Increased digitoxin cleavage by liver microsomes of spironolactone-pretreated rats

Pretreatment of rats with spironolactone caused an fourfold increased cleavage rate of the sugar chain of digitoxin (dt-3) in vitro yielding digitoxigenin-bis-digitoxoside. This was due to an enhanced, cyt. P450 dependent, formation of 15'-dehydro-dt-3, the intermediate which has to be formed before the terminal sugar can be split off. The second reaction catalysed by microsomal monoxygenases, the 12-beta-hydroxylation, was only increased by a factor 2. In contrast to the effects of spironolactone no increase of metabolism could be observed after phenobarbital pretreatment. From our results it may be concluded that the enhanced dt-3 metabolism in vivo is mainly caused by spironolactone inducible monoxygenases which catalyse the oxidation of the terminal sugar.