DECONJUGATION OF BILIRUBIN-IX-ALPHA GLUCURONIDES - A PHYSIOLOGICAL-ROLE OF HEPATIC-MICROSOMAL BETA-GLUCURONIDASE

Beta-glucuronidase is an acid hydrolase located in both the lysosomal and microsomal compartments of the hepatocyte. The function of the lat ter remains undefined. We postulated that microsomal beta-glucuronidas e may be responsible for the deconjugation of bilirubin-IXalpha glucur onides which are synthesized primarily in the hepatic microsomal compa rtment. We utilized two unique congenic strains of mice to characteriz e the role of hepatic beta-glucuronidase in the metabolism and disposi tion of bilirubin-IXalpha; the first exhibited less than 1% of total h epatic beta-glucuronidase activity (ATM), the second lacked only the m icrosomal enzyme activity (AT1). The biliary excretion of bilirubin-IX alpha conjugates was quantitated using reverse-phase high performance liquid chromatography. Under basal conditions, there was a 2-fold incr ease in the biliary excretion of bilirubin-IXalpha monoglucuronides an d total glucuronides in the AT1 and ATM mutants compared to the normal controls. When the plasma bilirubin-IXalpha level was increased to al most-equal-to 7 mg/dl to simulate hyperbilirubinemia, by intravenous a dministration of [C-14]bilirubin-IXalpha, mathematical modeling of the biliary excretion curves of bilirubin-IXalpha glucuronides revealed q ualitative differences between control and mutant animals, whereas bot h mutant groups were similar. Collectively, these data demonstrate tha t microsomal beta-glucuronidase modulates the net rate of bilirubin-IX alpha glucuronidation and glucuronide excretion in bile, under both ba sal and hyperbilirubinemic conditions, and that lysosomal beta-glucuro nidase has no such effects. Hepatic microsomal beta-glucuronidase appe ars likely to influence the biliary excretion and hence the hepatic el imination of endogenous and xenobiotic substrates (e.g. carcinogens) w hich undergo hepatic glucuronidation.