Substrate specificities of two tau class glutathione transferases inducible by 2,4,6-trinitrotoluene in poplar.

Background: The genome of poplar (Populus trichocarpa) encodes 81 glutathione transferases (GSTs) annotated in eight distinct classes. The tau class is considered the most versatile in the biotransformation of xenobiotics and is composed of 58 GSTs. Two of the enzymes, GSTU16 and GSTU45, have particular interest since their expression is induced by exposure of poplar tissues to 2,4,6-trinitrotoluene (TNT) and could potentially be involved in the metabolism of this toxic environmental contaminant.

Results: DNA encoding these GSTs was synthesized and the proteins were heterologously expressed in Escherichia coli and the purified enzymes were characterized.

Major Conclusions: GSTU16 assayed with a number of conventional GST substrates showed the highest specific activity (60μmolmin⁻¹ mg⁻¹) with phenethyl isothiocyanate, 150-fold higher than that with CDNB. By contrast, GSTU45 showed CDNB as the most active substrate (3.3μmolmin⁻¹ mg⁻¹) whereas all of the 16 alternative substrates tested yielded significantly lower activities. Homology modeling suggested that the aromatic residues Phe10 and Tyr107 in the active site of GSTU16 are promoting the high activity with PEITC and other substrates with aromatic side-chains. Nonetheless, TNT was a poor substrate for GSTU16 as well as for GSTU45 with a specific activity of 0.05nmolmin⁻¹ mg⁻¹ for both enzymes.

General Significance: GSTU16 and GSTU45 do not play a major role in the degradation of TNT in poplar.