Characterization of atrazine biotransformation by human and murine glutathione S-transferases

Atrazine is one of the most widely used herbicides in the United States and has been detected, occasionally, at low levels in drinking water sources. The biotransformation of atrazine in humans has not been fully characterized. Rodent studies suggest Phase I-dominated biotransformation with minor Phase II-mediated biotransformation by glutathione S-transferase(s) (GST). In human urine, mercapturates of atrazine are significant metabolites, yet the specific GST form(s) responsible for glutathione (GSH) conjugation have not been identified. Using recombinant alpha, mu, pi and theta class human GSTs, we demonstrated that only hGSTP1-1 displays significant activity toward atrazine (7.1 nmol/min/mg protein). We also confirmed that mouse GST Pi (pi) protein is responsible for the GSH-dependent biotransformation of atrazine in mouse liver; recombinant mGSTP1-1 had a specific activity of 7.3-nmol/min/mg protein. Furthermore, cytosolic fractions from mouse and human liver conjugated atrazine with glutathione at rates of 282.3 and 3.0 pmol/min/mg, respectively. Docking studies of the atrazine-GST conjugate in the hGSTP1-1 substrate-binding site were used to elucidate a basis for the dramatic difference in activity between mouse GSTP1-1 and GSTP2-2 (7.14 versus 0.02 nmol/min/mg protein, respectively). The inactivity of mGSTP2-2 appears to be attributable to an indirect structural disruption of the G-site by Pro12. Possible effects of the hGSTP1 polymorphisms were investigated. No significant differences in catalytic-specific activity were noted among purified proteins corresponding to the four hGSTP1 variants: hGSTP1(*)A (most common form), hGSTP1(*)B (Ile105Val), hGSTP1(*)C (Ile105Val, Ala114Val), and hGSTP1(*)D (Ala114Val). Overall, this work supports a physiological role for GSTs in atrazine biotransformation and indicates a novel diagnostic substrate for human and mouse GSTP1-1 proteins.