Glycosis in the eggs of the echiuroid, Urechis unicinctus and the oyster, Crassostrea gigas. Rate-limiting steps and activation at fertilization.

The content of glycolytic intermediates and of adenine nucleotides was measured in eggs of the echiuroid, Urechis unicinctus and the oyster, Crassostrea gigas, before and after fertilization. On the whole, the profile of the change in each glycolytic intermediate in Urechis eggs upon fertilization was found to be essentially similar to that in oyster eggs. Calculation of the mass action ratio for each glycolytic step from the amounts of glycolytic intermediates determined suggests that there are at least three limiting enzymes in the glycolysis system in unfertilized and fertilized eggs of each species examined. Phosphorylase (EC 2.4.1.1), phosphofructokinase (EC 2.7.1.11), and pyruvate kinase (EC 2.7.1.40) may be rate-limiting enzymes for the glycolysis system in Urechis eggs as well as in oyster eggs. These enzymes are thought to be activated upon fertilization, though even the reactions of the enzymes in fertilized eggs do not reach a state of equilibrium. In eggs of Urechis and oyster, phosphorylase is the first enzyme to be activated following fertilization. In Urechis eggs, pyruvate kinase is activated after the instant increase in the phosphorylase activity upon fertilization, followed by phosphofructokinase activation. In oyster eggs, however, pyruvate kinase and phosphofructokinase seem to be stimulated simultaneously, subsequent to phosphorylase activation upon fertilization. The mechanism controlling phosphorylase and pyruvate kinase activity is unknown, but the phosphofructokinase activity in both species may be regulated by the intracellular concentration of adenine nucleotides, since the enzyme activity is enhanced along with a decline in the phosphate potential in the eggs of both Urechis and of oyster.