Mechanistic insights into the inhibition of human placental glutathione S-transferase P1-1 by abscisic and gibberellic acids: An integrated experimental and computational study

The interactions of the classic phytohormones gibberellic acid (gibberellin A(3), GA(3)) and abscisic acid (dormin, ABA), which antagonistically regulate several developmental processes and stress responses in higher plants, with human placental glutathione S-transferase P1-1 (hpGSTP1-1), an enzyme that plays a role in endo- or xenobiotic detoxification and regulation of cell survival and apoptosis, were investigated. The inhibitory potencies of ABA and GA(3) against hpGSTP1, as well as the types of inhibition and the kinetic parameters, were determined by making use of both enzyme kinetic graphs and SPSS nonlinear regression models. The structural basis for the interaction between hpGSTP1-1 and phytohormones was predicted with the aid of molecular docking simulations. The IC50 values of ABA and GA(3) were 5.3 and 5.0 mM, respectively. Both phytohormones inhibited hpGSTP1-1 in competitive manner with respect to the cosubstrates GSH and CDNB. When ABA was the inhibitor at [CDNB](f)-[GSH] (v) and at [GSH](f)-[CDNB]v, V-m, K-m, and K-i values were statistically estimated to be 205 +/- 16 mu mol/min-mg protein, 1.32 +/- 0.18 mM, 1.95 +/- 0.25 mM and 175 +/- 6 mu mol/min-mg protein, 0.85 +/- 0.06 mM, 1.85 +/- 0.16 mM, respectively. On the other hand, the kinetic parameters Vm, K-m, and K-i obtained with GA(3) at [CDNB](f)-[GSH](v) and at [GSH](f)-[CDNB](v) were found to be 303 +/- 14 mu mol/ min-mg protein, 1.77 +/- 0.13 mM, 3.38 +/- 0.26 mM and 249 +/- 7 mu mol/min-mg protein, 1.43 +/- 0.07 mM, 2.89 +/- 0.19 mM, respectively. Both phytohormones had the potential to engage in hydrogen-bonding and electrostatic interactions with the key residues that line the G- and H-sites of the enzyme's catalytic center. Inhibitory actions of ABA/GA(3) on hpGSTP1-1 may guide medicinal chemists through the structure-based design of novel antineoplastic agents. It should be noted, however, that the same interactions may also render fetuses vulnerable to the potentially toxic effects of xenobiotics and noxious endobiotics.