Molecular characterization and taurine regulation of two novel CDOs (CDO1 and CDO2) from Carassius auratus

Cysteine oxygenase (CDO) is a mononuclear nonhemoglobin enzyme that catalyzes the production of taurine through the cysteine (Cys) pathway and plays a key role in the biosynthesis of taurine in mammals. However, the function of CDOs in bony fish remains poorly understood. In this study, we cloned CDO genes (CaCDO1 and CaCDO2) from Carassius auratus. The cDNA sequences of both CaCDO1 and CaCDO2 encoded putative proteins with 201 amino acids, which included structural features typical of the CDO protein family. Multiple sequence alignment and phylogenetic analysis showed that CaCDO1 and CaCDO2 shared high sequence identities and similarities with C. carpio homologs. Quantitative real-time polymerase chain reaction (qRT-PCR) results revealed that CaCDO1 and CaCDO2 were both broadly expressed in all selected tissues and developmental stages in C. auratus but had differing mRNA levels. In addition, compared to those of the taurine-free group, the in vivo mRNA expression levels of both CaCDO1 and CaCDO2 significantly decreased with increasing dietary taurine levels from 1.0 to 9.0 g/kg. Furthermore, in vitro taurine treatments showed similar inhibitory effects on the expression of CaCDO1 and CaCDO2 in the intestines of C. auratus. Our results also showed that the mRNA expression of CaCDO2 in the intestines was higher than that of CaCDO1 in response to in vivo and in vitro taurine supplementation. Overall, these data may provide new insights into the regulation of fish CDO expression and provide valuable knowledge for improving dietary formulas in aquaculture.