Stimulation of the glucuronidation of digitoxigenin-monodigitoxoside by liver homogenates from spironolactone-pretreated rats

Abstract

Pretreatment of rats with spironolactone (SPL) produces an increased formation of the glucuronide conjugates of digitoxigenin-mono-digitoxoside (DMD) present in the bile after administration of digitoxin. For the digitoxin series of cardiac glycosides, DMD is the preferred substrate for in vitro conjugation with glucuronic acid by rat liver homogenates. The present experiments were performed to determine whether SPL pretreatment could produce an inceased glucuronidation of DMD by rat liver homogenates. Three different substrate concentrations (3, 30, and 100 nmol DMD/g of liver homogenate) were incubated under identical conditions with homogenates from control and SPL-pretreated rats. At 3.0-nmol. treatment with SPL produced a slight, but significant, increase in the production of aqueous alcohol-soluble metabolites of DMD by the experimental homogenates. With 30 nmol, treatment with SPL produced a 1.57-fold increase in the formation of aqueous alcohol-soluble metabolites of DMD. At 100 nmol, treatment with SPL produced a 3.44-fold increase in the formation of the aqueous alcohol-soluble metabolites of DMD. Treatment of these aqueous alcohol-soluble fractions (30-nmol substrate incubations only) with β-glucuronidase converted nearly 99% of the activity to a nonconjugate fraction. For both the control and the SPL-treated groups, greater than 92% of this nonconjugate fraction was determined by high-pressure liquid chromatography (HPLC) to be identical to the parent substrate (i.e., DMD). Thus, by selective enzyme treatment of the apparent conjugate and subsequent HPLC identification of the released products, the aqueous alcohol-soluble metabolites were identified as glucuronide conjugates of DMD. Pretreatment with spironolactone increased the rate of glucuronidation of DMD by rat liver homogenates.