beta-glucuronidase latency in isolated murine hepatocytes

The physiological function of microsomal beta-glucuronidase is unclear. Sub strates may be either glucuronides produced in the lumen of endoplasmic ret iculum (FR) or those taken up by hepatocytes. In the latter case, efficient inward transport of glucuronides at the plasma membrane and the ER membran e would be required. Therefore, the potential role of beta-glucuronidase in ER was investigated. Isolated mouse hepatocytes and mouse and rat liver mi crosomal vesicles were used in the experiments. Selective permeabilization of the plasma membrane of isolated hepatocytes with saponin or digitonin re sulted in an almost dr 4-fold elevation in the rate of p-nitrophenol glucur onide hydrolysis, while the permeabilization of plasma membrane plus ER mem brane by Triton X-100 caused a further 2-fold elevation. In microsomal vesi cles, the e-nitrophenol glucuronide or phenolphthalein glucuronide beta-glu curonidase activity showed about 50% latency as revealed by alamethicin or Triton X-100 treatment. A light-scattering study indicated that the microso mes are relatively impermeable to both glucuronides and to glucuronate. On the basis of our results, the role of liver microsomal beta-glucuronidase i n the deconjugation of glucuronides taken up by the liver stems unlikely. H ydrolysis of the glucuronides produced in the ER lumen may play a role in s ubstrate supply for ascorbate synthesis or in "proofreading" of glucuronida tion. BIOCHEM PHARMACOL 59;7:801-805, 2000. (C) 2000 Elsevier Science Inc.