Mud crab Scylla paramamosain glutamate dehydrogenase: molecular cloning, tissue expression and response to hyposmotic stress

Glutamate dehydrogenase (GDH) is an important enzyme for the metabolism of glycine, proline and alanine, which are general osmolytes in aquatic animals. To explore the possible relationship between GDH expression and osmoregulation under hyposmotic stress, cDNA cloning of GDH and its expression, as well as free amino acid analysis in Scylla paramamosain, were investigated. GDH cDNA was first isolated from S. paramamosain and the 2269 bp sequence consisted of a 1622 bp open reading frame encoding 533 amino acids. GDH was predominantly expressed in muscle, which is a pool of amino acids. Under acute transfer from a salinity of 28 to 5 ppt, the haemolymph osmolality was significantly decreased at 6 h, and the expression of GDH in the muscle dropped sharply to 29.6% as compared to the control at 6 h (P < 0.05). Accordingly, the total free amino acid concentration decreased significantly in the muscle; this included glutamic acid, glycine, proline, glutamine and arginine, accounting for a large proportion of the total free amino acids (P < 0.05). In contrast, concentrations of total free amino acids in the haemolymph increased, partially compensating for a sharp reduction in extracellular fluid osmolality under hyposmotic stress, effectively maintaining the balance between extra- and intracellular osmolality. It was concluded that GDH plays an important role in controlling osmoregulation in S. paramamosain.