Identification of a sigma-class glutathione-S-transferase from the silkworm, Bombyx mori

A cDNA that encodes a glutathione-S-transferase belonging to sigma class (GSTS1) from the silkworm, Bombyx mori was cloned by reverse transcriptase-polymerase chain reaction and sequenced. The deduced amino acid sequence revealed 66%, 48% and 41% identity to sigma-class GSTs from Manduca sexta, Blattella germanica and Anopheles gambiae, respectively. The GSTS1 was also estimated to be close to those GSTs in a phylogenetic tree. GSTS1 mRNA was widely distributed in various tissues. The recombinant GST (rGSTS1) was functionally overexpressed in Escherichia coli in a soluble form, purified to homogeneity and characterized. The pH-optimum of rGSTS1 was around pH 8. The rGSTS1 retained more than 75% of its original activity after incubation at pH 4-9. Incubation for 30 min at temperatures below 40 degrees C did not affect its activity. rGSTS1 was able to catalyse the reaction of glutathione with 1-chloro-2,4-dinitrobenzene, the universal substrate for GST, as well as with 4-hydroxynonenal, the product of lipid peroxidation.