In vitro metabolism of simazine, atrazine and propazine by hepatic cytochrome P450 enzymes of rat, mouse and guinea pig, and oestrogenic activity of chlorotriazines and their main metabolites

1. The in vitro metabolism of chlorotriazines, simazine (SIZ), atrazine (AT Z) and propazine (PRZ) in liver microsomes from rat, mouse and guinea pig a nd the oestrogenic activity of chlorotriazines and their main metabolites h ave been studied. 2. The formation rates of products in chlorotriazine metabolism were determ ined by HPLC. The principal reactions catalysed by the cytochrome P450 (P45 0) system were N-monodealkylation and isopropylhydroxylation in all liver m icrosomes. As a result, 2-chloro-4-ethylamino-6-amino-1,3,5-triazine (M1) ( SIZ-M1 for SIZ and ATZ-M1 for ATZ) and 2-chloro-4-amino-6-isopropylamino-1, 3,5-triazine (M2) (ATZ-M2 for ATZ and PRZ-M2 for PRZ), and 2-chloro-4-ethyl amino-6-(1-hydroxyisopropylamino)-1,3,5-triazine (M3) (ATZ-M3 for ATZ) and 2-chloro-4-isopropylamino-6-(1-hydroxyisopropylamino)-1,3,5-triazine (M4) ( PRZ-M4 for PRZ) were detected as the metabolites. N-bidealkylation was not found in this system. 3. The formation rates of N-deethylated metabolites (SIZ-M1 and ATZ-M2) wer e generally higher in mouse than in rat and guinea pig. The formation rates of N-deisopropylated metabolites (ATZ-M1 and PRZ-M2) in guinea pig were th e lowest among the three animal species. The formation rates of isopropylhy droxylated metabolites (ATZ-M3 and PRZ-M4) were remarkably low in mouse com pared with rat and guinea pig. 4. The enzyme kinetics of chlorotriazine metabolism were examined by Eadie- Hofstee analyses. Some species differences in Michaelis-Menten parameters f or each metabolite were observed, and the ranking orders were Varied among the metabolites. 5. The binding affinity of chlorotriazines (SIZ, ATZ and PRZ) and their met abolites (M1-4) for recombinant human oestrogen receptor-a was assayed usin g the fluorescence polarization method. The binding affinity of M2 was sign ificantly higher than those of parent compounds and other metabolites, alth ough the oestrogenic activity was remarkably low compared with that of 17 b eta-oestradiol (E2). 6. These results suggest that the pattern of metabolism of SIZ, ATZ and PRZ by the P450 system differs extensively among rat, mouse and guinea pig, an d that M2 may be an activated metabolite of chlorotriazines.