GLUCURONIDATION OF PROPRANOLOL AND 4'-HYDROXYPROPRANOLOL - SUBSTRATE-SPECIFICITY AND STEREOSELECTIVITY OF RAT-LIVER MICROSOMAL GLUCURONYLTRANSFERASES

  • 1 January 1981
    • journal article
    • research article
    • Vol. 9  (5) , 466-471
Abstract
Liquid-chromatographic procedures were developed to assay the formation of diastereomeric glucuronides of the .beta.-blocker propranolol (PG) and 4''-hydroxypropranolol (HPG) by rat liver microsomes, with identifications performed by gas chromatography mass spectrometry 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, 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 are apparently glucuronidated by different forms of rat liver glucuronyltransferase. Propranolol glucuronidation occurs stereoselectively in vitro because of the different enzyme affinities for the enantiomers of the drug.