Comparative effects of chloroflavone congeners as inducers of hepatic microsomal monooxygenases in rats

The inductive effects of pretreatment with 10 synthetic chloroflavone isomers and congeners (80 mg/kg/d for 3 d, i.p.) on hepatic microsomal monooxygenases were examined with male Wistar rats. All chloroflavone congeners, except 3'-chloroflavone (CF), significantly increased (1.2- to 2-fold) the content of total cytochrome P-450 (P-450s) in microsomes. The effects of these congeners were evaluated based on the activities of microsomal aminopyrine N-demethylase, aniline hydroxylase and scoparone (6,7-dimethoxycoumarin) O-demethylase, and the CO difference spectrum and sodium dodecyl sulfate-gel electrophoretogram of cytochrome P-450s. The results were compared with those of phenobarbital (PB), 3-methylcholanthrene (MC) and PB plus MC. Based mainly on effects on the CO difference spectrum and ratio of the two O-demethylase activities of scoparone, 2'-CF and 2',4'-dichloroflavone (DCF) were categorized as the PB-type, 4'-CF and 6,8-DCF as the MC-type, 3',6-DCF and 3',5',6-trichloroflavone as weak MC-type, and 6-CF and 2',6-DCF as mixed (PB plus MC)-type inducers. A comparison of chloroflavone isomers and congeners indicated that (1) the presence of the 2'-chloro substituent of the flavone system is a minimal requirement for exhibiting the PB-type effect, (2) the absence of the 2'-chloro substituent is possibly that for exhibiting the MC-type effect and (3) the induction potencies by individual chloroflavone congeners may not be related to the degrees of chlorination.