STRUCTURE-ACTIVITY-RELATIONSHIPS FOR THE GLUTATHIONE CONJUGATION OF 2-SUBSTITUTED 1-CHLORO-4-NITROBENZENES BY RAT GLUTATHIONE-S-TRANSFERASE-4-4

In the present study structure-activity relationships (SAR's) are desc ribed for the experimentally determined kinetic parameters (K-m, k(cat ), and k(cat)/K-m) of the GST 4-4-catalyzed reaction between GSH and 1 0 2-substituted 1-chloro-4-nitrobenzenes. Steric, lipophilic, and elec tronic parameters were correlated with the kinetic parameters. Moreove r, charge distributions and several energy values were calculated for the substrates and the corresponding Meisenheimer intermediates with M eS(-) as a model nucleophile for the thiolate anion of GSH and used in the regression analyses. The correlations obtained were compared with the corresponding SAR's for the base-catalyzed GSH conjugation reacti on at pH 9.2. A high correlation coefficient was found between the kin etic parameter k(s) for the base-catalyzed reaction and the Hammett su bstituent constant(sigma(p)). Much lower correlation coefficients were obtained with k(cat) and sigma(p) and with k(cat)/K-m and sigma(p). M oreover, the reaction constant rho was significantly higher for the ba se-catalyzed than for the enzyme-catalyzed reaction. Also, high correl ations were found between the kinetic parameters and the charges on th e p-nitro substituent in the substrates. When k(s) was plotted against these charges, a linear relationship was found in which the slope was larger than the slope of a corresponding plot with k(cat)/K-m. The Ha mmett sigma(p) can be divided into an inductive (F) and a resonance (R ) component. With multiple regression between the kinetic parameters a nd F and R, higher correlation coefficients were obtained than with si gma(p) alone. Our observations suggest that the transition states for the base-catalyzed and the GST 4-4-catalyzed GSH conjugation reaction are different. Moreover, single classical physicochemical and computer -calculated molecular parameters and combinations of them can be an al ternative approach for examining SAR's for spontaneous and GST-catalyz ed GSH conjugation reactions.