title: Deconjugation of glucuronides catalysed by UDPglucuronyltransferase.
abstract: Evidence was found for UDPglucuronyltransferase-catalysed deconjugation of p-nitrophenol-, 4-methylumbelliferone- and phenolphthalein-glucuronides. The evidence is based on the following observations: 1, deconjugation is UDP-dependent and the reactions show Michaels-Menten kinetics with respect to UDP and glucuronide saturability; 2, UDP-glucuronic acid was identified as reaction product; 3, all studies were done in the presence of a beta-glucuronidase inhibitor; 4, induction profiles, using 3-methylcholanthrene and phenobarbital as inducing agents, were identical for conjugation and deconjugation reactions. Optimal deconjugation rates for p-nitrophenol- and 4-methylumbelliferone-glucuronides were at pH 5.1 and for phenolphthalein-glucuronide at pH 6.5. Only conjugation reactions showed latency; the corresponding deconjugation reactions were not latent. UDPglucuronyltransferase is a group of oligomeric isoenzymes with different molecular masses. The molecular masses of the isoenzyme species catalysing the forward and reverse reactions were determined by radiation-inactivation analysis. The molecular masses of the isoenzyme species mediating the catalyses of deconjugation reactions were significantly smaller than those mediating catalyses of conjugation reactions: 66 +/- 4 kDa vs. 109 +/- 7 kDa for p-nitrophenol; 82 +/- 8 kDa vs. 105 +/- 6 kDa for 4-methylumbelliferone; and 74 +/- 8 kDa vs. 159 +/- 14 kDa for phenolphthalein. This suggests that for catalyses of deconjugation reactions only part of a UDPglucuronyltransferase isoenzyme is needed, whereas for forward reactions the complete isoenzymes are required.
mesh_terms: D000818:Animals; D005965:Glucuronates / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006923:Hymecromone / Q000031:analogs & derivatives / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D008970:Molecular Weight; D010635:Phenolphthaleins / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism
chemical_list: D005965:Glucuronates; D007527:Isoenzymes; D010635:Phenolphthaleins; C016651:phenolphthalein glucuronide; C002730:4-nitrophenylglucuronide; D014535:Uridine Diphosphate Glucuronic Acid; D006923:Hymecromone; C034888:4-methylumbelliferyl glucuronide; D014453:Glucuronosyltransferase