Acceptor-specific glucuronyl transfer catalyzed by β-glucuronidase

Abstract

High purified rat liver microsomal or lysosomal β-glucuronidase (β- d-glucuronide glucuronosohydrolase, EC 3.2.1.31) catalyzes the specific transfer of glucuronyl residues from phenyl-β- d-[U- 14C]glucuronide to acceptor sugars. Specificity requirements of acceptor sugars are found to be: pyranose structure, 4C 1-conformation and equatorial position of C2 and C3 hydroxyl groups or pyranose structure, 1C 4-conformation and equatorial position of C3 and C4 hydroxyl groups. The acceptor capacities of 30 monosaccharides and glycosides including di- and tri- saccharides conform to this principle. The specificity of the β-glucuronidase catalyzed glucuronyl transfer is proved by the exclusive formation of β-glucuronyl (1–3)glycosidic linkages. Glucuronyl transfer rates increase with increasing donor substrate and increasing acceptor sugar concentration. In the presence of 1 M acceptor sugar the ratio of the transfer rate to the rate of enzymatic hydrolysis is about 2 : 1. An ‘acceptor substrate binding site’ on the surface of the β-glucuronidase molecule which brings the C3 hydroxyl function of the acceptor sugar close enough to the C1 atom of the glucuronyl residue, is postulated.