The changes in respiration and glycolysis of whole oocytes and homogenates of oocytes during oogenesis have been studied.The respiration rate of whole oocytes increases during oocyte growth and decreases during oocyte maturation. The respiration rate of homogenates also increases during oocyte growth and does not change during egg maturation. At all oogenesis stages the respiration rate of homogenates is higher than the respiration rate of whole oocytes.Respiration intensity increases during the small growth stage and decreases during the following stages of oogenesis. Respiration intensity of homogenates under optimal conditions changes in a similar way. Respiration intensity under physiological conditions diminishes during oogenesis from 70% at the small growth stage to 42% in unfertilised eggs.The rate of glycolysis in whole oocytes and homogenates of oocytes increases during the growth period of oocytes but does not change during egg maturation.Glycolysis intensity of the whole oocytes increases at the large growth stage-stage of cytoplasmic vacuolisation-and becomes less during the following stages. Glycolysis intensity in homogenates under optimal conditions is much higher than the glycolysis intensity of whole oocytes and it decreases slightly during oogenesis. The efficiency of glycolysis in oocytes under physiological conditions is very low. It increases from the stage of cytoplasmic vacuolisation (3.6%) to the stage at which vitellogenesis starts (20%) and diminishes at the following stages.The data obtained are considered in the light of the Prigogine and Wiame interpretation of a thermodynamic theory of development.
The oocyte at the end of oogenesis, mature egg and developing embryo of the loachMisgurnus fossilis L.) are characterized by indentical enzyme profiles of the Embden-Meyerhof chain, pentose phosphate cycle and key gluconeogenic enzymes. However, the carbohydrate metabolism in the oocyte differs substantially from that in the embryo.Oocyte maturation is followed by a complete loss of hexokinase (EC2.7.1.1),2-fold decrease of glycogen synthetase (EC 2.4.1.11) and 10-fold increase of glycogen phosphorylase (EC2.4.1.1) activity. This process is correlated with a gradual decrease of the ATP/(ADP+AMP) ratio from 4∶1 to 2∶1 and increase of the Fructose-6-Phosphate/Fructose-1,6-Diphosphate ratio from 0.27 to 2.0. Thus, oocyte maturation involves a number of changes in control mechanisms resulting in cessation of glycogen accumulation and a transition of carbohydrate metabolism from gluconeogenesis to glycogenolysis.
An acceleration of glycogenolysis after fertilization in loach (Misgurnus fossilis L.) eggs is not related to an increase of active phosphorylase (EC 2.4.1.1). The activity of "debranching" enzymes (amylo-1,6-glucosidase; dextrin-6-glucohydrolase, EC 3.2.1.33) considerably increases during early embryogenesis. These facts suggest that the enhancement of debranching enzyme activity is the controlling factor of glycogenolysis at early stages of embryonic development.Only 15-20% of the amylo-1,6-glucosidase and phosphorylase is found to be firmly associated with a particular glycogen fraction both in oocytes and embryos. The onset of glycogenolysis after fertilization is not accompanied by redistribution of the above-mentioned enzymes between different fractions of glycogen.
An acceleration of the rate of glycogenolysis in the early embryogenesis of the loach (Misgurnus fossilis L.) is accompanied by an increase of the content of hexose monophosphates, the rate of lactate formation and the rate of respiration. The unfertilized egg and the intact embryo of the loach have an identical activity of phosphorylase (EC 2.4.1.1.) and a constant ratio of the active/latent phosphorylase.Following the stage of 32 blastomeres, an increase of phosphorylase activity and the glycogen content occurs in the yolk-free embryo (blastoderm); this increase stops after the onset of gastrulation. In view of the facts that a) the blastoderm contains practically no latent phosphorylase, b) an elevation of phosphorylase activity is synchronized with an increase of the glycogen content, and c) this process is not related to an increase of the total phosphorylase activity and glycogen content in the intact egg, the authors suggest that glycogen-bound phosphorylase transfers from the yolk to the embryo at the stages of cleavage and blastula.In the loach oocyte, unfertilized egg and embryo the main activity of phosphorylase (more than 3/4) is associated with low molecular weight glycogen; this form of glycogen cannot be sedimented at 144000 g, and constitutes not more than 30 % of the total glycogen.Glycogen synthetase (EC 2.4.1.11) is, on the contrary, bound completely with granular glycogen. The oocyte maturation, ovulation and the onset of glycogenolysis after fertilization do not involve a redistribution of enzymes between glycogen fractions of different molecular weights.An increase of the glucose level in oocytes accelerates the conversion of active phosphorylase into its latent form. Physiological concentrations of glucose (up to 2 × 10 M) do not inhibit phosphorylase activity.
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