Biotransformation of coumarin derivatives (1). 7-alkoxycoumarin O-dealkylase in liver microsomes

The in vitro metabolic fate of 7-alkoxycoumarin was studied using liver microsomes. Microsomal enzyme catalyzed dealkylation of 7-alkoxycoumarin to 7-hydroxycoumarin in the presence of NADPH and molecular oxygen as cofactors was found to be one of the metabolic pathways. The metabolite 7-hydroxycoumarin was further metabolized to unidentified metabolite(s) in the presence of NADPH and O2 at a very slow rate, while the formation of the conjugate of 7-hydroxycoumarin with glucuronic acid was observed in the presence of UDPGA. Microsomal 7-alkoxycoumarin O-dealkylase activity was altered by the substitution of the alkyl group of the substrate, and the substitutions to either an O-propyl or an O-butyl group resulted in a decrease of the enzyme activity. Species differences were observed in the substrate specificity of microsomal O-dealkylation. The O-dealkylase activities in rat liver microsomes were stimulated by pretreatment of the animals with phenobarbital, regardless of the O-alkyl substituent at the 7 position of the coumarin ring. On the other hand, pretreatment with 3-methylcholanthrene or beta-naphthoflavone resulted in marked increase of O-deethylation. O-depropylation and O-debutylation activities, but not of O-demethylation activity. Pretreatment of animals with beta-naphthoflavone also resulted in remarkable stimulation of 7-hydroxycoumarin-glucuronide formation by the microsomal enzyme, while the conversion of 7-hydroxycoumarin to unidentified metabolite(s) was activated by the pretreatment of rats with only phenobarbital. The O-dealkylation activities in liver microsomes from intact and phenobarbital pretreated rats were inhibited markedly by the addition of hexobarbital to the incubation mixture, but no inhibition was observed with alpha-naphthoflavone. On the other hand, the O-dealkylation activities in microsomes from beta-naphthoflavone-pretreated rats were inhibited remarkably by alpha-naphthoflavone. These results confirmed that several microsomal enzymes, including the cytochrome P-450's and UDP-glucuronyltransferase, participate in the biotransformation of 7-alkoxycoumarin, and these enzymes are regulated differently by inducers.