Purification and characterization of glutatathione S-transferase cloned from pseudomonas sp. DJ77

Abstract

A new bacterial glutathione S-transferase phnC gene cloned from Pseudomonas sp. DJ77 was expressed in E. coli. The gene product PhnC was purified by glutathione-affinity chromatography. The results of SDS-polyacrylamide gel electrophoresis and gel permeation chromatography suggested that PhnC was a homo-dimeric protein, Mr of which is 42 kDa. The isoelectric point was 5.8 by isoelectric focusing. These Mr and pI values were similar to Mr (43200) and pI (5.5) predicted from DNA sequence of phnC. Furthermore, nine N-terminal amino acid residues determined by amino acid sequencer were the same as those predicted from DNA sequence. PhnC showed glutathione S-transferase activity toward glutathione and 1-chloro-2,4-dinitrobenzene (CDNB) and the specific activity was 10.6 $\mu$ mol/min/mg. The specific activity was higher than that of any other known bacterial glutathione S-transferase. PhnC was stable below $35^\circ C$ and between pH 6 and pH 9. The highest activity was shown at pH 8.0. An analysis of circular dichroism spectra of PhnC showed that the portions of $\alpha$-helix, $\beta$-sheet, $\beta$-turn and random coil structures in PhnC were 6, 13, 22, 49\%, respectively. The portions of secondary structures were somewhat different from those of known bacterial and mammalian glutathione S-transferases. Computer modelling of the inital-rate using nonlinear least-squares regression analysis favored a rapid-equilibrium ordered bi-bi mechanism with CDNB binding first, which was different from the kinetic mechanisms of mammalian GSTs.