The ontogenesis of catabolic abilities and energy metabolism during endogenous nutritional periods of tongue sole,Cynoglossus semilaevis

The ontogenesis of catabolic abilities and energy metabolism during endogenous nutritional periods of tongue sole was investigated. In this work, trypsin-like proteases (TRY) and triglyceride lipase (LIP) activities were measured to assess the capacities to catabolize proteins and lipids, respectively. Meanwhile, specific enzymes including pyruvate kinase (PK), glutamic oxalo acetic transaminase (GOT) and glutamate dehydrogenase (GDH), and hydroxyacyl CoA dehydrogenase (HOAD) as well as their ratios were assayed to evaluate the abilities to use energy substrates of carbohydrates, amino acids and fatty acids, respectively, for energy production. In addition, activities of citrate synthase (CS) and lactate dehydrogenase (LDH) and LDH/CS ratio were calculated to analyse the evolution of aerobic and anaerobic pathways. The study found that hatching occurred at 38.8 h after fertilization (HAF), mouth-opening day of eleuteroembryo appeared at 3 days after hatching (DAH), and the most rapid embryonic growth was observed in blastula stage before hatching. Enzymatic assay revealed that except for PK which appeared in cleavage stage onwards, all the other enzymes functioned after fertilization, preparing well for the coming embryogenesis of tongue sole. By comparing the average specific activity of enzyme in each period, it can be found that the highest value occurred at 3 DAH (for TRY, LIP, PK and LDH), 2 DAH (for GDH), fertilized egg (for GOT) and segmentation stage (for HOAD and CS), and the lowest value occurred at fertilized egg (for HOAD, CS and GDH), cleavage stage (for TRY, PK and LDH), gastrula stage (for GOT) and hatching day (for LIP). Based on the changeable patterns of metabolic enzymatic activities and ratios, it is concluded that metabolic capacities on three energy substrates displayed stage-specific traits, and the dominant energy substrate was fatty acids before segmentation stage, amino acids until hatching day and carbohydrate during eleuteroembryo period. As for energy production mode, aerobic pathway appeared to increase greater in fertilized egg and gastrula stage, whereas anaerobic pathway played a predominant role during cleavage stage, blastula stage, segmentation stage and eleuteroembryo stage. These results are valuable to elucidate the nutritional requirements of embryonic stages in tongue sole and to further understand their energy metabolic mechanisms.