Studies on glutathione S-transferase in Helicoverpa ( = Heliothis) zea

The amount of glutathione S-transferase activity present in various stages of development and in various tissues of an insecticide susceptible strain of Helicoverpa zea was determined. GSH S-transferase activity toward CDNB (1-chloro-2,4-dinitrobenzene) was detected in the egg, and increased throughout the larval stages and was the highest in the 2 day old fifth instar larvae. GSH S-transferase activity toward DCNB (1,2-dichloro-4-nitrobenzene) could only be detected in the 2 day old fourth instar larvae, and the 2 day old fifth instar larvae. Enzyme activity using CDNB and DCNB as substrates was found in different tissues of the 2 day old fifth instar larvae, i.e. fat body, gut and cuticle. The fat body contained the highest enzyme activity for both substrates. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for CDNB conjugation in different stages of larval development and in the different tissues of the 2 day old fifth instar larvae, was approx. 0.24 mM. The enzyme responsible for CDNB-conjugation was purified 48 fold by GSH-agarose affinity chromatography. SDS gel electrophoresis and chromatofocusing resolved only one isozyme which is a homodimer, consisting of two equal size subunits having 23,900 mol. wt, with a pI value of 8.0.