Abstract
In order to clarify the pathways of carbohydrate metabolism during embryonic development in Bombyx mori eggs, changes in oxygen consumption, free sugar content, glycolytic intermediates and selected enzyme activities were monitored during the nine days of embryonic development. Two distinct phases of metabolism were found - a phase of differentiation (phase I), from day 1 to day 4, and then a phase of organogenesis (phase II), from day 5 to day 9. A gradual degradation of glycogen during phase I resulted in the production of trehalose, fructose and sorbitol. In this phase, phosphofructokinase was inhibited due to a sharp fall in levels of its activator, fructose-2,6-bisphosphate. Glycolytic flux leads to lactate formation during phase I. During phase II, activity of the mitochondrial enzyme, NAD-isocitrate dehydrogenase increased eight-fold, and oxygen consumption rose sharply by six-fold. These changes correlated with increased TCA cycle activity and the activated biosynthesis of mitochondria in phase II. Sharp increases in phosphofructokinase (eight-fold) and pyruvate kinase (18-fold) activities were observed during phase II. The increased glycolytic flux correlated with decreases in substrates (glycogen, trehalose, sorbitol and fructose), and the increased carbon flux was funneled into the TCA cycle as indicated by increased oxygen consumption and decreased lactate levels between days 3 and 6. The intense activation of glycolysis apparently overwhelmed mitochondrial capacity from days 6 to 9, as indicated by the marked increase in glycolytic intermediates including phosphoenolpyruvate, dihydroxyacetone phosphate and pyruvate, and the production of lactate even in the presence of high oxygen uptake. Levels of auxiliary enzymes changed during development; NAD-sorbitol dehydrogenase activity increased during phase I and trehalase activity increased during phase II. Glycerol-3-phosphatase activity decreased slightly and fructose-1,6-bisphosphatase-1 increased slightly over the nine days of development. In summary, phase I (differentiation) is fueled by glycogen and characterized by low glycolytic flux under largely anaerobic conditions, whereas phase II (embryogenesis) is characterized by increased glycolytic flux under aerobic conditions fueled by glycogen, in addition to the mixture of sugar substrates.
