A cDNA that encodes a sigma class glutathione S-transferase (GST; bmGSTS2) from the silkworm (Bombyx mori) was cloned by reverse transcriptase polymerase chain reaction and sequenced. The deduced amino acid sequence revealed 68%-63% identity with the sigma class GSTs from other insects. bmGSTS2 mRNA was widely distributed in various tissues. The recombinant enzyme was functionally overexpressed as a soluble form in Escherichia coli, purified to homogeneity, and characterized. The optimum pH of bmGSTS2 was approximately pH 7.0, and bmGSTS2 retained >75% of its original activity after incubation for 12 h at pH 6.0-8.0. Incubation for 30 min at temperatures below 40°C did not affect the enzymatic activity. bmGSTS2 was able to catalyze the reaction of glutathione with hydrogen peroxide, and 4-hydroxynonenal. These results indicate that bmGSTS2 may play a role in antioxidant defense in the silkworm.
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