sulfide in echiuran worm Urechis unicinctus

Animals living in coastal burrows are periodically exposed to the sulfide, a mixture of H2S, HS− and S2 −, during low tide. Mitochondrial sulfide oxidation is an important strategy that allows organisms to avoid injury from sulfide exposure, and sulfur dioxygenase (SDO) plays an essential role. In this study, we characterized the SDO expression and the total SDO-specific activity (T-SDO SA) in different organs of Urechis unicinctus, which inhabitU-shaped burrows in intertidal and subtidal mudflats. The SDO expressions at both mRNA and protein levels were highest in the anal sac, followed by the midgut, and were extraordinarily low in the body wall and hindgut; SDO was located mainly in the epithelial cells of all organs by immunohistochemistry. Moreover, the T-SDO SA was different in the detected organs, but with no significant differences and SDO SAs were strong positive correlation with GSH contents. Furthermore, we investigated the responses of the SDO in the midgut and hindgut of U. unicinctus during sulfide exposure. The SDO contents increased significantly at 48 h and 72 h, respectively, in both the midgut and hindgut when the worms were exposed to 50 and 150 μM sulfide. However, the T-SDO SA was no significantly different in the midgut except that at 72 h for 150 μM sulfide treatment, meanwhile in the hindgut, the T-SDO SA increased significantly after 24 h exposure for 50 and 150 μM sulfide treatments. We concluded that the hindgut plays more important role than the midgut in sulfide tolerance for U. unicinctus.