Hydroxylation and Subsequent Glucuronide Conjugation of Desmethylimipramine in Rat Liver Microsomes

Abstract

1. We have studied the hydroxylation and subsequent glucuronide conjugation of desmethylimipramine (DMI) as a two-step reaction in isolated rat liver microsomes, using cofactors for both hydroxylation and glucuronide conjugation. 2. The unconjugated hydroxylated intermediate metabolites accumulated more rapidly and reached higher steady-state levels at higher microsomal protein concentrations than at lower. The glucuronides formed from DMI by the two metabolic sequences appeared at increasing rate with increasing protein concentrations. 3. The total amount of accumulated metabolites was independent of the concentrations of UDPGA, showing that the hydroxylation rate was not influenced by the glucuronide conjugation. The proportion of glucuronides increased with increasing levels of UDPGA, even up to very high concentrations (10⁻² M). 4. SKF 525-A and high concentrations of 2-hydroxydesmethylimipramine (2-OH-DMI) inhibited the hydroxylation of DMI, possibly due to competition for binding to cytochrome P-450. 5. A type I spectral change induced by 2-OH-DMI in liver microsomal suspension diminished with time in the presence of UDPGA but not in the absence of this cofactor. This disappearance was more rapid in microsomes treated with digitonin.