Carbohydrate metabolism in sea urchin eggs

In sea urchin eggs, the production rate of glucose 1-phosphate (GIP) from glycogen and inorganic phosphate (Pi) in Phosphorylase reaction, one of the rate-limiting reactions in glycogenolysis system, was enhanced following egg activation by sperm and by A23187. The increase in the production rate of G1P in eggs activated by sperm or A23187 was blocked by W 7 but was not affected by H7 and HA1004. Increase in the activity of Phosphorylase a in eggs following fertilization or A23187 treatment was also blocked by W7 but normally occurred in eggs kept in the presence of H7 and HA1004. Upon egg activation, phosphorylation of Phosphorylase b by Ca²⁺ calmodulin dependent protein kinase is probably augmented by an increase of cytosolic Ca²⁺ level, resulting in an increase in the Phosphorylase activity. In egg homogenate, W7-sensitive phosphorylation of Phosphorylase was observed in the presence of calmodulin and Ca²⁺. Respiration, known to be enhanced in its rate following insemination or adding A23187, is sensitive to rotenone, antimycin A (AMA) and CN^- in unfertilized eggs and in those 15 min after egg activation. Mitochondria isolated from unfertilized eggs underwent respiration at the similar rate to that in mitochondria isolated from fertilized eggs. The respiratory rate in mitochondria was reduced by Ca²⁺ in proportion to intra-mitochondrial Ca²⁺ concentration, which became high in relation to extra-mitochondrial Ca²⁺ concentration and was higher at pH 6.6 than at pH 7.2. The increase in the respiratory rate following egg activation in sea urchin eggs seems to result from canceling Ca²⁺-caused inhibition of electron transport in mitochondrial respiratory chain due to an increase in intracellular pH, which results in a decrease of the Ca²⁺ amount in mitochondria.