4-SUBSTITUTED 1-CHLORO-2-NITROBENZENES - STRUCTURE-ACTIVITY-RELATIONSHIPS AND EXTENSION OF THE SUBSTRATE MODEL OF RAT GLUTATHIONE-S-TRANSFERASE-4-4

In the present study, eleven 4-substituted 1-chloro-2-nitrobenzenes we re tested for their GSH conjugation capacity when catalyzed by base or rat glutathione S-transferase (GST) 4-4. Kinetic parameters (k(s) and K-m, k(cat), and k(cat)/K-m) were determined and subsequently used fo r the description of structure-activity relationships (SAR's). For thi s purpose, eight phgsicochemical parameters (electronic, steric, and l ipophilic) of the substituents and five computer-calculated parameters of the substrates (charge distributions and several energy values) we re used in regression analyses with the kinetic parameters. The obtain ed SAR's are compared with corresponding SAR's for the GSH conjugation of 2-substituted 1-chloro-4-nitrobenzenes, previously determined [Van der Aar et al. (1996) Chem. Res. Toxicol. 9, 527-534]. The kinetic pa rameters of the 4-substituted 1-chloro-2-nitrobenzenes correlated well with the Hammett sigma(p)(-) constant; the Hammett sigma(p), constant corrected for ''through resonance'', while the corresponding kinetic parameters of the 2-substituted 1-chloro-4-nitrobenzenes did not. The base- and GST 4-4-catalyzed GSH conjugation reactions of 2-substituted 1-chloro-4-nitrobenzenes depend to a different extent on the electron ic properties of the ortho substituents, suggesting the involvement of different rate-limiting transition states. The base- and GST 4-4-cata lyzed conjugation of 4-substituted 1-chloro-2-nitrobenzenes, however, showed a similar dependence on the electronic properties of the para s ubstituents, indicating that these substrates are conjugated to GSH vi a a similar transition state. Multiple regression analyses revealed th at, besides electronic interactions, also steric and lipophilic restri ctions appeared to play an important role in the GST 4-4-catalyzed GSH conjugation of 4-substituted l-chloro-2nitrobenzenes. Finally, the 4- substituted 1-chloro-2-nitrobenzenes were also used to extend the prev iously described substrate model for GST 4-4 [De Groot et al. (1995) C hem, Res. Toxicol. 8, 649-658], by which a specific steric restriction of substrates for GST 4-4 became clear.