Investigations on the metabolism of viable and nonviable gilthead sea bream (Sparus aurata) eggs

The present study investigated selected biochemical parameters in viable and nonviable eggs of the gilthead sea bream, Sparus aurata. During embryogenesis, S. aurata eggs had a balanced and stable energy metabolism as the levels of adenosine nucleotides and acetyl-CoA, and the adenylate energy charge (EC), remained constant. Mg2+-dependent ATPase, which is involved in membrane-driven ion transport during oxidative phosphorylation, increased in activity. In nonviable eggs, the levels of ATP, acetyl-CoA, the adenylate energy charge, and the activities of malate dehydrogenase were significantly decreased in comparison to viable eggs. Viable eggs had high Na+/K+-ATPase activity which remained constant during embryogenesis while Ca2+-ATPase activity increased. These enzymes were similarly high in nonviable eggs indicating that the ability for ion transport and for osmoregulation did not differ. However, nonviable eggs contained nonphysiological high levels of magnesium and calcium ions indicating ion influx from the seawater. As the phospholipid levels were significantly lower in nonviable eggs, this ion influx is thought to be related to changed composition of the oolemma. Activities of glucose-6-phosphate dehydrogenase, transaldolase, phosphofructokinase, and pyruvate kinase were constant in viable eggs of S. aurata during embryogenesis. Pyruvate carboxylase increased in activity in the embryonic stage. The occurrence of these enzymes indicated the presence of the enzymatic system for glycolysis for gluconeogenesis and for the pentose phosphate pathway. The monosaccharide levels (i.e. total amount, glucose, fructose, galactose) increased steadily during egg development. Monosaccharides are necessary for nucleic acid synthesis levels, which increased during embryogenesis, and may also play a role as osmotically active compounds. In nonviable eggs, levels of all assayed sugars as well as activities of pyruvate carboxylase and transaldolase were very significantly decreased. Enzymes involved in the catabolism of proteins and amino acids (proteases, aspartate aminotransferase, glutamate dehydrogenase) were constant in the viable eggs with the exception of aspartate aminotransferase, which increased significantly in the embryonic stage. Nonviable eggs had lower activities of glutamate dehydrogenase than viable eggs, while the other enzyme activities were similar. Amino acid levels and inorganic phosphate levels were lower in nonviable than in viable eggs.