Variations Of The Redox Status In The Silk Gland Of The Silkworm (Bombyx Mori) During The Fifth Instar

The silk gland of the silkworm, Bombyx mori, is not only the organ for thesynthesis and secretion of silk protein but also the important bioreactor for exogenousprotein. The redox status of cell participates in the regulation of division, differentiation,growth and programmed cell death. The ratios of reduced glutathione （GSH） andoxidized glutathione （GSSG）, and ascorbate acid and dehydroascorbate acid are themajor dynamic indicators of redox status, which declines mean the shift of the redoxstatus to the pro-oxidation conditions. Changes in the redox status of cell are closelyrelated to the metabolisms of hydrogen peroxide （H₂O₂）, GSH, AsA and nicotinamideadenine dinucleotide phosphate （NADP）. In this study, variations of redox status and itsrelated metabolisms were investigated in silk gland of the silkworm at the differentstages of the fifth instar. The main results obtained in this study are reported as follow:1. The redox status of the silk gland shifts into the pro-oxidation conditionsfrom the middle stage of the fifth instarThe ratios of GSH/GSSG and AsA/DHA respectively decreased from the late stageand the middle stage of the fifth instar, level of H₂O₂continuously increased, activity ofcatalase （CAT） continuously decreased, and levels of GSH and AsA significantlydecreased. The above results showed the oxidative shift of the redox status from themiddle stage of the fifth instar, which are mainly caused by the increase of H₂O₂level,the decline of CAT activity and the declines of GSH and AsA levels.2. The decline of GSH level in the silk gland is mainly due to the decreasedabilities for the synthesis and regeneration of GSHCoordination with the decline of GSH level, level of total glutathione （GSH+2GSSG）, activity of glutamate cysteine ligase and activity of glutathione S-transferasedecreased, level of DHA increased, and activities of thioredoxin reductase andglucose-6-phosphate dehydrogenase decreased, respectively. The above results indicated that the decline of GSH level in the silk gland is mainly due to the decreased abilitiesfor the synthesis and regeneration of GSH.3. The decline of AsA level in the silk gland is mainly due to the decline of totalascorbate acid （AsA+DHA） and the stronger oxidation of AsACoordination with the decline of AsA level, level of total ascorbate acid decreased,level of DHA increased, and no significant variation of dehydroascorbate reductaseactivity was observed. The above results showed that the decline of AsA level in the silkgland is mainly due to the decline of total ascorbate acid （AsA+DHA） and the strongeroxidation of AsA.Therefore, the redox status of the silk gland in the silkworm shifts into thepro-oxidation conditions at the middle and late stages of the fifth instar, in which thecorresponding variations of the metabolisms of H₂O₂, GSH, AsA and NADP areobserved.