DOPA Decarboxylase (DDC) in Pacific Oysters: Characterization and Role in Tyrosine Metabolism and Melanogenesis.

DOPA decarboxylase (DDC) plays a crucial role in the physiological functions of animals by participating in the dopaminergic system. However, the functions of DDC in shellfish remain poorly understood. The Pacific oyster (Crassostrea gigas) is an extensively cultivated shellfish. In this study, we characterized a DDC gene, designated CgDDC, from C. gigas. The CgDDC gene encodes a protein that contains a Pyridoxal_deC domain, which features specific binding sites for pyridoxal-5'-phosphate (PLP) and L-DOPA. CgDDC exhibits a significantly higher expression level in the black shell oyster strain than the white strain. In vitro enzymatic reaction assays demonstrated that CgDDC catalyzes the conversion of L-DOPA to dopamine. In vivo experiments revealed that inhibiting CgDDC activity reduced the expression of genes associated with tyrosine metabolism. Furthermore, the knockdown of CgDDC caused a decline in cAMP level and reduced transcription of genes involved in the cAMP-mediated melanogenesis. Additionally, treatment with L-α-DOPA inhibited CgDDC enzyme activity and cAMP-mediated melanogenesis; however, dopamine supplementation countered this inhibition, maintaining gene expression and melanin content at baseline levels. Collectively, our findings suggest that CgDDC is intricately involved in regulating tyrosine metabolism and melanogenesis in C. gigas.