Comparison Of Energy Related Metabolism In The Silk Glands Of The Silkworm(Bombyx Mori) Between The Protein Synthesis And Secretion Phases

Silk glands of the silkworm, Bombyx mori, are major organs for silk protein synthesis and secretion. The respiratory oxygen consumption rose rapidly in fifth instar of silkworm larvae(silk protein synthetsis phase), on the contrary, dropped significantly in spinning stage(silk protein secretion phase). Changes of oxygen consumption showed that there may have different characteristics of the energy related metabolism between protein synthesis and secretion phases in silk glands. Therefore, in this study, energy metabolism related indicators: oxidative phosphorylation, respiratory electron transport chain(ETC), the krebs cycle(TCA) and glycolysis(EMP) in silk glands have been determined. The main results are reported as follow:1. ATP content increased at silk protein synthesis phase, on the contrary, gradually reduced at silk protein secretion phase.The trends of ATP, ADP, AMP, ATP/ADP ratio, ATP/AMP ratio, energy charge and Complex Ⅴ activity were significantly different in silk glands between the phases of silk protein synthesis and secretion. ATP content increased at silk protein synthesis phase, on the contrary, gradually reduced at the secretion phase. However, changes of ATP can not be perfectly explained by variations of ADP, AMP, ATP/ADP ratio, ATP/AMP ratio, energy charge and Complex Ⅴ activity.2. Consistent with ATP content, Complex Ⅰ activity increased at silk protein synthesis phase, on the contrary, gradually decline at the silk protein secretion phase.Activities of Complex Ⅰ, Ⅱ and Ⅲ exhibited the different profiles in the silk glands between the phases of silk protein synthesis and secretion, whereas the same profiles of Complex Ⅳ and Ⅴ were observed. ComplexⅠactivity increased at silk protein synthesis phase, on the contrary, gradually reduced at the silk protein secretion phase, the same with the trend of ATP content. In addition, the trends of oxidation of coenzymeⅠ(NAD+) content and ComplexⅠactivity are the same. The results show that the change of ATP content in the silk gland mainly depends on the change of ComplexⅠactivity in ETC and NAD+ content. NAD+ content is significantly affected by TCA flux.3. Consistent with the change of ATP content, the activities of pyruvate dehydrogenase(PDH), mitochondrial isocitrate dehydrogenase(ICDHm) and the content of citric acid(CA) increased at silk protein synthesis phase, on the contrary, gradually reduced at silk protein secretion phase.The trends of PDH, citrate synthase(CS), CA, ICDHm and α ketone glutarate dehydrogenase(α-KGDH) except the content of acetyl-CoA and the activities of succinate dehydrogenase(SDH) are significant different between the phases of silk protein synthesis and secretion. Consistent with NAD+ content, activities of PDH, ICDHm and content of CA increased at silk protein synthesis phase, on the contrary, gradually reduced at silk protein secretion phase. The results show that TCA flux mainly depends on the activities of PDH, ICDHm and content of PA.The content of PA is resulted from the EMP flux.4. Consistent with the change of ATP content, activities of hexokinase(HK) and fructose phosphate kinase(PFK) increased at silk protein synthesis phase, on the contrary, gradually reduced at silk protein secretion phase.At silk protein synthesis and secretion phases, the trends of pyruvate kinase(PK) activity and pyruvic acid(PA) content are the same. But, consistent with the change of ATP content, HK and PFK activities increased at silk protein synthesis phase, on the contrary, gradually decline at silk protein secretion phase. The results show that the change of EMP flux mainly depends on HK and PFK activity.Therefore, the trends of energy metabolism in silk glands are opposite between the phases of silk protein synthesis and secretion, in which the content of ATP depends mainly on the activities of the key regulation enzymes: HK, PFK, PDH, ICDHm and ComplexⅠ.