Glucuronidation of propranolol and 4'-hydroxypropranolol. Substrate specificity and stereoselectivity of rat liver microsomal glucuronyltransferases

Liquid-chromatographic procedures were developed to assay the formation of diastereomeric glucuronides of propranolol (PG) and 4'-hydroxypropranolol (HPG) by rat liver microsomes, with identifications performed by GC/MS techniques. Propranolol was conjugated at a rate 10% of that determined for 4'-hydroxypropranolol. Glucuronyltransferase activity increased slightly (10-17%) in the presence of MgCl2. Inclusion of 0.04% Triton X-100 produced a 55% inhibition of PG formation, but increased HPG formation greater than 2-fold. Pretreatment of animals with phenobarbital resulted in a 4-fold increase in PG formation, but did not affect HPG formation unless MgCl2 was also present. Under these conditions, a 50-60% increase in HPG formation occurred. Pretreatment with 3-methylcholanthrene did not affect the formation of either glucuronide. (R)-(+)-Propranolol was glucuronidated 2-fold faster than the (S)-(-) enantiomer at substrate concentrations below 0.1 mM, and 1.3-fold faster at substrate concentrations above 2.0 mM. The estimated Vmax, 0.67 nmol/mg/min, was identical for both enantiomers. The dissociation constants were significantly different, however, being 0.57 mM for (R)-(+)-propranolol and 0.87 mM for (S)-(-)-propranolol. No stereoselectivity was observed in the formation of HPG when 4'-hydroxypropranolol was used as substrate, or when propranolol was incubated in the presence of an NADPH-generating system. Propranolol and 4'-hydroxypropranolol were used as substrate, or when propranolol was incubated in the presence of an NADPH-generating system. Propranolol and 4-hydroxypropranolol are apparently glucuronidated by different forms of rat liver glucuronyltransferase. Furthermore, propranolol glucuronidation occurs stereoselectively in vitro because of the different enzyme affinities for the enantiomers of the drug.