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Abstract:
Rat renal microsomes catalyzed the glucuronidation of l-naphthol, 4-methylumbelliferone and p-nitrophenol, whereas morphine and testosterone conjugation were not detected. In contrast, all five substrates were conjugated by hepatic microsomes; the activity was typically 5-10 times greater than with renal microsomes. Renal microsomal UDP-glucuronyltransferase toward l-naphthol was fully activated (six-fold) by 0.03% deoxycholate while the hepatic enzyme was fully activated (eight-fold) by 0.05% deoxycholate. Full activation of hepatic UDP-glucuronyltransferase occurred when microsomes had been preincubated at 0 C with deoxycholate for 20 min. This effect of preincubation was not observed with renal microsomes. The presence of 0.25M sucrose in the buffers during renal microsomal preparation resulted in a two-fold greater rate of l-naphthol conjugation in both unactivated and activated microsomes than renal microsomes prepared in phosphate buffers alone. Preparation of hepatic microsomes with or without 0.25M sucrose had no effect on UDP-glucuronyltransferase activity. Unactivated (-deoxycholate) renal enzyme was activated when incubations were done at a low pH (5.7), whereas fully activated (0.03% deoxycholate) renal microsomal UDP-glucuronyltransferase displayed a pH optimum at 6.5. Renal microsomal UDP-glucuronyltransferase activity toward l-naphthol, p-nitrophenol and 4-methylumbelliferone was induced by pretreatment of rats with beta-naphthoflavone and trans-stilbene oxide but not by phenobarbital or 3-methylcholanthrene. These data demonstrate that renal UDP-glucuronyltransferases are different from the hepatic enzymes with regard to biochemical properties, substrate specificity and in response to chemical inducers of xenobiotic metabolism.